At what mm do photos start to get rounded edges?

Barrel distortion depends on the lens pure and simple. It used to be that fixed lenses distorted less than zooms, but nowadays that is not necessarily so. However, even the best wide angles are going to distort, there is no getting around that. You need to consult the stats for each lens in order to form an opinion. With digital and its need for extra wide lenses there is of course much greater potential for distortion, but some of the better lenses do a great job handling this, and the milder form can be corrected easily enough in photoshop.

“With digital and its need for extra wide lenses there is of course much greater potential for distortion”

wouldn’t the crop factor minimize this – or is lens distortion proportionally linear ?

there’s no simple answer to this question. lenses are designed differently:

for example, a RECTILINEAR wide angle lens distorts a lot less than a FISH-EYE lens, which distorts a whole lot more. A 15mm rectilinear, therefore, distorts less than a 16mm fisheye, even though the one is wider than the other.

then you get into RETROFOCUS vs. NON-RETROFOCUS designs for wide angle lens. most modern wide-angle lenses for SLR cameras are retrofocus so that the rear lens element does not need to be too close to the focal plane; no need for mirror lock-up, etc. however, non-retrofocus designs, such as the old Nikkor 21mm f/4 or Canon 19mm f/3.5, will distort less than their retrofocus cousins. but they will vignette. also modern retrofocus lenses, at least the high end ones, have been corrected more.

really expensive wide-angle lenses should distort less than their cheap alternatives. The Leica 28mm f/2.0 Aspherical in M mount, for example, distorts so little that you might think that the photos shot with it were made with a 35mm or even 50mm lens.

with digital and ultra-wide lenses, you have to worry about more than just distortion. because of the angle at which light hits the sensor, wider lenses don’t perform well on digital. the wider you get, the more fringing, halo-ing, and other such image degrading factors you’re going to get. that’s why some companies have made specific “digital” lenses supposedly corrected and optimized to reduce these problems. it’s also the reason why more digital cameras aren’t “full frame.”

then there’s technical vs. apparent distortion. Even a 35mm lens, barely even considered a wide-angle these days, distorts more than a normal 50mm. but we’re so used to seeing photos made with 35mm lenses that we don’t consider that level of distortion to be “too much.” And, the same is true with a good 28mm. So it’s fair to say that apparent or noticeable distortion starts with 25mm and 24mm lenses. that would be 16mm or 17mm for digital cameras with 1.5x crop factor.

20mm lenses, which become 30mm on 1.5x cropped digitals, are probably the widest you can go without it beginning to look like “wide-angle.”

Thank you all, espescially Alan, very informative posts.

one positive aspect of digital and the not-so-full-frame sensor is the fact that it tends not to show the barrel distortion on most lenses.

see the 5D review at dpreview. they actually compare this issue by putting images side-by-side made with the 5D and 20D.

also if you don’t shoot wide-open you will limit some barrel distortion. also adobe camera raw has a function that allows you to remove some of this after the shoot.