The ratio in which a work of art is presented has a host of artistic, practical, and — for some, anyway — even mystical considerations. As photographers, we come across ratios every day without giving them much thought. Let’s remedy that, and think about them a bit, shall we?
An aspect ratio is simply a proportion that describes the measurements of something in terms of its proportionality of length to width. So, in other words, something that measures four feet by three feet would have the same aspect ratio as something that measured 20 miles by 15 miles. In former times, some people got awfully worked up about ratios, claiming some to be not only better than others, but that one ratio in particular (1:1.6, known also as the “Golden Ratio” or “Golden Mean”) was divinely inspired. If you think those people were silly in comparison to artists today, you haven’t been on an internet discussion board lately. But I digress.
Where were we? Ah, yes. Theory. Enough of that. Let’s get practical. Nikon and Canon sensors often use a 3:2 aspect ratio, which is roughly the same ratio as a frame of 35mm film; this holds true whether you’re using a full frame camera (i.e. the sensor is also the same size as a frame of 35mm film) or a cropped sensor, which is going to be somewhat smaller. Later digital cameras have tended to use a 4:3 aspect ratio, which is the same — coincidentally or not — as the aspect ratio used by many computer monitors and standard-def TV screens. Medium and large format cameras also frequently make use of 5:3, 5:4, and 1:1 ratios, while HD video is either shot at or scaled to a 16:9 or 16:10 ratio to fit the size of the screen on which it’s being shown. Remember, this has nothing to do with the size of the sensor, since two sensors — or two of pretty much any object — can make use of the same ratio with very different measurements.**
Let’s get even more practical, since this determines not only how your photos are made, but also how they’re displayed. Many computer monitors utilize the 4:3 ratio, as mentioned earlier. If we stop to think about common print sizes, 6×4 format is 3:2, 10×8 is 5:4, and 7×5 doesn’t quite fit any of the aspect ratios in common use. What that means for practical purposes is that if you’re going to print your photos, you now know the relation between your camera’s sensor and the print sizes you’re likely to be using. You also realize, if you’ve given this a second’s thought, that in a good number of cases, the printing process is going to involve some kind of trickery to match the sensor or film’s native aspect ratio to whatever it is you’re using to print. So if you’re using a 4:3 (or four thirds) camera, you’ll have to stretch (or shrink) the print to fit, put a border of some sort around the print, or crop out some part of your image so that everything stays proportional between the two media. The latter two instances are generally safer bets, since the first option generally leads to objects in your photo looking misshapen to one degree or another (you’ve probably already seen how video solves the problem of differnt aspect ratios, especially if you’ve seen something shown in letterbox format).
Some cameras — and this is especially true of compacts — will allow you to shoot in different aspect ratios than the one for which the sensor was designed (which you’ll sometimes hear referred to as the “native” aspect ratio). The sensor’s aspect ratio might be 4:3, in other words, but the camera will still allow you to shoot in 3:2 or 16:9 formats. You’ll notice something when you’re not shooting at the camera’s native aspect ratio, however: those photos will generally have smaller file sizes than the “normal” ones (see photo at left). The reason for this is that your camera’s sensor isn’t somehow changing its proportions just because you’ve selected a different setting; it is instead telling a certain number of pixels to step out for coffee and donuts while the rest do the heavy lifting. In other words, you’re not shooting at full resolution.
Hopefully this clears up the question of what an aspect ratio is. If I haven’t been clear, let me know in the comments, and I’ll try (again) to clear the air.
*It should be noted that these ratios are expressed in what we’d think of as landscape format, so it’d be 4 wide by 3 long; if you’re looking for the proportion in portrait format, just invert the numbers.
**Put differently, some digital cameras use a 1:1 sensor that might have dimensions of 1″x1″, for a surface area of one square inch. Now, if you also happen to bake, that 9×9 pan you’re using for your brownies has a 1:1 aspect ratio too, but has a surface area of 81 square inches (while your 13×9 is an almost-but-not-quite 4:3, but if people are paying closer attention to the formatting of your brownies than their taste, I humbly suggest you find another recipe).
Postscript: Maybe you came here looking for information on cinematic aspect ratios. If that’s the case, you’ll probably find this article helpful: http://www.thelooniverse.com/movies/west/aspectratio/aspectratio.html