Let's just deal with some common aspect ratios used to capture and display images for a moment:
- 1:1 — square, something that some digital cameras can do by throwing away information, and which some old film formats used as their standard.
- 3:2 — rectangular, and the format that was pionered in 1950’s television and that's used for most still cameras today.
- 4:3 — squarish, and the format used by m4/3 cameras and Medium Format digital today.
- 5:4 — squarish, and the format that was used in prints in the US (and is still used in some prints and frame sizes).
- 8:5 — wide, and a commonly used format for LCD monitors and tablets, though this isn't strongly enforced.
- 16:9 — wide, the agreed upon standard for FullHD, the primary television broadcast method.
Of course, none of those actually conform with US (letter) or European (A4) paper sizes, though letter comes close to 4:3 and A4 comes close to 3:2.
I was in an art museum recently. No uniformity of aspect ratio to the paintings hung there, either, though I did note that artists are more likely to use a vertical format than I think most photographers do (might have to do with the ease at which a vertical placement of a large canvas works better for painting, while a horizontal format is the way most cameras are designed to be held).
Much of my formative training years were spent pursuing Hollywood norms (e.g. feature films), thus I have a strong tendency towards 1.85:1, the most often used widescreen ratio during that time. The 4K DCI standard is a very close 1.9:1, and is usually expressed as 17:9.
You may have noticed that the photos I post on the main page of this site tend to be 16:9 or something relatively close to that. Besides my proclivity towards 17:9, there's a not commonly discussed related thing called "above the fold." That is a newspaper term. Headlines and photos that appeared at the top of the folded front page were what you saw when the paper was presented in a newsstand. Most newspapers had standards about what should be seen above the fold. Generally, those were the most important thing you needed to know or see (according to the editors). Note that the “fold” is a rectangular area that’s essentially the top half of a vertical format.
My Web site is designed and produced for a computer monitor, and these days, for what works out to probably about half the width of the computer monitor (long lines of text aren't as readable as more moderate length ones). The site is designed so that the weekly image and at least some of the most recent article titles appear above the fold (i.e., you don't have to scroll to see them).
Of course, many of you are now reading on smartphones and tablets, and Google forced us all to create responsive sites that adjust to the screen they're viewed on, which pretty much makes "above the fold" random. Thanks Google.
We’ve wandered a bit away from photography, but it was important to do so. Your camera is a “capture device.” The real question as concerns aspect ratios is what is your “output device”? Not enough photographers think about this, and end up at some point being caught by it.
For instance, I know that the few media outlets I currently work with typically use images mostly horizontally at something between 3:2 and 5:4 (and rarely using my crop ;~). That has to do with historical standards of design for them that they don’t really want to change. For my own Web site, as I mentioned above, I tend to use 16:9. For presentations I use 16:9 because that’s usually what the projector I’ll be using wants. For prints on my studio walls, I tend to want vertical 3:2.
Thus, the answer to the headline’s question is simple: “none.”
Which brings me to another point: from time to time you’ll see someone ask “why don’t they make square image sensors?”
First, I see no difference between square aficionados cropping from a 3:2 capture than I would 3:2 afficionados cropping from a 1:1 capture. Which final use would be more prevalent? 3:2, not 1:1. So that’s a pretty good reason for what we get in our cameras.
More importantly, large square chips tend to create more waste on the circular wafer used to make image sensors, raising costs. At some wafer and image sizes, 1:1 might produce a more efficient use of the wafer, but at the sizes currently used, it tends to be less efficient. Efficiency directly relates to cost.
One reader pointed out that most presentation displays these days are 16:9 and horizontal. Of course, people hold their near-16:9 smartphones vertically. We get a real clash going on now. Smartphones held on the narrow axis create vertical video, which looks terrible on horizontal displays. Vertical 3:2 images displayed on a 16:9 horizontal display also waste much of the display space. In both cases, using vertical capture on a horizontal output loses visual impact.
Another reader pointed out that for prints you can use a mat to make the result any aspect ratio you want. Of course, some pre-done mats come in sizes that introduce yet another aspect ratio, so this works best if you are creating your own mat and making your own custom-sized frames.
I’m not agnostic with aspect ratios, as I noted above. I prefer wide aspect ratios (photographed horizontally). You’re probably not agnostic about image ratios, but maybe haven’t thought through the reasons why.
So my final point is simple: if you have a strongly perferred output, you’re best off if you have your capture match it, or coming as close to matching it as possible. Cropping 1:1 out of a 4:3 medium format sensor is better rationalized than doing so out of a crop sensor that’s 3:2, for instance. Likewise, cropping 16:9 out of a 3:2 sensor is better rationalized than doing so out of one that’s 4:3.
Update: corrected a misstatement, added some additional thoughts.
