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B&H Photo - Video - Pro Audio


Image Resize - Cropping, Resampling, Scaling

For anyone just starting with digital images, or having trouble getting started, here is a review of the first basics we need, about using our digital images, about preparing them for printing, or for viewing on the video screen. This is those first basics about resizing images (which is about the very least we need to know about using digital images). Please appreciate that this is like learning to drive, in that you learn it one time, and it's not difficult, anyone can do it. And then you can use it for the rest of your life, when knowing how will make a huge difference. You just gotta know this too. Doing anything at all with digital images REQUIRES knowing at least this much. It is quite easy, if you let it be (if you 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 material.

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? Once we accept that pixels actually exist, it's all quite easy. And there is already another Summary of Digital Basics, so here, let's start about 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.

Printing - Before getting into resizing details, first some reasons for them - some generalities about printing. There are two common methods of printing our images.

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 typically, we can grab an image corner (in that print box) and drag it larger on the paper to print a different size. This is also scaling. As you drag this, you see the dpi number changing, as it scales the image for you. Or if you send the pixels out to be printed, and specify 5x7 inches, they will, by scaling it 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 below, at Scaling.


Cropping

 
Example just to show the concept of pixels. Above is an enlarged view of a tiny area (the stem just below center at left) of 58x58 pixels, shown at 800% size in order to see the pixels. In contrast, the view at left is a reduced even smaller here than than 16.7% size it reports.

Each pixel is digital data, NUMBERS describing ONE RGB COLOR for that tiny pixel area, much like one colored tile in a mosaic tile image. Our eyes see the reproduced image. Pixels are all there is in a digital image, we must think of it that way. It will make sense when you do.

Adobe: Above is the Photoshop Marquee tool (the "marching ants", moving theater marquee lights - you have to see it). The Marquee Tool and there is also a "Crop Tool" too work the same in Adobe Elements and Photoshop (both tools in both programs can provide the Fixed Ratio below). This is about both programs, and about most programs. And there are other ways too... I often do this crop in Adobe Camera Raw (select its Crop Tool, then right click in the image to see the aspect ratio menu).

There are usually two purposes for cropping. Compositional enlargement, like zooming in the camera, so we can remove any surrounding blank space, trimming away any uninteresting, or distracting areas around our image, concentrating the subject larger, leaving only what we want to see, for a better and more pleasing view (this image for example). And also very important, we can make our image shape fit the paper shape (or the desired video screen shape), so something important wont' be cut off, past the edge, off of the paper. You can see the two different shapes here, right? The overall image is aspect ratio 2:3 (taller narrow shape), and the marked rectangle is 4:5 (shorter wider shape).

The Marquee Tool is the rectangular icon symbol selected and marked here in yellow near the top of the Photoshop tool bar (right click it for other shape options). Then when this tool is selected, we can simply draw a rectangle on the image, by dragging the mouse over the image (dragging: with left mouse button held down).

When crop area is selected (drawn to include the image we want to include), then the menu Image - Crop will trim away all else, leaving only the area we marked to be included. Save it to a file, and we have it. Use "Save As" to specify a new file name - Do NOT "Save" to overwrite your only copy of the original, if you expect to ever need it again. The operation is not recoverable (except on Raw images).

Fitting the image to the paper size (or screen size):

When we select a tool on that vertical tool bar, we get a secondary menu bar, under the main menu bar. This secondary menu bar has options which apply to the tool we have selected. Here, the Marquee tool bar shows Style, which selects Normal by default. Normal means we can draw any shape box we wish. My choice was Style - Fixed Ratio, and I specified Width 4 and Height 5, which means any box I draw will be forced to be the same shape (not size, but shape) as a 4x5 or 8x10 inch paper. In this tool, if we wanted 5x4 (landscape instead of portrait orientation), we have to specify 5x4, but the left/right arrows between Width and Height will swap the fields. NOTE there are no units in Fixed Ratio. This Aspect Ratio is the simple ratio of the dimensions, and 4x5 is about the shape, not about the size. The idea is to match the "shape" of the paper we will print on. The size will be the pixels remaining after drawing the box, whatever they are (we will fix that in next step below). A minimal amount of practice will make this be clear.

You can move this selected rectangle AFTER drawing it by simply dragging it, or nudging it with the arrow keys. You can move it WHILE drawing it by holding the space bar to move it, and then continue drawing it. Or after other operations, then you can move the selected area by selecting the Move Tool (icon above the Marquee tool), and then dragging it. To change its size, the Marquee tool requires redrawing it, but other tools allow us to drag one edge. You can remove the marked rectangle several ways, usually just clicking someplace else on the image removes it, or the menu Select - Deselect. There are of course other options, but these are the basics.

A warning, about the formal Adobe Crop Tool . It has additional powerful options, and does all that too, but it has one feature to watch out about. It's menu bar also shows image width, height and printing dpi. If Width and Height are entered, it is aspect ratio, and becomes the only shape you can mark. If dpi is also entered, it will resample, so that width and height become actual inches on printed paper. If using this actual Crop Tool, and if you enter a Resolution value there (like 300 dpi), then it will crop as marked, and then the Crop Tool WILL also RESAMPLE your image, to provide the number of pixels to print the size and resolution that you specified. If that's what you wanted, great. This can combine this Cropping step and the next Resampling step too, meaning, if you want 8x10 inches at 300 dpi (2400x3000 pixels), just say so, and that's exactly what you will get. It is good to be aware however. If you merely want to crop, then simply DO NOT enter a resolution value in the Crop Tool. Just leave Resolution blank if only cropping, so it will not resample. Or, I instead just use the Marquee Tool above, which simply marks a selection (to be cropped), and will never resample. The Image Size dialog resamples.
Said again: Crop Tool will resample if you enter a dpi value (might resample to be smaller, or to be larger, which is good if you intended that, but you should not be unaware). If you use the Crop tool, but merely want to Crop, then you must leave the dpi field blank, and then the Crop tool will only crop to shape, and will not resample. Playing with it a little will quickly show this.

Aspect Ratio

Aspect Ratio is the "shape" of the image - the simple ratio of the images long side to its short side. For example, 4x6 is long and thin or 8x10 is short and wide (Aspect Ratio). It is naturally important when printing images on paper that the image shape (aspect ratio) matches the specified paper shape (its aspect ratio). Otherwise the image shape will not fit the paper shape exactly. Then most likely, the photo lab will print it so that it fills the paper, but some of the image is cropped away, missing off the edges of the paper. It is of course far better if you see and plan and do this crop yourself, instead of the print lab doing it randomly (We have photo editor tools to make this be easy work). Another possibility if printing at home is that some of the paper area is left as blank white space (if shapes don't match).

This is just saying, a 4x6 image cannot fit 4x5 paper, simply Not the same shape. This is not about Size, size can be enlarged as necessary, but Shape can only be cropped. It is a very simple concept (like square pegs in round holes), but not all beginners seem to take notice yet. A 2:3 image will enlarge to 4x6 or 8x12, but it will not fit 8x10 (unless first cropped to 4:5, one way or another). It is a complication that most paper sizes are all different shapes (except ISO metric paper, but which is still different than our images).

Aspect Ratio simply compares the image width and height dimensions, for example 4x6 inch paper is ratio 4:6, which lowest common denominator reduces to 2:3. Or a 3000x2000 pixel image is 3000:2000, which is reduced to 3:2 (same ratio as 2:3, just rotated, but still fits the paper). Or the aspect ratio can be described as the division into the larger dimension, so 2:3 is 3/2 = 1.5, and the ratio compared to "1" as 1.5 to 1... same shape as 2:3.

8x10 inch paper is 8:10, which is reduced to call it 4:5. The big point is that 2:3 and 4:5 are simply different shapes of paper. Obviously, we cannot print on a different shape paper without losing some area (cropped away, off the paper).
Said again, 4:5 aspect ratio can be scaled to fit 8x10 inch paper, but a 2:3 image will require 8x12 inch paper (else on 8x10, something is cropped off and lost). Or, the idea is that you can do that cropping yourself first, intelligently, your choice of what is cropped.

When printing the image, you often need to crop it first, to fit the paper shape. Because, 4x6, 5x7, 8x10 inch paper are all different "shapes". 4x6 is a more elongated shape than 4x5 (so the same one image cannot possibly fit both shapes of paper). 4x5 or 8x10 shapes are a little wider, so to speak. Shape and size are different properties. Images can be scaled or resampled to different sizes, but the shape follows in proportion. 8x10 can print on 4x5 paper, but not on 4x6 paper (because 8x10 shape is 4x5). If the image and paper are different shapes, the paper will necessarily crop it - some of the image will be off the paper and lost, or there will be blank unfilled white space - so the correct shape is important. It seems reasonable and necessary to decide this yourself about how to crop your image, before you print it. And frankly, a little tighter cropping often helps many images, like this one above, making the remaining subject area larger and more important.


This Cropping step above shows how to crop to fit the paper.

Some common Aspect Ratios are: (ratio of the two dimensions, which is "shape"):

1:1 1 to 1 Square
4:5 1.25 to 14x5, 8x10, 16x20, 20x25 (inches or cm, doesn't matter).
11x14 1.27 to 1 Near 4:5
8.5x11 1.29 to 1American Letter size
4:3 1.33 to 1 Most compacts, phones, and non-widescreen monitors
5x7 1.4 to 1 Also 2.5x3.5 inches wallet size
Metric ISO1.414 to 1Metric paper sizes, A4, A6, etc. √2 :1
3.5x5 1.43 to 1 Many old snapshot photos, also 9x13 cm
2:3 1.5 to 14x6, 8x12, 35mm film, and most DSLR cameras
11x17 1.54 to 1 Near 2:3
16:10 1.6 to 1Recent wide LCD monitors, near Golden Ratio 1.618
16:9 1.78 to 1HDTV 1920x1080, 1280x720, and camcorders

Note: Wallet size can vary. Traditionally 2.5x3.5 inches (measure your wallet picture insert sleeves), which is 1/2 of 5x7 size - meaning, 5x7 images will scale to fit wallet size with no cropping - often printed four on 5x7 paper, borders may make them 2.25x3.25. FWIW, Walmart advertises their wallet size as 2x3 inches, but they measure close to 2.5x3.5.

These shape differences are the reason people complain that the photo lab didn't print all of their picture area. To prevent the lab from cropping your image in an surprising way (but it is expected, the image shape must fit the paper shape), you should crop to 2:3 shape BEFORE you order 4x6 inch prints, or to 4:5 shape BEFORE you order 8x10 inch prints. Then what is cropped is your own choice. If you don't, and if you send the lab images that do not fit the requested paper size, the lab will have to enlarge it enough to fill the paper, cutting off a little of your expected image. Or sometimes you can request they just print it leaving blank white space where it does not fill. It is best if you tend to this before you send it, cropping the way you want it so that the image shape fits the paper shape without surprises. Note the obvious, in these "red" images above at right, we can of course move our own cropping selection to be at or near either edge, to better center our actual subject in the final area.

Most paper sizes are each a different shape (except metric is consistent, all actual ISO metric sizes (A, B, C series... A4, A6, etc) are the same shape, but not same as our image). In the old days, a human operator might see and select a better print cropping, but the machines are fully automatic today, so forget that, at least for discount price. This is something you surely want to see and choose first, your way, instead of letting the printer do it automatically by machine, their way (often unseen by human eye).


Resampling

The image above has been cropped now, to show the image that I decided I wanted to show, and also to fit paper shape of 4:5 aspect ratio. This step gets it to 8x10 inch size when printed.

Also shown is the Adobe Image Size dialog box (same in Photoshop or Elements ). It is menu Images - Image Size, or keys CTRL L pops it too (speaking Windows). Most serious image editor programs necessarily do about the same thing, and work about the same way. And BTW, the Best Help available for Adobe stuff is simply to do a Google search for the words you want to know about.

This tool does two very different resize functions: Resample, or Scale for printing. The mode switch on this tool is the Resample Image check box (bottom), checked so it will Resample. If NOT checked, this box will instead Scale for printing (next part below), and then, the top portion of the box (pixel dimensions) is grayed out, image size cannot be changed. But if checked, it will resample, and the pixel dimensions are changed.

Resampling changes the image size, specifically, it changes the dimensions in pixels to be other dimensions you specify, other new pixels. Several reasons why and when we do this, but our camera images have become larger than most uses need, so we generally need to make them smaller, to be the image size needed for our purpose. For example,

    900x600 pixels - an arbitrary size for video, a large image on a web page, or email
1920x1080 pixels - HDTV 1080i (new monitors are becoming this size too)
1800x1200 pixels - for printing 6x4 inches (300 pixels per inch)
3000x2400 pixels - for printing 8x10 inches (300 pixels per inch)
4288x2848 pixels - 12 megapixel camera image

There are two ways: We can just change the Pixel Dimensions directly (top part of Image Size box), which is very handy for images for video screen purposes, since we are working with dimensions in pixels. But for printing, there is a method to compute this size requirement for the goal of "pixels per inch" for printing, in the middle Document Size fields. For example, to print 8x10 inches at 250 dpi, will need:

(8 inches x 250 dpi) x (10 inches x 250 dpi) = 2000 x 2500 pixels.

The resample tool is a calculator that will do this multiplication, just enter 8x10 inches and 250 dpi, and the dimension numbers change so that this new image size is what you get - designed for this specific printing purpose.

After the first crop above (done for shape and content), this tool shows this image size now is 2514x3143 pixels (4:5, or 3143 x 4/5 = 2514). Resampling smaller is a drastic operation, which works great, but there is no going back after the excess data is discarded. First archive your largest original data somewhere unmodified (Raw is great for this).

A diversion about bytes. Image size is dimensioned in pixels, but data size is in bytes of memory to store the RGB data for those pixels.

Note the number on the top line of this tool - the 22.6M is the data size in megabytes, 22.6MB. This is not file size, and it is not image size. This is size of the image data when uncompressed and open in computer memory. It is a computed number, from the image size (pixels). For 8-bit images (24 bit color), this number is always 3 bytes per pixel (it is the RGB data for each pixel). Or 6 bytes per pixel for 16-bit images. So any generic 12 megapixel image is 12x3 = 36 million bytes of RGB data. True of any/every 24-bit RGB 12 megapixel image. But this one is now cropped to be smaller, 7.9 megapixels now (2514x3143).

This 22.6 MB number is close, but not exactly (2514x3143 x 3) bytes. Megabytes is an odd situation. One megabyte (1024x1024 bytes) holds nearly 5% more bytes than one million bytes (1000x1000), so there are about 5% fewer megabytes. See More about that. But this size in memory simply depends on the pixel dimensions, as shown, and it is the size of our data, usually three bytes per pixel.

Image data size is not file size. JPG compression makes the data smaller while in the JPG file. File size is variable with the JPG Quality option (High JPG Quality is larger files and better images, Low JPG Quality is smaller files at the cost of worse images). File compression size varies with image content too - featureless image areas (walls, sky, etc) compress smaller than detailed image areas (tree leaves, etc). We speak of JPG Quality losses, which means the addition of JPG artifacts, see More about JPG. The JPG file size might be tolerable quality if around 1/8 to 1/12 of image data size (very vague numbers, but larger files are better images). The image is the full original size again when opened and uncompressed into computer memory (same pixel dimensions), but quality might suffer from JPG compression. The top line of the resize box shows this uncompressed data size, in MB. This is computed at three bytes per pixel (for 24-bit images). That is simply how large our image data is (JPG files tend to give us other false notions about it).

In this Image Size box - with the Resample Image check box checked - we can Resample to change image size (pixels), done in a couple of ways:

You do want to pay attention to be sure you are resampling smaller, and not resampling larger. For printing, instead of resampling 20% larger, which just makes the image softer (nothing can add detail from the original scene once out of the camera), then it is surely better to simply print it scaled to 20% less resolution (pixels per inch), which increases printed size the same, but leaves the pixels unaffected (which is Scaling, next below).

Bring up this tool and experiment by typing some numbers. It should become very clear, very fast. It is just a simple calculator, about pixels per inch.

The last list box at bottom is the resampling algorithm to use. For photos, Bicubic Sharper is a fine choice for reducing the image size. Bicubic Smoother can be better for increasing image size, but increasing size is generally not a great option. Note that all images need a moderate bit of sharpening after resampling (all new pixels then). Any sharpening done before resampling is generally pointless, as those pixels will be replaced with different pixels... Sharpening ought to be done last, the final operation. Large images for printing need a bit more sharpening amount than smaller images for video. Very small images cannot stand much sharpening. We sharpen for the viewing goal.


Scaling

This option is ONLY to prepare for printing the image, to cause it to print the size we want on paper. Whereas... the Resample option above is about changing the pixel dimension of the image, for any purpose, but for example, maybe to make it display smaller on a video monitor. Scaling is very simple, all we do is change the dpi number, specifically without changing any image pixel in any way.

Video and printing are very different. Video monitors simply show pixels directly. If we want to see a different size (maybe the image is too large for the screen), then the image must be resampled, to create a different or smaller image size, the pixels of which are always shown directly on video. Whereas printers operate very differently - printers fill inches of paper by spacing the pixels at different "pixels per inch" rates. Scaling simply changes that dpi number, so the same pixels will cover a different size in inches (on paper). The scaled image will appear unaffected on the video screen, which ignores the dpi number, and shows pixels directly.

Scaling does NOT change image dimensions (in pixels). It never affects any pixel at all, and it has zero effect on the video screen. It is only for preparing to print, and technically, scaling only changes the Resolution field "Number" (pixels per inch), which is just a number. Some new dpi number just causes the printer to compute to print the same pixels to cover a different size in inches on paper (pixels per inch). The dpi number simply "scales" the printed size on paper, by changing the printed pixel spacing to make the SAME pixels cover a different dimension of paper, in inches. The pixels stay the same, totally unchanged. 3000 pixels printed 300 pixels per inch will cover 10 inches of paper, or printed 200 pixels per inch will cover 15 inches (same 3000 pixels). Scaling is very reversible, can be done at will, since there was no actual image change at all (just this one dpi number stored away separately - it is just a number).

We initially cropped this image to 8x10 inch shape (4x5 aspect ratio), so now lets suppose now we want to print it 8x10 inches. If we just feed this image (in first part above, after cropping) to our printer, what this said is 8.38 x 10.477 inches at 300 dpi, and that is how the printer will print it (It is how our local home printer will print it. If we take to Walmart or such, they will print it any size we ask for, i.e., they will scale it for us - they don't look at the dpi value).

Here, same tool, but I unchecked the Resample Image check box, so now it cannot/will not resample. This selects Scaling mode. The top fields become grayed out, the pixel dimensions cannot be changed. All it does is change the separate dpi value, but there is this specific option for it, this is how we do it (it saves the new number in the file, if we save the file). Notice again, the top part (Pixel Dimensions) is grayed out now, we CANNOT change the pixel dimensions in any way. We are not resampling, and we cannot change the image size. All we can do is to just change the dpi number, which changes the printed size, after it is on paper. The purpose is to scale the image size (pixels) to the paper size (inches).

However, we can change the dpi value, or change the printed dimensions in inches, and then the other one will track it. It is just a calculator. I also typed 8 inches for image width in the center at Document Size. This is printing size. The 10 inches automatically changed (because this is the aspect ratio), and the 314 pixels per inch is just the simple calculation: 2514 pixels / 8 inches = 314 pixels per inch. It is still the original pixels, but this dpi value will cause our home printer to print it 8x10 inches.

This 314 dpi is close enough, it is not a critical or exact number. For best printing quality, we would always like it to be around 250 to 300 dpi for printing. Within 10% or 20% is fine (when necessary, when that is the pixels we have), and will be undistinguishable. The places you send it out to for printing normally have capability around 250 dpi, and you would like to have that much. If you don't have enough pixels, then a 150 dpi result is possibly acceptable (not terrible), especially for large prints which are not viewed too closely. My own goal is to provide 300 dpi, "just to be sure"... but 400 dpi is pointless, won't help.

So if we wanted to print this one 4x5 inches, this 2514x3143 pixel size is simply far too many pixels this time. Scaling to 4x5 inches will come out 2514 pixels / 4 inches = 628 pixels per inch, which is simply ridiculous for printing color photos (but is fine and good if for line art). Realistically, printing at 250 to 300 dpi size is all that can benefit the printer (any printer, for color photos), so resampling smaller is often necessary and prudent. It would print if we left it scaled to 628 dpi, but the file and upload would be 4x larger than necessary, for no reason at all.

And when we print, many printer drivers today will allow us to specify the printed size there too. This is the same scaling operation (changes value of dpi to fit the image to the page size). We still need to have the shape right however.

Bottom line, if the image size is halfway close to proper printing size, there is no reason to modify and affect the pixels. Just leave them alone, use what you've got, and scale the image to print the exact desired size. If the printing resolution is near the ballpark of 250 to 300 dpi, you're good. This Document Size tool is just a simple calculator, which computes: pixels / inches = pixels per inch. And when you click OK, it stores this dpi number in the file, to retain it. Your local printer device uses that number to size the image pixels on paper, so this is how you make it print a specific size in inches. The dpi number is stored in the file (is just a number), but the inch numbers are not stored. There is zero effect on the image pixels if we simply change the dpi number. The unmodified existing pixels just print a different scaled size image on paper.


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