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How Shift Lenses Change your Life
the above shift stuff, except that the only lenses that tilt are the Canon TS-E's. What does tilt do for you?
Normally, everything in a plane a set distance from, and perpendicular to your lens will be in focus.
When you use lens tilt, you forego the "perpendicular" constraint. You can tilt that plane quite severely
if desired. The plane of the back element of the lens, the film plane, and the focus plane will all intersect.
The classic example is for landscapes. You have wildflowers at your feet and Alps at infinity. f/22 won't
give you the depth of field you need (well, it would at 24mm but at the sacrifice of sharpness). As its just
after sunset there's not much light, and there is also a wind moving the flowers so a long exposure won't
work. Fast shutter dictates big aperture, which means lousy depth-of-field... normally.
As you tilt the lens, you make the focus plane tilt. However, a small tilt of the lens creates a huge tilt in
the focus plane. Just 10 degrees of lens tilt can rotate the focus plane to the point that wild flowers a
meter away, at the bottom of the frame, are perfectly focused as are distant alps.
There are very mathematical explanations to calculate all the exact effect, but I don't know how you
could use them accurately in the field. It would be nice if there was a focusing screen with a half-prism
focusing aid at both the top and bottom of the frame. But there's not. The camera could also calculate
this information for you and display it.
The other tilt trick is just the opposite. Say you want everything out of focus except for the subject. Say
there is a row of columns ahead of you, running left to right. Tilt like mad, focus one column, and the
equally distant neighbors to the left and right will be fuzzy.
Why You Can't Get a Good Exposure
Wide angle lenses get darker the further from center you get. This is called "cos^4 vignetting" and is due
to the light hitting the film at an angle. This is a problem when the lens->film distance is small compared
to the image width.
On SLR's, wide angle cameras use a "reverse telephoto" group at the back, so the rear of the lens is far
enough away to leave room for the mirror. This extra glass hurts quality, but means that even a
EF14mm/2.8L is about 40mm from the film, which is 36mm wide... which means "cos^4" is NOT
visible on SLR wide-angles. Rangefinders like Leica M and Contax G and Mamiya 7 have no telephoto
group, so their images are sharper - but the back of the lens is VERY close to the film, and the corners
are much darker.
So what does this have to do with your shift lens? Everything! Shift lenses make a picture much wider
than a normal lens, but the distance from lens to film is still about 40mm. Your lens shifts 11mm, so the
image is 36mm (film) + 11mm * 2 = 58mm or so. Unshifted, only the center 24mmx36mm of the image
hits the film, and has no more cos^4 vignetting than a normal 35mm. However, as you shift, the darker
part of the total image hits the film.
http://www.photo.net/photo/canon/tilt-shift (6 of 9)7/3/2005 2:24:04 AM