DOES ANYONE KNOW IF THE PLUS SERIES BACKS WILL ADDRESS THE MOIRE ISSUE?
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Unlike all current DSLRs, most digital backs do not have an anti-alias (A/A) filter and are therefore susceptible to moire to some degree or another. I doubt the P+ backs will be any different. For many users, the lack of an A/A filter is a good thing, since the image is a good bit sharper right out of the back when no A/A filter is smearing the image. But for some users, especially fashion users, moire is a problem that's going to be hard to eliminate with current lower MP backs. Higher MP backs are another story, as described below.
Moire is simply a digital sampling artifact. Any time the spatial sampling frequency of the back is lower than the spatial frequency found in the subject, moire can happen. The spatial frequency of patterns in clothing material is very high, and is often higher than the spatial sampling frequency of most backs, so fashion photography is especially susceptible to moire. An A/A filter, if present, lowers the spatial frequency in the subject below the sampling frequency of the backs (it's a low pass filter and smears or softens the image a bit), eliiminating (or mostly eliminating) moire.
As backs become available with higher and higher MP, the photosite size gets smaller and smaller, and the spatial sampling frequency of the back goes up and up. In other words, higher MP backs with smaller photosites sample at higher spatial frequencies, and moire becomes less and less likely. This is why people with lower MP P20/H20 backs see a lot of moire, and people with higher MP P30 and P45 backs see less moire. Interestingly, at higher spatial frequencies, lenses start to act like weak A/A filters (the contrast in the higher frequency regions of the MTF curves starts to tail off). So, to reduce or eliminate moire with digital backs, use a higher MP back with smaller, more densly packed photosites, and/or use poor glass with louse MTF specs :-) Another person in this thread mentioned that you can reduce moire by changing the aperture - this is basically just the lens acting as a weak A/A filter. In effect, changing the aperture changes which part of the MTF curve you are operating the lens, either increasing or decreasing the high spatial frequency MTF falloff.
Regards,
Hans.