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Image Resize
Cropping, Resampling, Scaling

(This is about the Least that we must know about using images)


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, safety concerns, and the rules of the road, etc. But you learn it one time, and it's not difficult, anyone can do it. Then 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, and safer too. :) We do need to know 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 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), so a 4000x3000 pixel image is 4000x3000 = 12 megapixels.


The concept of pixels: This is an enlarged view of a tiny 58x58 pixel area of this picture (right end of stem just below center), shown at 800% size to be able to see the pixels.

This is a 400x500 pixel image, 0.2 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).

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.

Shapes are a major printing issue

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 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 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.

Cropping to Fit Image Aspect Ratio to Paper

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.

Cropping for Printing

Existing image size: x pixels

Compute crop to fit a paper size ratio?

Yes
No
aspect
"Other" paper size: x
inches
mm

The calculator allows comparing an image to a paper shape ratio. Clicking Yes will select the ratio dropdown box, and "Other" in it selects the two WxH value boxes below it (for paper size).

The largest Crop possible (maximum dimensions) to fit the paper shape is shown for the two situations of fitting the long or the short side to the paper shape.

Summary of
Shape Mismatches

Dimension percentage difference

The table is a summary of common calculator results, four image shapes (aspect ratios) on eleven paper shapes. Comparing shapes, the second column computes dimensions of a print on each paper shape. Most camera images won't fit the paper without cropping. Results are better if we crop it ourself, choosing what is to be cropped off.

The percent error is the percentage of the cropped off image, or the percentage of the blank unfilled paper (depending of which dimension is fitted to the paper). Both alignments have the same percent numbers.

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 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:

(6 inches x 300 dpi) x (4 inches x 300 dpi) = 1800 x 1200 pixels needed (2.16 megapixels)

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 method is one thing you really need to know, it should be second nature to you, considered when printing any image.

Desired Image Size Goal

To print   x  
inches
mm

at dpi resolution  

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.


A printing tip:   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, and the machine is dumb (cannot recognize what the image is). So... I create all of these separate copies (from that one same original). If there are different sizes of same image, I also add the size to 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 it, still same shape.

This is easy trivial stuff to do, and then I know exactly how it will come out, and the results are always perfect. My notion is that the more expensive shops might have an 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. But if you crop it to paper shape yourself, then of course you get your choice. Image shape is a primary concern about printing.

Possibly you are the only human that will ever see it until after it is printed. But their web site probably does offer a Crop tool option for you to do it online first, after you upload it (however, that is still 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, 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.

Resizing Images

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.

  1. Crop it - to simply cut away some at the edges, to include less area in the final image. A little like zooming in a little tighter, but done afterwords. Same action as scissors on paper, so to speak, except if we still have sufficient pixels remaining, we can still enlarge our image later, to print on larger paper.

    • Cropping tighter can improve the composition by removing empty or uninteresting or distracting side detail that contributes nothing, and draws attention away from the subject.
    • Cropping tighter increases the size of the subject in the frame, making it more dominant.
    • Much less arbitrary, cropping is often required to make the image Shape fit the print paper Shape.

    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.

  2. Resample it - to create a new image of different image dimensions (in pixels). Resampling might for example replace 4000 pixels across with only say 1000 new pixels across, still the same scene view, but a much smaller image. In this case, every four pixels of width and four pixels of height (16 pixels) is combined into one new image pixel... 16 colors averaged into one new color (a pixel is just a color of an area). Reasons would be to make an excessive size image smaller, maybe to show it smaller on the video screen, or to send it as email, or to print a smaller paper print. The plan is to make the image size more appropriate for a purpose of using it. There is no going back, so do not overwrite the original - this second one should be a copy, with a different file name. Resampling is not reversible, resampling smaller discards pixels (detail) in order to be smaller. The smaller copy has enough pixels for the smaller size of course, but less than before. More detail on next page.

    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.


Resampled image, 500x333 pixels, 0.166 megapixels.
Original was 7360x4912 pixels, 36 megapixels, 3:2 aspect ratio. 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 other paper shapes.

Cropped image 450x563 pixels, 0.253 megapixels.
Different view and shape. It is 4:5 shape now, which "shape" could fit 8x10 paper (needing 2400x3000 pixels). We might debate the cropping choice, but cropping can fit the paper shape. Cropping can also often improve images, to be better pictures.

To emphasize the shape differences, note that 2:3 is a longer and thinner shape, 4:5 is shorter and wider.


Resampled image, 300x200 pixels, 0.060 megapixels.
Fewer pixels in a smaller image, but same view and same shape as the uncropped image.
  1. Scale it - The third way to resize is to scale your existing image for print paper. Scaling does NOT change the image pixels or the image shape in any way (no resample, no crop), so scaling does affect the video screen (which does not use dpi). Scaling works when printing images on paper. Its only action is to change the single number for dpi (ppi), which is an arbitrary number that is simply stored separately in the image file. It is only used by the printer, and it only changes the size this image will print on paper (at so many pixels per inch). The number has no effect on images on the computer screen. The camera has no clue what size you might print the image, and it just makes up some number. We fix it before printing (called scaling, which simply edits the dpi number). FWIW, some like to say ppi now, but I started when it was only said as dpi (same thing, and interchangeable if about images instead of ink), so dpi is second nature to me.
    Word definition: One use of the word is that a scale is a graduated measurement, like the scale of a map, and scaling is creating a proportionate size or extent. 100 miles of map drawn at 20 miles per inch will cover 5 inches of paper. In photo printing, scale is pixel distribution relative to the paper dimension. Scaling is computing that 3000 pixels printed at 300 pixels per inch will scale to cover 3000/300 = 10 inches of paper. Setting the 300 dpi number "scales" the 3000 pixels to print 10 inches on paper. The dpi number fresh out of the camera might say 72 dpi, 180 dpi, 240 dpi, 300 dpi, etc, but which was arbitrary, it simply does not matter what it is. The camera has no clue what size we will print it, and this number called dpi has absolutely no meaning until we actually do this proper scaling for the one specific future printing purpose.
    So scaling stretches the image on the paper, without changing any actual pixels. It changes only the actual dpi number (printing resolution, the spacing of the pixels on paper), which changes only the number of inches the same image pixels will fill on paper (spaced at so many pixels per inch). Because, the image simply prints larger with a smaller dpi number (wider pixel spacing, fewer pixels per inch), or prints smaller with a larger dpi number. Your PC Print menu normally offers this with its Fit To Paper choice, but again, it changes only printed size, and NOT shape.

    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:

    • Scaling it to 300 dpi (simply changing the number to 300) would tell the printer to fill 2000/300 = 6.67 inches of paper.
    • Scaling it to 250 dpi (simply changing the number to 250) would tell the printer to fill 2000/250 = 8 inches of paper.
    • Scaling it to 200 dpi (simply changing the number to 200) would tell the printer to fill 2000/200 = 10 inches of paper, etc.

    I learned it as dpi, but some people say ppi now. If about image pixels, it has the same meaning, it can only mean pixels.

    Scaling it simply is 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.

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 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:

  1. First process to correct the image exposure and white balance, make it be perfect. It can really come to life when just right. If you might want to print multiple sizes, Save a large processed master copy before any cropping and resampling (so it will still be capable of any future action).
  2. Crop the image for better composition, for example to eliminate excess surrounding blank space detracting from the subject. It also makes the subject larger in the image frame. And if to be printed, specifically crop it to match the desired print paper shape (aspect ratio on next page)).

    Steps 1 and 2 can routinely improve our image tremendously.

  3. Then Resample the image appropriate for about 250 or 300 dpi print resolution size.
    For example, the image size goal to print these print sizes at 300 dpi is:

    • 4x6 inches - 1200x1800 pixels    (this is simply pixels = inches x "pixels per inch")
    • 5x7 inches - 1500x2100 pixels
    • 8x10 inches - 2400x3000 pixels
    • 11x14 inches - 3300x4200 pixels
    • 16x20 inches - 4800x6000 pixels
    • The pixel numbers need not be exact (± 10% is fine, it still prints within ± 10% of 300 dpi), but the ratio should be close (there is no reason ratio cannot be about exact). The largest poster prints for the wall, viewed from a distance, can make do with less resolution, maybe only 150 to 200 pixels per inch, if that is all the pixels you have.
  4. Then write a high quality JPG for printing (at least Adobe JPG Quality 9, ... why not?) Save this to a new file name (or use Raw), never destroy your original image. You may need it for something else from it in the future, maybe after your skills grow.

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.


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