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).
Please appreciate that this can be compared to learning to drive... sure, there are a few details, how to operate the car, and the rules of the road, etc. But you learn it one time, and it's not difficult, anyone can do it. It becomes second nature to us, we always know how. So then you can use the skill for the rest of your life, when knowing how will make a huge difference.
Images are easier to learn, seems less to it. We do need to know that digital images are composed of pixels, and that image size is dimensioned in pixels. You just gotta understand this, and it is simple, and this one detail will be repeatedly covered here.
So image size is dimensioned in pixels, for example a 4000x3000 pixel image. Megapixels is the total image area (image width x height multiplied), so a 4000x3000 pixel image is 4000x3000 = 12 megapixels.
Digital images are binary data, meaning, each pixel is just NUMBERS, data describing ONE RGB COLOR for that tiny area, a tiny dot of color, much like one colored tile in a mosaic tile picture. Our eyes recognize the reproduced image in those pixels or tiles. Pixels are all there is in a digital image, so we must think of it that way. It will make sense when you do (a few more details at digital basics).
But relax, we don't need to know much technical depth. To use our digital images properly and skillfully, mostly we only 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. 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 vary, see digital basics).
There is not much advantage of fitting the image shape to the screen shape. It need not be full screen, few images are. Whatever shape it is, we can view it.
Viewing the images on a video screen - The digital camera takes large images, maybe most are generally 10 to 24 megapixels size. But the computer screen size, and also the HDTV screen size, are only about 2 megapixels in size... and cell phone screens are maybe 0.2 megapixels. Sure, you can ignore everything, and simply show the 12 megapixel image as is, but then the video system has to first resample it down to fit on a 2 megapixel or 0.2 megapixel screen size, which takes more time and storage space, slower waiting to access each one. (Many models of HDTV have a USB port, which easily shows our JPG images BIG from a USB memory stick... but still, 1920x1080 pixels is all the screen can show.) For more than a one time show, you can instead simply properly prepare a copy of the images to be the right size, perhaps to fit the 1920x1080 pixel HDTV screen, which will then become fast and peppy.
Printing the image on paper is much more critical, the image shape needs to match the paper shape. If it does not all fit on the paper, some of the image is cropped off and lost. One big issue is that the image SHAPE is often NOT the same SHAPE as the print paper, until we crop it to match. Size is a factor too (number of pixels), and important, but speaking of SHAPE right now (aspect ratio).
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 is 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 monitor or HDTV screen, but cutting heads off of paper prints needs more attention.
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. 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 of course perfectly obvious, but we tend to forget that images also have a shape. So we simply first crop the image to 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 sides - if the long side is 6 inches and if the short side is 4 inches, then the long side is 1.5x the short side, and the sides are the ratio of 3 to 2, which we call aspect ratio. It describes the shape of the paper. Images from a DSLR camera (and also from 35mm 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, and more. But all 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.
Aspect ratio is only critical when matching an image shape to 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 enlarge the image size, it stays the same shape. A 4:5 image can fit 4x5 or 8x10 paper, but not the shape of 4x6 or 5x7 paper.
If you use a 3:2 DSLR (or 35mm 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 an available paper shape. So these images suffer a similar problem of needing to be cropped to fit the desired paper.
Most printing labs (where we send images to be printed) set up their machines to fill all of the paper. They won't underfill the paper, they will instead cut something off the image if necessary. If you order 5x7, you will get 5x7, but you may not get all of your image. This means if you print your images without first preparing them (cropping them to the correct shape to fit the paper ordered), you will 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, simply not the same shape as the print paper. However back then, there was a human operator watching and making decisions and adjusting things for film images. Digital machines are more automatic (inexpensive prints), but digital does make it easy for us to crop it right first. Some online photo printing web sites offer a crop tool there, and will warn 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 any 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 size itself is going to crop it, a different shape simply will not fit... something has to go. And of course 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. :)
The editor resample box (Resample section on next page) will be your most useful tool. It is a calculator, and you can try different numbers to see the result numbers (then click Cancel then instead of OK, to Not actually change your image). It does not crop, and you do want "Constrain Proportions" always On, to avoid image distortion.
Specifics: Size and shape are very different things. We cannot fully print a 4x6 image on 4x5 paper. The purpose of this 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 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).
Calculator: Specify your image size and your paper size (numbers only). Calculations assume borderless printing. The calculator just computes the precise numbers which fit the paper. The most detailed option is to enter "Other" for paper, and then enter actual paper dimensions (adds dpi information). Image size need NOT be exact for printing. 300 dpi is the usual goal for photos, but 240 to 360 dpi might be acceptable for printing. But the shapes of the image and paper do need to match fairly closely.
The exact percentage numbers are not the point here. They are what they are, and any difference is not good. We know of course that a 4x6 image simply will not fit on 4x5 paper. And images and paper are often not the same shape, which of course means the image must be cropped to be the same shape as the paper. We have good tools making this 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 how-to about cropping to aspect ratio on next page.
Just a few pixels is normally no big deal, but otherwise, we of course 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 (2000 in this case). Math does like the exact numbers. If our image had been 5998x4000 pixels, then it 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:
Image Size: 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 size does NOT need to be exact at all, 300 dpi or 282 dpi does not really matter, 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 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.
OK, an example 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 much smaller. Now perhaps about 5 inches wide on some screens here (screens vary), but the FX sensor image was only 1.4 inches wide, a DX sensor image is almost 1 inch wide, and a compact image sensor might be less than 1/4 inch wide. Like film sizes (which are generally small too), that's a considerable enlargement. But here, this example is about other properties, like shape.
To emphasize the difference between size and shape, 2:3 is a longer and thinner shape, 4:5 is shorter and wider.
If we wanted to print an image that is 2400 pixels tall, then:
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.
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 an issue. We like to send 300 dpi, but in fact, almost all of these Fuji or Noritsu type machines are set up to print at 250 dpi. 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 - Save For Web, 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 insure 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 insure 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 will do the same calculations, and will 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).
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 at Scaling on next page.
OK, here we go. Next Page is the Cropping, Resampling, and Scaling topics.