Question about small sensors and short lenses

[burtharrris]

First, two facts:
a) Shorter lenses distort the image, giving it the wide angle feel. It usually gets noticable at 24mm and below.
b) On a small sensor camera, it takes a crop out of the larger availible image.

So using this logic, even though a 30mm lens will give the standard view (like a 50mm on a full-frame sensor), it will have some image distortion. As I use smaller and smaller lenses, it should get worse. Is this correct?

And lastly, if I were trying to get a full-frame equivalent of 28mm, I would need to use an 18mm lens. This is usually down in fisheye range, with incredible perspective change. Do wide-angle digital specific lenses correct for this fisheye distortion?

As always, thanks!

 

[Big Mike]

I have kind of wondered this myself...so I'm not sure about it. But remember that wide angle distortion is most pronounced at the edges of the frame...and since the smaller sensor essentially crops off the outer edges of the frame...we are left with the centre portion of the image, where the distortion is less. So following that logic...we don't really get as much wide angle distortion with the smaller sensors.

I haven't taken a really close look at it...but 18mm (widest digital only lens) on my digital doesn't seem to have as much distortion as 24mm (my widest lens for full frame) on my 35mm camera.

 

[Digital Matt]

Fisheye lenses are made differently than wide angle lenses. In a fisheye, the distortion is not corrected. You pay for that effect. In a wide angle, you want your horizon to be as straight as possible, so companies exert a great effort to correct the distortion.

I have a Tamron 17-35 which I have regularly used with a 35mm film camera, and while there is some obvious barrel distortion (which all wide angles have to some degree) it's certainly not fisheye.

 

[fmw]

Really well corrected fixed focal length wide angles don't have any barrel distortion at all. What you refer to as fisheye distortion is called barrel distortion. I had a wonderful 18mm f2.8 Nikkor for 35mm that produced perfectly straight lines everywhere (rectilinear is the term for this.) So did my 24mm f2.8. There was some image stretching at the edges and corners but no barrel distortion.

With zooms, of course, you will get some barrel distortion, particularly at the short end of the zoom range. This is because the designer has to correct a moving target - namely the focal length.

Don't get confused about comparing digital only lenses with lenses for 35mm. They have a smaller image circle so you can produce the same angle of view with no more need for correction than you would with a lens for 35mm made with the same angle of view. As an example, my wide angle zoom goes from 12-24mm which is pretty close in angle of view and performance to 18-35 on a 35 mm camera. At each end of the zoom it behaves and produces images that are equivalent to the longer focal lengths on 35mm gear. Since the image circle is smaller, the focal length can be shorter with no additional correction.

Nikon's full frame fisheye is 16mm in focal length and my 10.5mm Nikkor produces the same 180 degree field of view across the diagonal that my 16 did on 35mm. It also displays the full, uncorrected barrel distortion as the lens it emulates.

Also one doesn't pay for barrel distortion with full frame fisheyes. It is simply a trade off between straight lines and image stretching. If you attempt to make a rectilinear wide angle with a 180 degree angle of view, the subjects located near the edges of the frame would be stretched beyond recognition. So they don't correct barrel distortion in order to prevent the severe stretching. Rectilinears aren't often found beyond 120 degrees of view angle for this reason. That's why 14mm is about as wide as rectilinears get in the 35mm world. Everything in lens design is a compromise of some sort.

I should mention that all the wide angle zooms I've tried with my DSLR's have been less than satisfactory. This is because I was spoiled by using fixed wide angles in the 35mm days and the zooms just don't cut it for me. The distortion these zooms can produce is really pretty amazing. As an example, if you shoot a building head on and raise the camera above level, you expect to get some perspective distortion called keystoning. But with these zooms you also get some at 90 degrees to vertical which drives me up a wall. Lateral keystoning is not pretty. The manufacturers simply haven't addressed wide angle photography with digitals very effectively yet. My fingers are crossed because I'm a wide angle fanatic.

Hope this helps clear some things up for you. If not, you can re read it and become just as confused as I am.

 

[burtharrris]

Fred, you give the best answers ever.

I'd say you should write a book, but then I would have to read it to get answers. This way is easier cause I can just ask whenever I want.