Aspect ratio is the SHAPE of the image, and is described as the ratio of the image's width and height dimension. Aspect ratio is NOT about size, it is only about the shape. An 10x8 inch print is the aspect ratio of 10:8 (10/8 = 1.25), but the fraction is reduced to be expressed as 5:4 (1.25), which is the same SHAPE as a 5x4 inch print.
If the image and paper are the same aspect ratio, then the image can be scaled to fit the paper. But if the image shape is 3:2 and the paper shape is 4:5, then the image and paper are different shapes. The point is that we cannot print a 3:2 shape image to exactly fill 5:4 paper shape, or vice versa. The shapes are simply different - 3:2 is more long and thin, and 5:4 is more short and fat, relatively. We must crop the image and print the area that fits within the paper shape. The only other alternative is to instead leave larger white margins on two sides.
Common examples of aspect ratio shapes are:
3:2 aspect ratio (3/2 = 1.5, a longer shape)
35 mm film, 36x24 mm
6x4 inch prints
Most SLR Pro digital camera images
4:3 aspect ratio (4/3 = 1.33, medium shape)
Monitor video screens, 800x600, 1024x768, 1280x960 pixels
Standard television screens, NTSC or PAL
Most non-SLR consumer digital camera images
5:4 aspect ratio (5/4 = 1.25, a fatter shape)
8x10 or 4x5 inch prints
1280x1024 pixel video monitor screen *
* Some video boards provide the standard video 4:3 ratio 1280 x 960 pixel setting, which I prefer to use on a 19 inch CRT monitor because it enlarges text fonts slightly more. But any LCD monitor should only be operated at its native resolution (like 1280x1024).
Other shape examples are:
8.5 x 11 inch paper is 1.29
5x7 inch print is 1.4
Metric paper sizes (A, B, C series) are 1.414 (square root of 2)
HDTV, 16:9 is 1.78 (real long)
These shape differences are the usual reason beginners frequently 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 (it is expected, the image shape must fit the paper shape), you can crop to 3:2 shape BEFORE you order 6x4 inch prints, or to 5:4 shape BEFORE you order 8x10 inch prints. Then what is cropped is your own choice to make. (see this about cropping JPG for the television screen)
The Adobe Elements and Photoshop Marquee Tool has an optional toolbar Mode of Fixed Aspect Ratio. You can set it to 6:4 or 3:2, so that is the only shape you can mark to crop, regardless of dimension. You can move that crop box to position it just right. Or maybe set it to Fixed Size like 640x480 pixels if that is the goal. Or their Crop Tool similarly allows setting the Width and Height in inches which may be more convenient for printing (but check the resulting ppi number to be sure what you end up with).
Irfanview Batch Conversion mode, free from irfanview.com
Printing 6x4 inches at 300 ppi needs 1800x1200 pixels. Crop larger images to 3:2 or 2:3 shape. Then at the Irfanview menu File - Batch Conversion - Advanced , specify to resample multiple files to 1800x1800 pixels (shown above). Also specify Preserve Aspect Ratio so that this 1800 pixels is a maximum. Then either landscape and portrait image orientations will come out just right, either 1800x1200, or 1200x1800 pixels (if already 3:2). Photoshop menu File - Automate - Fit Image does the same thing in batches.
When resampling, if we halve the width of a photo image, we must exactly halve the height too, otherwise we distort the image, causing a tall skinny or short fat image, not the same proportion as the original - a circle becomes an ellipse.
So when resampling photos, there will be a check-box labeled Constrain Proportions or Retain Aspect Ratio. This choice automatically maintains the original aspect ratio, so that if you change one image dimension, the other dimension tracks it automatically, to accurately maintain the same aspect shape. You surely always want to use this, to avoid distorting the shapes of the objects in the photo image.
Changing the aspect ratio necessarily requires either resampling or cropping the image, but always differently in the two directions. Resampling differently will distort the image, but cropping differently does not, it merely discards some of it.
Scaling cannot and does not change the aspect ratio. When you uncheck Resample to do scaling (page 74), the Constrain Proportions button is surely grayed out, as it is not a factor when scaling (the pixels are never changed when scaling).
If printing photos at full page size, you may prefer printing on metric A4 paper size (aspect 1.414). It is closer to the shape of 35 mm film or 6x4 inch print shape (1.5) than is 8.5x11 paper (1.29). Less cropping is needed to fit the paper shape.
The Aspect Lock is a very handy feature, don't overlook it. In Microtek ScanWizard 5 & 6, it is the Keep Proportion field. In Microtek ScanWizard 2, it is the little padlock symbol to the left of the Input dimension window (shown at right).
The Minolta Dual II software has the same little padlock, and it is the same thing. In Umax VistaScan 2.4x, it is the "Keep Aspect Ratio" button in the Image Size window.
When scanning for a desktop wallpaper image, you require an image with aspect ratio of 4:3, nothing else will really do it right. Similarly, if printing on paper that is 6x4 inches, you want an image with aspect ratio that is 6:4 or 3:2 (which also matches 35 mm film full frame aspect ratio of 36:24 mm or 3:2). 11x8.5 paper is about 1.3 to 1 ratio, and metric paper is 1.414 to 1.
Aspect ratio is sometimes a fundamental consideration for sizing the image.
Well, it takes trial and error repetitive tries to set BOTH a resolution and the exact dimensions of a desired area to give you the scan you want... The area dimensions change with resolution changes. The first try isn't large enough, so you increase resolution. Then you have to change the dimensions... Still not large enough... Do it all again. And again, until you get it exactly like you want it... You can do it, but it's a pain. It's unnecessary work anyway, there's a better way.
Instead just set the aspect ratio you want in the Settings Window. The first step is to enter the ratio without regard for real size, that's handled last. If you want a 3:2 ratio, then enter this for an Input size, like Width = 3 and Height = 2. Or if scanning film, 3x2 inches is larger than the film size and is not allowed, so just specify 0.3 and 0.2, it's all the same ratio. Pay attention to your units, if you're showing pixels, don't enter 3 x 2. Enter a realistic size, like 300x200 in that case.
If printing to 8.5x11 paper, you can enter 11 x 8.5, or 1.1 x 0.85 for film. If printing to 6x4 PhotoSmart paper, just enter 6 x 4, or 0.6 x 0.4 for film. Note there are two choices for paper ratio, portrait or landscape mode, i.e., either 4 x 6, or 6 x 4.
Then when the ratio is specified, click the lock symbol to the left to lock it. These windows go dark, you cannot change them again. Then you can type the final size in the Output field. Or using the mouse in the preview window, you can change the image size any way you want it, and no matter how you change it, it will automatically keep the aspect ratio you specified. You change width, and height will automatically track it. You simply center the result. This makes setting the exact size be very trivial, especially in combination with resolution changes.
Don't forget to unlock the aspect lock next time you want to change the size conventionally, otherwise you cannot change it.