For anyone just starting with digital images, or having trouble getting started, here is an review of the first basics we need, about how to USE our digital images, about how to resize them for viewing them on the video screen or for printing. This is about those first basics of resizing images (i.e., the necessary steps to be able to USE and show your images). These methods apply to digital camera images, and scanned photos or documents, any digital image.
Why might you want to know? There is good reason:
The problem if Printing: So, you took some important pictures with your digital camera, and you will send them to Walmart or CVS or wherever to be printed, say some 5x7 inch size and a few 8x10 inch size. How do you prepare that? If you do nothing except simply just send them in, they will print them at specified size you ordered, but humans won’t see them first. Meaning, their printing machine can handle resampling the SIZE, but if they are not the same SHAPE as the paper (and they won’t be), then there will be some cut off areas that won’t fit on the paper shape. This could mean cut-off heads and not centered and such. Not the best results. The machine can’t look, and doesn’t care. The print sizes 4x6, 5x7, 8x10 inches are three different shapes. DSLR cameras are 3:2 aspect which at least does match 4x6 paper shape, but the compact and phone cameras are 4:3 aspect which doesn’t match anything. Camcorders vary, but their stills might be 16:9 or 4:3 shape. The odds are “few to none” that any picture can ignore needing this close attention (and the printing machines can’t look and don’t care).
The solution is that some human (meaning you, to be your choice) first processes each image to prepare it for printing, cropping its shape to match the desired paper shape (centering the scene as you choose) for the best fitting the important scene area into that cropped result. This is really easy to do, maybe only several seconds each after you get the hang of it, but it takes a human who cares about best result. Instead of cutting into the subject, you simply crop away the uninteresting or distracting or blank wasted areas, giving due importance to the desired subject area, also with the result matching the intended paper shape. Details about how are on the next page here, but basically you choose a Crop Tool box the shape of the paper (say 4x6, 5x7 or 8x10, or maybe width x height is 6x4, 7x5, or 10x8 for landscape orientation), and then simply mark that crop box on the image with the mouse, and position and size it as seen to be the best view result that you prefer. This box can change Size as desired, but this tool remains the same Shape (but there is a similar option that does NOT remain the same shape). Then you are expected to say “Wow, that was easy”, and this will be even easier and faster to do the second time you see it. And the print will come out just as you marked it.
One caution: Do NOT overwrite your original file, which would interfere with later future plans. The SAVE operation should specify a different file name. If you will print it on two sizes of paper, you should prepare two appropriate crops (each from the original), and label the final file name as which shape it is so you will know when selecting them to be printed. Otherwise, the printing machine will simply crop them as necessary to fit the paper shape.
This much is all that is actually required to prevent bad cropping results. If necessary, the print shop will resample it to the smaller size required to print, but you may be uploading huge 16 or 24 megapixel images when you only need about 2 megapixels for best 4x6, or about 3 megapixels for 5x7 prints, or 7 megapixels for 8x10. It does need to be large enough (about 250 to 300 pixels of size per inch of print), but too large is simply a waste. It is trivially easy for you to resample them to appropriate size yourself while you’re at it. I’d add that as a desirable plan (this resampling is also on the next page).
Or if planning a slide show on a HDTV television set: First, there are no inches or cm defined in video images. Video images show on any size screen, which are also dimensioned in pixels. HDTV screens are 1920x1080 pixels or 1280x720 pixels. That is the image size, and the shape is Aspect 16:9, which is 1.778 times wider than tall. If you don’t know about the TV that will show them, you could just plan on either size and all will be fine (some TV channels send one size, others send the other size). But 1920x1080 will be optimum for all cases. But if you try to show the original huge images, it will seriously slow the TV loading each one. And if not 16:9 shape, they won’t fit the screen shape, so there will be black bands at both sides. The black bands are maybe not so serious for video, except they cause showing a smaller picture than you could show if you cropped the shape to 16:9 for the HDTV showing. If the images will be shown more than once, for guests other than yourself, a little preparation is certainly worthwhile. Images vary, some include wasted space easily cropped to 16:9, but some are already filled tightly, so that 16:9 just won’t work for them, and black side bands may be better for some. So there are choices to be made. But again, the situation is the same as the printing above — with different numbers, but which is to crop to TV shape (16:9) and then resample to TV size (1920x1080 is about 2 megapixels, or 1280x720 is about 1 megapixel).
Again, a very good plan is to Never overwrite your original camera file. This work just mentioned will be awesome for your current plan, but the future could have different plans for the image, unknown now. I’d suggest only making copies, and Never overwriting your original camera file. They are not too big to store some archived place. Maybe on a CD or DVD disc in a desk drawer.
So that is what this article is about, how to do that easily and correctly. Which is how to first crop to shape, and then resample to size, for viewing or printing. In truth (at least for me), our pictures didn’t always frame the scene the best way, so a little additional tighter cropping improves most pictures anyway (getting rid of distracting areas, and excess nothingness wasting space at the edges). Kodak’s Better Photo Tips always said “Get close to your subject”, and cropping is one way, especially when reducing Size for printing or video. And for most of us, this is a fun part of the photography hobby; the creation ain’t done until it is your best result. This actual instruction is on the next page, but first, some more reasons and background are shown here. Knowing why tells you how.
Please appreciate that this concept might be compared to learning to drive... sure, there are a few details to learn: how to operate the car, the laws and the rules of the road, safety concerns, maintenance needs, and maybe how to change a flat tire, etc. But you learn it one time, and it's not difficult, everyone does it. Then it becomes second nature to us, very easy, and we always know how. So then you can use this skill for the rest of your life, to great advantage, when simply knowing how will make a huge difference.
This much of image processing (preparing for printing or viewing) is easy to learn and is described on next page. The terms might be new, but the concept is just about size and shape of the image. Good viewing and printing skills are an advantage to learn. We do need to realize that digital images are composed of pixels, and that image size is dimensioned in pixels. Not inches, not bytes, but pixels. You just gotta understand this, and then it is simple, and this one detail will be repeatedly mentioned here.
Continuing here with some background explanations:
So image size is dimensioned in pixels, for example a 4000x3000 pixel image. Megapixels is the total image area (image width x height), so a 4000x3000 pixel image is 4000x3000 = 12 megapixels.
Digital images are binary data, meaning, each pixel is just NUMBERS. The numbers are digital data describing ONE COLOR for that tiny area of the one pixel, to be shown as a tiny dot of color, rather like one colored tile in a mosaic tile picture is one color of a tiny area. The big difference is that the tile is not digital — the tile is an actual color instead of numbers. Either way, our brain recognizes the reproduced image in the arrangement of those pixels or tiles. Pixels are all there is in a digital image, so we must think of it that way. It’s dimensional sizing (X×Y pixels of Width×Height) will make sense when you do (a few more details are at digital basics).
What is a pixel again? A pixel is only digital data numbers which describe one color (one of 16.7 million possible RGB color shades in JPEG image files). Digital means that each pixel is just numbers which represent the magnitudes of the three primary color components of Red, Green, and Blue, called RGB color. For example, we know red and green make yellow, so a yellow pixel’s color is large numbers for red and green, but very little if any blue. It is called digital data because it is just numbers of data — there is nothing of that actual color that can be seen (until the viewing device uses the numbers to create and show the color). When viewing the image, printing or the video screen knows its own method to create the right color from those numbers. There are more details, but which are not necessary just to order prints made. The RGB color system is important to technicians, and some degree of detail is always nice to know, but only a few details are essential for just printing or viewing images (primarily the size and shape of the image, the topic here). But each pixel is only digital numbers describing which one of the possible color shades that its tiny area should be.
So relax, again, we don't need to know much technical depth just to use our camera. To use our digital images properly and skillfully for printing and viewing, we do need to realize that pixels do exist, in fact, pixels are all that does exist in our digital images. Primarily, we must understand that our images are in fact dimensioned in pixels, an image is so many pixels wide, and so many in height — which describe both a size and a shape. We can see them enlarged above. Just accept it, and think in terms of pixels. :) Then things can proceed properly.
There are two general methods to view a digital image: (details of these two do vary considerably, see digital basics).
Images have both size and shape. There is not always great advantage of exactly fitting the image shape to the screen shape, unless you want it to show larger, full screen size, which is an advantage for your home slide shows on the HDTV. But very few web images are shown full screen, too large becomes a strong disadvantage on the web. But whatever shape it is, we can view it on the screen, whereas printing photos normally should fit the paper shape.
Viewing the images on a video screen — The digital camera takes large images, maybe most are generally 10 to 24 megapixels size. But the HDTV screen size is only about 2 megapixels in size... and cell phone screens are maybe 0.2 megapixels. Sure, you can ignore everything, and simply show the 24 megapixel image as is, but then the video system has to first resample it down to fit on a 2 megapixel HDTV screen size, which takes more time and storage space, much slower waiting to access each one.
HDTV screens are either 1920x1080 or 1280x720 pixels of screen size. Some models of HDTV television have a USB port, with a menu to easily show JPG images from a USB memory stick. Another option to show images on HDTV is that the streaming media players which also have a USB port (Roku 3, etc) have similar options to show image files (and these may show your video movie files - but check for compatible video file formats, like .mp4 or .mov or .mkv files).
For work more critical than a one time show, you can instead properly prepare an improved copy of the images to be the right size, to fit the size and shape of a 1920x1080 pixel HDTV screen, which will then become fast and peppy and will show larger, filling the screen. Hugh images will load slow, and is wasted effort since the screen size is 1920x1080 pixels.
4K video: 4K TV sets are available, of size 3840x2160 pixels (which is called UHD size, Ultra HD). However no broadcast channels or cable channels offer 4K, they do 1280x720 or 1920x1080 pixels. However, 4K Blu-ray DVD and some streaming and satellite channels are offering some UHD 4K media. UHD fits HDTV screen shape.
Printing the image on paper is much more demanding about shape. At least for borderless photos, the image shape routinely needs to match the paper shape. Our photo paper is often 4x6, or 5x7, or 8x10 inches, which are three different Shapes, probably not matching your image shape. We can easily resize the the image Size, but if the Shape does not fit the paper, some of the image is cropped off and lost. So a very big issue is that the image SHAPE is often (usually) NOT the same SHAPE as the print paper, until we crop it to match. Size is a factor too (number of pixels), and is important, but this is speaking of SHAPE right now (aspect ratio). The best procedure is to crop it to desired shape first (centering your scene as you best choose), and then resample to desired size.
That's really about it, but you just gotta think about pixels. Doing anything at all with digital images REQUIRES knowing at least this much. It is quite easy, if you let it be (if you will simply think "pixels"). If your complaint has been that the photo lab does not print all of your picture area (cuts off heads, etc), then you really need to learn this easy material. It is easy, but we all have to know this much.
We may not be greatly disturbed if our image has some unfilled blank margins on the side of the video monitor or HDTV screen, but cutting heads off of paper prints needs more attention.
There are two main ideas here:
More introduction still below, but some direct links to page 2 topics:
I wouldn't say it's a problem, it's just an issue which we must address. It is a problem if we don't address it. Images do have a shape, and paper has a shape, and the image and print paper sizes are very often NOT THE SAME SHAPE until we correct it. For example, we simply cannot make a 4x6 inch image fit 4x5 inch paper, not without losing an inch off the end, perhaps the top off of someones head. This fact is perfectly obvious, but we tend to forget that images also have a shape. So we simply first crop the image to be the same Shape as the paper goal, easy to fix, and an important step.
Aspect Ratio is the shape of things — the shape of the image and the shape of the paper. The rectangular shapes may be long and thin in various degree, or might be more nearly square. There is absolutely nothing fancy or complicated about this. Aspect Ratio is the simple ratio of the two side dimensions.
This Crop to fit the paper can be a trivially easy operation (see Cropping on next page). We don't need any math skills. Better photo editors (Adobe Elements for example) should have a simple cropping option to declare the desired paper aspect ratio to be cropped (enter crop as 8x10 shape for example), and then any image crop box we can draw on the image will be this exact shape (meaning shape, not size). We simply mark its crop size to include what we want to include, and it will be that shape (and we can move it around as desired). As for size, our crop result does need to have sufficient pixels to print at about 250 to 300 pixels per inch of paper (e.g., 1000x1500 to 1200x1800 pixels to print 4x6 inches). More detail on next page.
Images from a DSLR camera (and also from 35 mm film) are aspect ratio of 3:2. We can enlarge it to any SIZE, but this means for example that the SHAPE of the 3:2 uncropped images will enlarge to print 4x6 inches, 8x12 inches, 16x20 inches, etc. Regardless if the image is oriented portrait or landscape (vertical or horizontal), all of these are still SHAPE of 3:2. This shape will print "as is" on 4x6 paper, but all other paper sizes probably are all different shapes. It is just a way to describe a rectangular shape with numbers, the simple ratio of the lengths of the two sides.
There are two similar ways to describe this shape in numbers. For example, for a 6000x4000 pixel image, the ratio of the two sides is 6:4, which we reduce to say 3:2. This is also the ratio of 3/2 = 1.5, or 1.5:1 when one side is compared as "one". Same thing, and we may encounter it either way (typically we say it as 3:2, but we compute with 1.5).
A few very common Aspect Ratio numbers are: (more detail on next page)
|4:5||1.25:1||4x5, 8x10 inches, 16x20, 20x25|
|4:3||1.33:1||Most compact cameras, phones, and non-widescreen old movies and monitors|
|5:7||1.4:1||5x7 inches, 13x18 cm. Also 2.5x3.5 inches wallet size|
|3:2||1.5:1||4x6 inches, 8x12, 10x15 cm, 35 mm film, and most DSLR cameras|
|16:9||1.78:1||HDTV format and camcorders, 1920x1080 or 1280x720 pixels|
We might call an aspect ratio as being either 3:2 or 2:3. These are the same paper "shape", just depending on which way the image is rotated. The usual "correct" ratio is when the image is viewed in its proper upright orientation, then aspect ratio is Width:Height, whichever that is. We all understand it either way, but by convention, camera sensors (images yet untaken) are generally named landscape orientation (largest first, 3:2 or 4:3 or 16:9 or 5:4), but print paper purchased still blank is normally named as portrait orientation (smaller first, 4x6 or 5x7 or 8x10 or 8.5x11). Obviously the paper can be rotated either way for printing, and we match that by specifying either Portrait or Landscape orientation in our printer driver options, which then simply rotates the image correctly for that paper orientation (load the paper the right way).
Aspect ratio is only critical when matching an image shape to one printed paper shape, or maybe to full screen monitor shape. Only one ratio fits another shape. And since many shapes exist, no one ratio number is very important, except for your current match, when it is all important.
Size and Shape are different things. Cropping can change the image shape (to fit the paper shape). When we only enlarge the image size, it stays the same shape. A 4:5 image can fit 4x5 or 8x10 paper, but is not the shape of 4x6 or 5x7 paper.
If you use a 3:2 DSLR (or 35 mm film) and only print 6x4 prints, these are the same shape, so you may never realize there is any problem. But all other combinations will see a problem.
Photo images from a compact or phone camera are 4:3 shape (long side is 1.33x the short side, 4 to 3 ratio). However, 4:3 is NOT a common paper shape. Some photo printing labs offer 4x5.33 inch prints which do fit them (if you specify it). And 6x8 or 9x12 inch paper may be available a few places.
A rough visual guide in the camera viewfinder, for estimating the planned crop lines for future printing:
Note that the cropping need not be centered, it could be all at one side or the other, just positioned as needed to frame the important view. And note that if cropping it smaller than this (retaining same ratio), both width and height will be reduced.
Cropping serves two purposes, for the best appearance, and to fit the print paper.
In general, cropping a little tighter often pictorially improves many images anyway (removes empty, uninteresting, or distracting surroundings, and makes the subject a bit larger). Often it can be a big plus.
Kodak's standard tips for Taking Great Pictures always emphasized "Get Closer". Cropping tighter later can correct to serve the same purpose (if it leaves sufficient pixels remaining), and cropping is usually a necessary step for printing anyway (to match the paper shape to the image shape). Opinions will always vary about specific details, but cropping is very easy to do. We can crop to paper shape any way we want (include a larger or smaller area, and/or move it around to a different location on the original image), so why not give a little thought to improving your prints too? The biggest benefit is to first give it a seconds thought in the viewfinder. Slightly changing camera position (to choose background) can be a big help sometimes. The purpose of the viewfinder is to examine your image BEFORE you click the shutter button.
Most printing labs (where we send photo files to be printed) set up their machines to fill all of the paper. Normally they won't underfill the paper to leave white space, they will instead cut something off the image if necessary. If you order 5x7, you will get 5x7, but you may not get your entire image. This means if you print your images without first preparing them (cropping them to the correct shape to fit the paper ordered), you may see surprises about parts of your image cut off, not showing in the final print.
This is nothing new with digital cameras, film was always the same thing. The film negative has a shape too, generally simply not the same shape as the print paper. However back then, there was a human operator watching and controlling and making decisions and adjusting things for film images. Digital machines are automatic (inexpensive prints), but digital does make it easy for us to crop it right first. Many online photo printing web sites offer a crop tool there online, and also ordering sometimes warns when our image is not the same size as the selected paper. And some processors offer an option to "Print Full Image", meaning, they won't crop anything off, but will instead leave blank white space (borders) where it doesn't fit the paper. You can trim the paper smaller then.
But generally, the paper shape itself is going to crop it, and a different shape simply will not fit... something has to go. And the point is that when we crop it ourself, then we see it, and we can judge and decide ourselves which portion of the image is to be cropped away... probably we can choose to crop the edge opposite from the head we want to save. :)
Size and shape are very different things. We cannot fully print a 4x6 image on 4x5 paper. The purpose of this next aspect ratio calculator is to help give an idea of the crop for the aspect ratio of the paper you want it to fit. The calculator is a minor thing (see details in Cropping section on next page for the actual procedure), but perhaps the numbers can make the major point that cropping to fit the paper shape is usually very necessary. The paper is usually NOT the same shape as the image, so we can choose to fit the long side to the paper, or to fit the short side to the paper. I am trying to emphasize the difference between SHAPE vs SIZE, but both are important. The point of the calculator is to show that image shape and paper shape are often NOT the same shape (so we must crop the image shape to fit the paper). Most camera images won't fit the paper without cropping. Results are better if we crop it ourself, for our choice about what is to be cropped off.
This one is about shape (aspect ratio). It does not matter if you use inches or mm in this one, since it only computes (maximum / minimum, or width / height) to determine aspect ratio, which is shape. It compares paper shape to image shape, and reports any mismatch.
Here, specify your image shape and your paper shape numbers. Calculations assume borderless printing. The calculator just computes the precise image numbers with shape which fit the paper shape. Another option is to enter "Other" for paper, and then enter any actual paper dimensions. But the shapes of the image and paper do need to match fairly closely. Again, you can enter it as inches or mm or cm. Entering 5x4 or 10x8 or 50x40 or 100x80 will all give the same answer, because it is Not about size, this one is only about shape (aspect 5:4 in this example, all of which here divide out to be 1.25:1 aspect ratio). Inches or mm are NOT USED THIS TIME (only their shape). The only dimensions used here are paper aspect ratio and image pixels (trying to emphasize the concept of image and paper SHAPE).
The Summary Chart assumption is that the print shop will fill the specified paper shape. The percent error is the cropped off percentage of the image when the paper is filled. The words End or Width here refer simply to the cropped long End or the narrow paper Width (both Portrait or Landscape Orientation can fit). But if only one dimension is fitted leaving blank space on the other side, the same percentage applies to the blank space.
Most example paper sizes it shows are common to inches, but the same numbers could be mm sizes too, since this one is only about shape.
The procedure to prepare the image for printing:
The NEXT PAGE is the easy way about How To Do the Crop to Fit Paper Shape.
For example, to print 6x4 inches at 300 dpi, resample to
(6 inches × 300 dpi) × (4 inches × 300 dpi) = 1800x1200 pixels.
For example, to print 15x10 cm at 300 dpi, resample to
(300 dpi × 15 cm / 2.54) × (300 dpi × 10 cm / 2.54) = 1772x1181 pixels... (2.54 cm per inch)
(Or see Image Size Goal for Desired Image Size calculator below)
After this concept is realized, then see Another Calculator for a more elaborate working tool (about the image you will need). But actually, that is overkill too (still trying to show the concept), since once the concept is understood, then most photo editor resize boxes alone ought to be fully sufficient for preparing for printing (see next page, crop to shape, then resample to size).
The exact percentage numbers are not the point here. They are what they are, and any difference is not good. We know that a 4x6 image simply will not fit on 4x5 paper. And images and paper are often not the same shape, which means the image must be cropped to be the same shape as the paper. This is just trying to emphasize that point here. We have good tools making this crop very easy to do, and it's an extremely important skill to learn. The point is, if you crop it yourself first, then how it turns out is your own choice. More about cropping to aspect ratio on next page.
Just a few pixels is normally no big deal, but otherwise, we should choose the best way to crop the image to the correct SHAPE first (to match the paper shape), and after that (if image is too large), then we might resample the image SIZE to print at about 300 dpi on this paper. I worry that novices not yet familiar with images may miss the point. Do realize that cropping to the paper shape is an extremely important issue when printing an image.
Aspect ratio is just the simple ratio of the two sides. 6000x4000 pixels is 6000:4000 = 3:2 = 1.5:1 ratio. This ratio represents the Shape, the simple ratio of the sides. Like all math fractions, we normally reduce the 6000:4000 to express it as the ratio 3:2, using the greatest common divisor method discovered by Euclid, 4th century B.C. (GCD is 2000 in this case). Math does like exact numbers, and if our image had been 5998x4000 pixels, then greatest common divisor is 2, so reduction comes out 2999:2000, not exactly 3:2, and not as easily recognized (the calculator may show rounded minor differences).
So image Shape is important, and also, image Size is important. We can crop the shape, and resample the size (to be smaller — resampling larger does not add needed detail). Discussed on next page, but for printing goals, the necessary procedure should be to first crop image to match the paper shape, and then resample smaller to produce image size of about 300 dpi. That means for example, to print a 6x4 inch print at the ideal 300 dpi resolution computes:
Sufficient pixels to print at 250 to 300 dpi is optimum to print photo images. More pixels really cannot help the printer, but very much less is detrimental to quality. This is very simple, but it is essential to know and keep track of. This simple little calculation will show the image size needed for optimum photo printing. This calculation is so simple, and is one thing you really need to know, and it should be second nature to you, to be considered when printing any image.
This simple little calculation shows the concept, and can serve two purposes:
Or same thing, 3000x2400 pixels printed 8x10 inches will be printed at 300 dpi resolution.
There is a larger dpi calculator that knows about scanning, printing, and enlargement.
It's important to realize that an area scanned at 300 dpi will create the pixels necessary to also print the same size area at 300 dpi. The concept either way is pixels per inch. Scanning and printing at 300 dpi is likely what you want for a photo copy job (a line art scan of black text or line drawings can use 600 dpi well).
However, this size does NOT need to be exact at all, 330 dpi or 300 dpi or 272 dpi does not really matter much, but somewhere near this size ballpark (of 250 to 300 pixels per inch) is a very good thing for printing.
If 3000 pixels are printed 10 inches size, then the printing dpi is 3000 pixels / 10 inches = 300 dpi.
If 2820 pixels are printed 10 inches size, then the printing dpi is 2820 pixels / 10 inches = 282 dpi.
When viewing images on a monitor screen, we're used to images not filling the entire monitor screen. So while artistic cropping is still important, there is no paper shape to match. HDTV is 1920x1080 pixels (or 1280x720 pixels), and 6x4 inch prints at 300 dpi is 1800x1200 pixels, close enough to same for either to show well on the HDTV screen (assuming Landscape orientation), but printing on paper really needs the correct shape.
A printing tip: It is often artistically good to consider cropping a little tighter in camera, when taking the picture (just meaning, show what you want to show, but think about it. Empty or extra space may not offer much, other than distractions from the subject).
However, for printing, realize that is not quite the final result, because your best effort will still need to be cropped a little more to fit the paper shape. It's necessary to think ahead, and provide this little extra for the later cropping.
3:2 camera images match 6x4 paper, but printing 5x7 or 8x10 involves cutting a bit of the long ends off (6.7% or 16.7% of total length, respectively). Of 6000 pixels, 16.7% is 1000 pixels.
From 4:3 cameras, 8x10 cuts a bit of the long end off (6.3%), but 5x7 or 4x6 trims some of the long side off (4.8% or 11.1% of total width, respectively).
So it can be very awkward when you really have not planned any extra there to be trimmed off to fit the paper. If you don't know what size you may print, it's a good plan to leave a little extra all around. Then plan on cropping it better when preparing images for screen or printing. We have so many megapixels now, this is not normally an issue.
Just a little attention will quickly make this be automatically intuitive. Which it needs to be.
However, we may not know yet what size it might be printed, so leaving a little space for cropping can be a good thing (so long as you do crop it).
When preparing for printing a few different sizes from one image (say for portraits of one image printed 4x6, 5x7, and 8x10 inch), what I do is crop it three times and create three proper output images to upload. If I don't crop it, the printer's machine will, but the machine is dumb (cannot recognize what the image is). So... (if printing multiple sizes) I create all of the appropriate separate copies (from that one same original). If there are different sizes of same image, I also add the size into the file name to help me keep it straight when selecting them online for printing.
For 8x10 inch prints, I crop 4:5, resample to 2400x3000 pixels, and add "8x10" to that output file name.
For 5x7 inch prints, I crop 5:7, resample to 1500x2100 pixels, and add "5x7" to that output file name.
For 4x6 inch prints, I crop 2:3, resample to 1200x1800 pixels, and add "4x6" to that output file name.
For wallet size prints, we can simply use the 5x7 image, since they are the same "shape" (next page). The shop will resample its size, still same shape.
This is easy trivial stuff to do, and then I know exactly how it will come out, and the crop results are always perfect. My notion is that the expensive shops possibly might have a paid employee to verify acceptable cropping of each print, but the least expensive shops (one employee to work the counter while the machine prints) just feed it into the machine, which doesn't know, can't decide, and doesn't care. Even if there might be a person preparing for the printer, they cannot know your particular preference, how you actually want it to look. But if you crop it to paper shape yourself, then you always get your choice. Image shape is a primary concern about printing.
Possibly you are the only human that will see it before the print is returned to you (meaning the machines will print it automatically, in their way). The printer's web site probably does offer a Crop tool option for you to crop it online first, after you upload it (however, that can still be a problem if you need multiple print sizes of one). Today, this crop to paper shape is Your Job to do it. I simply just do it at home, and then upload all the proper images, and then no surprises. The rest of this article (next page) is about how to do it, and this is about the least that we need to know about using digital images.
When doing anything with digital images, the first question is "What size is the image?" Digital image size is dimensioned in Pixels. Pixels is what it is all about, and digital is very different than film. If any mystery about pixels, here is a short primer: What is a Digital Image Anyway?, or a more detailed summary at Pixels, Printers, Video - What's With That? Once we accept that pixels actually exist, it's all quite easy.
Resize is a term too vague and ambiguous, it has no specific meaning until we say what it means. There are three very different ways to "resize" an image, and all three have very different meanings and results. These topics are on the next page, but first a summary.
Cropping discards those trimmed pixels, making the image pixel dimensions smaller, but primarily, it changes the scene included, and often the shape too. Different paper sizes (4x6, 5x7, 8x10 inches, are each a different shape — therefore we also often crop to make our image shape match the paper shape. Our camera always makes its images of the same one shape (aspect ratio, which is width:height), but our intended use often needs other shape(s), to fit it to the printed paper size or viewing screen size. And frankly, a little cropping often improves the composition of many images, removing distracting or uninteresting blank nothingness around the edges, concentrating the actual subject larger (zooming tighter, so to speak). More detail at Cropping.
Examples before we get into how to do it. This was a D800 camera image, 36 megapixels, 7360 x 4912 pixel dimensions. To show it here on the web screen (our screens are no more than 2 megapixels size, and many are not even that), it was resampled to an arbitrary 500x333 pixel size, 0.167 megapixels. And by the way (a different subject), do note that even this small image is still quite enlarged here, because the lens image on the camera sensor was very much smaller. But now perhaps near 5 or 6 inches wide on some screens here (screens vary), but the FX sensor was only 1.4 inches wide, a DX sensor is almost 1 inch wide, and a phone or compact camera sensor is less than 1/4 inch wide. Like film sizes (which are generally small too), that's still a considerable enlargement that we see.
Original was 7360x4912 pixels, 3:2, 36 megapixels.
Your screen needs to be at least 500 pixels wide to see it as 500. It is still 3:2, but a vastly smaller image now. The original size is quite capable to print large sizes, but the shape will need cropping for various paper shapes.
To emphasize the shape differences, note that 2:3 is a longer and thinner shape, 4:5 is shorter and wider.
The camera has no clue what size you might print the image, if at all, so it just makes up some dpi number (it does not affect the pixels). We fix it before printing (called scaling, which simply edits the dpi number).
Cropping or Resampling are NOT reversible operations (pixels do get changed), however Scaling is completely reversible, it merely changes the separate dpi number that will be used to adjust pixel spacing (resolution) when printing on paper. Scaling does not change pixels, it merely spaces them differently.
If we wanted to print an image that is 2000 pixels tall, then:
PPI: I learned it as dpi (dots per inch), but some people like to say ppi now (pixels per inch, which it is). When I started, it was only said as dpi, so dpi is quite second nature to me, and to many. It may have been jargon, but a pixel is a dot of one color, which is normally printed as a few colored ink drop dots (usually of 3 or 4 ink colors to simulate the pixel color). If dpi is used referring to image pixels, it is about pixels, so interchangeable terms if about images instead of about ink drops. If about images, dpi can only mean pixels. We need to understand either use.
Scaling the image is simply changing the dpi number in the file that tells the future printer to print "X pixels per inch". It will be the spacing of "pixels per inch" on paper (same pixels, just different spacing). Scaling the dpi or ppi number does not change any pixels in any way, and it does not change the appearance of image seen on our video monitor, not in any way (resample certainly does though, but our screen video systems don't even look at the dpi or ppi number, they just show pixels directly, one for one, but possibly having to resample a too-large image smaller first to fit it on the screen).
Scaling is sometimes called Resize, and Resample is even sometimes called scaling, (not really unreasonable), so the terms can be questionable (what they actually mean in the given usage). My definition of Scaling is about changing the size the image will print on paper (inches), specifically WITHOUT any pixel resampling. It is about declaring the dpi number, in preparation for printing a certain size on paper. This is by far the simplest operation, but sometimes a bit harder to grasp it.
Printing - Before getting into resizing details, first some reasons for them. Printing at home is different than sending the work out.
We upload an image, which is some size and shape, and tell them to print it on some paper size and shape, like 8x10 inches. If we tell them 8x10 inches, that is what we get, regardless of any other information about the shape or size of our image. If we have not already cropped the image properly (to match 8x10 paper SHAPE), the edges of the paper will crop it. Hopefully we have used the resize methods described here to be certain the image is appropriate size and shape to be printed that size. Because if we don't crop the images (shape) correctly for the paper size (shape), the shops printer will crop them automatically, often in surprising unwanted ways, cutting off tops or sides of the image. We should also be sure our image has sufficient pixels to print the size we specify. That image dimension number should ideally have been prepared to be at least 250 pixels per inch, or better 300 pixels per inch, of print dimension, mentioned next below.
Saying again, if you send the printing out and order 5x7 inch prints, it does not matter what the actual dpi number in the image file says, because they will rescale whatever you send to print the 5x7 inch print that you ordered. And the paper will crop it, one way or another. Your image dimensions (the resample) should be sufficient so that they necessarily will compute a dpi number around 250 to 300 dpi... around 1500x2100 pixels for a 5x7 print at 300 dpi. Sending more pixels cannot help. Many fewer can be a problem — low resolution, but ± 10% or 15% difference on dimensions and resolution is not much issue. We like to send 300 dpi, but in fact, most of these Fuji or Noritsu type machines are set up to print at 250 dpi. We get good results either way.
Photoshop for example, if you saved the image with the File - Save As menu, it does embed the dpi number that was set in the Image Size dialog, and will default to print that size. But if you used File - Export - Save For Web (which was File - Save For Web in older versions), it strips out the Exif, so there is no dpi number saved, and Photoshop will then assume 72 dpi, which won't be helpful. There probably is still an option to Scale to fit media size (the selected paper size) — you just need to ensure that resulting resolution will up near 250 or 300 dpi then. But if that file number has been scaled to say 300 dpi, it will scale the image to cover one inch for every 300 pixels, which is what determines print size. A 3000 pixel dimension at 300 pixels per inch will then print 10 inches (this is called scaling).
With either method, a little of our attention first will be naturally be necessary to ensure the desired results (sections on next page). If we want 8x10, we need to crop it to 8x10 shape, and resample it and scale it to 8x10 size. This is not probably going to happen unless you do it.
A printing calculator might be of some help, at least to see the scaling concepts the first time. But the photo editor Image Size tool should do the same calculations, and should do all you need.
My own notions about how to prepare our megapixel size camera images for printing are this:
Steps 1 and 2 can routinely improve our image tremendously.
When we print in photo editors (for example in Adobe Elements or Photoshop), we specify paper size, and the default print size in inches is computed from the pixel dimensions and the value of the dpi number stored in the file (3000 pixels at 300 dpi, then 10 inches, which hopefully we prepared for this printing). But we can also print it a different size. This is also scaling. Scaling is changing the dpi number so it prints a different size. Or if you send the pixels out to be printed, and specify 5x7 inches, they will scale it to 5x7 for you. But the printer device itself needs that dpi number when it prints images (which is the only use for this dpi number).
Adobe has maybe one quirk. The menu File - Save As - JPG will save the Exif data (not all of it, but some of it is selectable), but always including the printing resolution dpi information, so the file can print properly. However, the menu File - Export - Save for Web can save the same image, but does Not save any Exif or dpi information (saves a few bytes, and video monitors have no use for dpi). Then subsequent showing of this file in Adobe will show it as 72 dpi, which is a fake number, which only simply means "no dpi information is present".
Any cropping or resampling changes are definitely seen on the video screen too. However, scaling does NOT affect the image seen on the video screen in any way. We will see no change at all, because video totally ignores any dpi number and shows pixels directly. But scaling will print a different size on paper if the dpi number changes. More detail on Scaling on next page.
OK, here we go. Next page is the Cropping, Resampling, and Scaling topics.