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p.1 #13 · p.1 #13 · Does Focus Shift mean anything on mirrorless (non-Leica) bodies? | |
Steve Spencer wrote:
If you use the aperture ring on a lens and stop down to the aperture you intend to use, then focus shift should will not lead to out of focus images. It does not, however, mean nothing. The usual cause for focus shift as Michael pointed out is quite a bit of spherical aberrations. So a lens with focus shift probably has that sort of aberrations, which means it will have a loss of contrast wide open that will resolve as you stop down. It also tends to have it worst effects at closer shooting distances. So it is a good guess that a lens with focus shift will be kind of soft wide open and especially so at close distances. It is not all bad, however, because under corrected spherical aberrations also can add a softness to the bokeh behind the focal plane, so many of the lenses that have focus shift and high spherical aberrations also have very pleasant bokeh.
One thing to look for in lens design is a lens with a floating element design. This design tends to reduce focus shift quite a bit and they tend to perform much better at close range (in fact Nikon calls this sort of design close range correction). It does have a drawback for lenses that use an adapter, however. Lenses with floating element design tend to be finicky about adapter and are particularly sensitive to adapters that are too thin. Even a good lens with a floating element design and a thin adapters will tend to show chromatic aberrations that would not typically be seen and may lose sharpness as well....Show more →
Thanks for that.
But don't rangefinder shooters complain that focus shift often results in *missed* focus?
Regarding your comments, let's imagine I'm shooting toward a ruler -- I'm wide open -- and I'm focused on 1 meter. For the corresponding narrow DOF, roughly 1/3 in front of 1 meter will be in focus, and 2/3 behind it.
So, focus shift means that if I step down, 1 meter will remain in focus, but the increase in DOF will not maintain that ration of focus around 1 meter.
That more like it?
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