Lens question: wide angle, sensor size and distortions

Wondering about this: using a 17mm (from the 17-40mm f/4L) on a 1.6 crop gives me heavy distortions. Vertical lines are anything but. Church towers are all the tower of Pisa.

If I would go with a 35mm sensor and a 30mm lens, would I get the same distortions or would it be better?

Sample pic with 40D and 17-40L:

Are you talking about how the lines converge as they move away or are you talking about the slight bend to the straight lines?

Like the lines converge when it goes up in the case of that picture.

uOpt wrote in post #12452132
Wondering about this: using a 17mm (from the 17-40mm f/4L) on a 1.6 crop gives me heavy distortions. Vertical lines are anything but. Church towers are all the tower of Pisa.

If I would go with a 35mm sensor and a 30mm lens, would I get the same distortions or would it be better?

The "distortion" you are seeing is not true lens distortion (such as pincushion or barrel distortion) but perspective distortion. This has NOTHING to do with focal length or the camera format but everything to do with where the camera is located relative to the elements of the scene in front of it. Had you been a lot further from the building, you would not see the same "distortion", though you would have very likely wanted to use a longer focal length to frame the building. Also, if you had been able to get the camera close to halfway up the height of the building, you would not see the same convergence of lines.

For more information on perspective and how to control it, please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance?.

One possible, though expensive solution is using TS-E lenses. Check e.g. this. It has good figures.
Search the net for more.

OP: with the same framing, you'll get identical "distortion" due to perspective effects with 1.6x crop camera with 17mm lens and with 17x1.6=27.2mm lens on a full frame camera. So you were right.

The exaggerated perspective distortion is an intrinsic property of ultra-wide angle lenses, and is often used on purpose. If you want to avoid it, then (1) use a longer FL lens, and move farther, or (2) try to find a vantage point at the half height of the target (say building on your photo), (3) use software to correct for the distortion (Adobe Camera Raw, PTlens, or free Hugin), or (4) use (expensive) TS-E lenses.

pulsar123 wrote in post #12453461
OP: with the same framing, you'll get identical "distortion" due to perspective effects with 1.6x crop camera with 17mm lens and with 17x1.6=27.2mm lens on a full frame camera. So you were right.

The exaggerated perspective distortion is an intrinsic property of ultra-wide angle lenses, and is often used on purpose. If you want to avoid it, then (1) use a longer FL lens, and move farther, or (2) try to find a vantage point at the half height of the target (say building on your photo), (3) use software to correct for the distortion (Adobe Camera Raw, PTlens, or free Hugin), or (4) use (expensive) TS-E lenses.

Or (5) Do all four.

In the example shot, it would have been hard for any one of these techniques by itself to overcome the "keystoning" effect. I'd have shot it from the highest possible vantage point with my 45mm TS-E with that lens shifted as far as possible (which really isn't that much... not as much as you can do with a view camera, for example) and I'd have used Photoshop perspective correction on the image afterward. Likely there would still have been some keystoning, but it could have been reduced to the point where it's less bothersome.

I know a TS-E addresses it. I always love an excuse for just another lens ... set.

While I was aware that perspective distortion is unavoidable I did not know whether the wider angle lens and smaller sensor makes it worse.

For a given lens, it's the larger (not the smaller) sensor that makes the distortion "worse". (I'd rather use the word "stronger", because people spend a lot of cash to get UWA lenses which produce a very strong perspective distortion effect.)

uOpt wrote in post #12454038
While I was aware that perspective distortion is unavoidable I did not know whether the wider angle lens and smaller sensor makes it worse.

NEITHER the focal length of a lens or a different size film or digital sensor (camera format) directly affect the perspective distortion in an image. The exception to this would be the choice to use a lens such as a "fisheye" lens which does distort the lines in the scene.

It's the simple fact that with a shorter focal length lens or a larger format camera, the photographer who wants the perspective distortion in the image gets closer to the subject or chooses a position that emphasizes the convergence of lines, etc. The camera equipment merely lets the photographer record more or less of the scene as desired.

TS-E lenses are lots of fun. I've heard you can do similar things by using regular lenses taped to the front of your camera with electrical tape and supported by bent coat-hangers (sounds messy to me). Try renting one of the tilt shifts and see if you like it.

17mm on FF gives equally wonderful distortion

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