D800 AA filter

Started Jan 19, 2012 | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 53,151
Re: D800 AA filter

mhammon wrote:

OK, I'm going to ask a very new guy question. As I understand it, the AA filter assists in reducing moire and MAYBE some chromatic aberration. http://www.photography.com/articles/digital-photography/aliasing-antialiasing/ That seems like a good thing to me.

So why would one want to shoot with a D800 without the AA filter?

There is a problem with AA filters, that is that they are far from perfect. A perfect AA filter would let evrything through below the Nyquist frequency and nothing above it. What we actually get is this:

(linked from Lester Wareham's page at http://www.zen20934.zen.co.uk/photography/Anti-Alias.htm )

These AA filters are presumably set for a Nyquist og 80lp/mm. You can see how the birefringent (quartz) filter is better then the simple diffuser (plastic) which is why manufacturers go to the very considerable expense of the more sophisticated filter. However, you can also see that it is still removing huge amounts of contrast at higher spatial frequencies, so for instance the 50% contrast (MTF50) point is at 44lp/mm with the AA filter and 109 without. 60lp/mm details will be rendered at 32% contrast with the AA filter and 76% contrast without, which is one reason why AA-less cameras seem so much 'crisper' (as well as the bogus 'crispness' produced by artifacts, which very definitely will happen in this case, sinc at Nyquits, the unfiltered lens is yielding 68% contrast. The right answer, in the absence of a perfect 'brick wall' optical low pass filter (which would anyway produce artifacts of its own) is to sample at sufficient frequency that the nyquist is well clear of the regions of interest. The D800 doesn't even begin to get there, it's resolution will only provide about 420 pixels per inch on an A3 print, which isn't a whole load above the 300 ppi standard printer resolution.
--
Bob

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