Architecture - Distortion - Lenses

[Canon88]

So; Yeah I know about tilt-shift lenses and their astronomical costs (I'm a college student - everything is astronomically expensive).

The Information:
Going to Vancouver for the first time - wanting to shoot the architecture there.

I have:
Canon Rebel Xt w/ a Tamron 11-18mm Lens.

The question:
Why does there seem to be SO much more distortion on a DSLR at 11mm but, a point shoot can take amazing architecture shots at 7mm?

And -- Why 'really' invest in tilt-shift?

[AndrewCCM]

7mm on a point and shoot isn't 7mm in reality. There is a serious sensor crop factor to consider.

BTW: Try shooting architecture with a less wide lens... it isn't always necessary. I shot some of my best city pics with a 24mm...

[johnastovall]

A tilt and shift will let you do things on with a 35mm body which require a view camera. It can also reduce the amount of correction you have to do in post.

Speaking of post have you looked at getting some of the open source pano tools for distortion correction?

[jeffkohn]

There are two types of distortion that you'll encounter that may be objectionable for architectural photography in particular.

The first is perspective distortion, sometimes called keystoning. This not really distortion per se, but just a matter of how perspective works as you get closer to subjects. This is the effect that causes converging verticals when you point your camera up to photograph something tall, and is also the reason that parallel lines such as railroad tracks seem to converge in the distance. Perspective distortion has generally been considered undesirable in architectural photography when it's obvious. A little bit is probably OK depending on the audience you're shooting for, because we're used to seeing this effect with our own eyes (just not to the extent that extremely wide lenses show it).

The way to avoid perspective distortion is to keep the camera's film/sensor plane as close as possible to parallel with your subject (which would mean your lens is pointing perpendicular to the subject). Without a shift capability this will often mean chopping off the top of your subject though, unless you can find a height half as high as your subject to shoot from. What the shift does is allow you to avoid tilting the camera back by moving the lens' image circle up, so that you avoid the converging verticals (it's somewhat hard to describe if you haven't actually used a shift lens).

BTW the tilt functionality really has nothing to do with this, it's for controlling the plane of focus to manipulate depth of field. Not really and issue for most architectural photography, but often very useful for landscape and macro photography.

The other type of distortion is geometric distortion. This is caused by the lens itself and is always undesirable. With geometric distortion, even if you have your camera perfectly square with the subject you'll get curved lines that should be straight, most often near the edges of the frame. The two basic types of geometric distortion are barrel, which is when straight lines bow outwards away from the center of the frame; and pincushion, which is when straight lines bend inwards towards the center of the frame. Sometimes you get a combination of the two, often called complex or mustache distortion, which is difficult to correct in software.

Speaking of software, both perspective and geometric distortion can be corrected in post-processing with the right tools. Photoshop has the Lens Correction plugin, and I would guess other image editors have something similar. There are also more specialized tools such as PTLens that may be needed for complex distortion such as mustache. Depending on how bad the distortion is, correcting it may signicantly degrade image quality. essentially you're applying the reverse distortion, which can mean interpolating a lot pixels and stretching parts of the image. You may also lose a significant portion of the image to cropping, so it's best to frame your compositions loosely if you're planning to correct in software.

As for point-n-shoots having less distortion, I don't think that's necessarily true. Perspective distortion has nothing to do with focal length or sensor size, it's strictly a matter of how close/far you are to your subject and how much you have to tilt the camera. Some point-n-shoots with really good lenses may have less geometric distortion than the typical 35mm/DX ultrawide, simply because it's easier to design a lens with minimal distortion when you're only covering a tiny sensor. On the other hand many of the "ultrazoom" point-n-shoots have quite a bit of distortion throughout their focal range.

[jeffkohn]

7mm on a point and shoot isn't 7mm in reality.

Last I checked 7mm was a measurement of length that had a fairly precise meaning.

BTW: Try shooting architecture with a less wide lens... it isn't always necessary. I shot some of my best city pics with a 24mm...

This is certainly true, if possible backing up is preferable to correcting in software, because the farther you move away from the building the less you'll have to tilt the camera (if you move far enough away, you won't have to tilt it at all). That's not always an option though depending on what else is around that you might not want in your shot. The narrower field of view will also change your composition somewhat with respect to other objects/bulding around the subject.

[edpouso]

Most images can be fixed in Photoshop using the transform tool.
I use this to fix many things when I can't us a 4x5.
example boxes, buildings, skylines, stairs. etc

[ramblinray]

Why buy a lens when you could probably rent the lens and see if you really like the way it functions? Should be way cheaper than buying one.

[Canon88]

Where can I rent a tilt-shift lens in the Dallas area? Can I take it out of the country? What's the cost?