D800 AA filter

Started Jan 19, 2012 | Discussions thread
apaflo Veteran Member • Posts: 3,854
Re: D800 AA filter

bobn2 wrote:

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.

For this application, if all other characteristics were equal, the plastic filter would produce significantly better results in many, if not most, circumstances. Of course all else is not equal, but the red curve is better in this application.

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.

Whatever the curve is below the Nyquist Limit (we are presuming 80lp/mm) can be reshaped with a software filter applied to the digital data. But post sampling there will not be any frequencies in the data above the Nyquist Limit and instead, everything above that frequency will have been folded to a frequency below the Nyquist Limit in the form of aliasing distortion that cannot be removed with software. Look at the chart, where at 120c/mm the quartz filter is allowing significant signal and the plastic filter virtually none (approximately 0.26 compared to 0.1). That difference results in signal distortion at 40c/mm that cannot be removed with software, as opposed to the differences in the curves below the Nyquist Limit than can be corrected with software.

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.

There will always be detail at higher frequencies. The level will depend on the scene of course, and on the lens, and finally on the AA filter and the resolution of the sensor.

Producing sharp crispy artifacts can be avoided with appropriate balance of characteristics, though it won't always be perfect for every scene.

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