Moiré disaster on the E-5

Started Jul 10, 2013 | Discussions thread
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Great Bustard Forum Pro • Posts: 36,922
By "weak AA filter"...

Airmel wrote:

John Sheehy wrote:

Airmel wrote:

Great Bustard wrote:

...the simple fix is more pixels. which so many fight against. The more pixels the sensor has, the weaker the AA filter needs to be to avoid aliasing.

All other things being the same, the use of a higher resolution sensor ("more pixels") requires Anti Aliasing with a correspondingly higher cutoff frequency.

It's hard to tell why you're saying this. You said the same thing as the Great Bustard, but in different words. An infinite cutoff frequency would be no filter at all.

The issues are so simple when you put your DSLR or 4/3 lens on a camera like the Pentax Q with an adapter; you can simulate what you system would be like (at least in the center of the frame) with hundreds of MPs; no significant spatial artifacts, and a virtually analog capture of the lens projection with only lens issues remaining. The images look much, much more natural and artifact-free, than cropping from the same lens with the camera made to work with the lens (large sensor with coarse pixels).

The fact is, in their sweet spots with good technique, DSLR and 4/3 lenses can have far too much resolution for their sensors. Those who claim otherwise are generally people who ENJOY aliasing artifacts, and can't tell them from accurate, natural imaging due to a weak right hemisphere.

Well as a matter of fact, what I said is quite different that what GB said. An anti-alias filter with a higher cutoff frequency is not "weaker", it simply utilizes a higher cutoff frequency.

The required cutoff frequency is well defined by the sampling theorem. It is directly related to the sampling frequency and is nowhere near infinite. The more pixels per unit distance, the higher the cutoff frequency of the anti-alias filter will need to be. Very simple really.

Now as a practical matter, it turns out that virtually all optical anti-alias filters are very poor performers. The technology simply doesn't exist to build a cheap optical low pass filter which has anywhere near the performance of its electronic signal processing counterpart. For this reason you might rightly refer to all optical anti-alias filters as "weak".

However so, there is no validity to the notion that an anti-alias filter with a higher cutoff frequency is "weaker" in some way. If you adjusted your sports car's speed limiter from 125 MPH to 155 MPH, you wouldn't say the speed limiter is now "weaker". It would simply be tuned to a different speed limit. The speed limiter's performance above and below the new limit would be the same as that seen above and below the previous limit.

...I mean that the size of the blur is less as a proportion of the image.  For example, if an AA filter had a blur radius of 1.5 pixels, then a 24 MP sensor would have half the blur (twice as weak) as a 6 MP sensor.

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