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Join Date: Dec 2000
Location: Venice, Florida, USA
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How does lens shift affect sharpness?
Basically lenses for the most part perform best when the image exiting them exits from lens center and the smaller the exiting image the better. This spot is called the G, whoops, the sweet spot. As one uses lens shift, the image which stays fixed entering the lens, hits a different entering point on the back of the lens, because one is shifting the lens (moving it) causing the exit point on the front of the lens to differ. One other goodie to remember is that white light is comprised of many frequencies and each of these frequencies diffract differently as they exit the lens into the air surrounding the lens.
Lens generally are flattest in the center and tend to curve as one moves away from center. Lens manufacturers coat their lenses to try and compensate for the different bending of each light frequency, but this compensation is fiqured for light at lens center and not on the curved part of the lens glass. The more from center, the more the curve and the greater the different degree of bending. Because of the bending, red, green, and blue light exiting the lens will not hit exactly where you and the lens designer would like them to go. This error is called Chromatic Aberration or CA. It looks if you put up a grid of white lines this error looks exactly like misconvergence except that the error will vary across the grid.
Besides CA, lens just don't focus as well in the curved (non G spot, whoops again, non sweet spot) area. The more one is into the curved area and the longer the throw, the more unsharp will be the image. Of course, sit far enough away and one can't see anything on the screen except a blob of light. Then again, one can sit real close and still have impaired vision. So there is no real answer without knowing a lot of detail about ones set up and eyes.
But a rule of thumb is, the less lens shift one uses, the better. Lens shift is a convenience feature. The better and wider the lens the more it can be used to shift away from center without hurting the image.
Now there is something to be said from moving off of dead center a click or two vertically. Never horizontally because ye old 16/9 or greater is wider horizontally than vertically and is much closer or on the curved area already. Some lenses are less than ideal and light bounces off the inside barrel and result in some ghosting if exactly centered. Use lens shift if you need to, but don't use it to the extent possible just because its posssible.