How much ff resolution to not be "weak link" with Otus?

Started 5 months ago | Questions thread
hjulenissen
Senior MemberPosts: 1,617
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Re: How much ff resolution to not be "weak link" with Otus?
In reply to Jack Hogan, 5 months ago

Jack Hogan wrote:

Mark Scott Abeln wrote:

Jack Hogan wrote:

Mark Scott Abeln wrote:

John Sheehy wrote:

  • How about APS-C sensors with 24 mpx and no AA filter? I believe I've read that these sensors present the greatest challenge to the center of premium lenses.

Nikon D7100s are aliasing with consumer-grade lenses.

But could some of that aliasing be from the demosaicing algorithm? Especially since this doesn’t have an AA filter?

No. A good demosaicing algorithm will have the opposite effect, behaving like an upsizing followed by mixing information from more than one photosite location, smearing spatial detail.

From what I understand of the Nyquist theorem, pre-blurring analog data before digital capture is optimal for eliminating artifacts. You can blur the data after digitizing, but you need to do more blur in demosaicing than if you originally blurred the analog signal.

Not that this means that an antialias filter is always needed in a camera, or that existing AA filter designs are optimal, etc.

I see what you mean, although technically you cannot completely make up for a missing/weak AA automatically during conversion (other than effectively painting the area in manually)

And generic demosaicing's blurring can be thought of as a low pass filter of sorts, so if anything it will reduce aliasing not produce it. Some advanced algorithms try to 'guess' at the missing information (effectively 'painting' it in), sometimes getting it wrong. Perhaps this is what you are referring to? If so I agree.

We should perhaps distinguish between aliasing and imaging. Improper (pre-) filtering in the ADC (or downsampling) stage can introduce aliasing components anywhere in the spectrum (even at "DC"). Improper (post-) filtering in the DAC (or upsampling) stage can introduce imaging components at multiples of the sampling frequency.

From a Nyquist point of view, the Demosaic processing should perhaps be seen as an upsampling stage. If you do interpolation filtering in the separate color planes (perhaps not state-of-the-art, but pedagogic), you are "filling in the dots" in order to recreate a "smooth" 2-dimensional waveform. Nevermind that those samples were not created with "proper" filtering in the first place, and some "correction" is probably warranted.

My point is that aliasing in the capture stage can cause DC or near-DC errors (think tiled roof at a very high frequency, causing slowly fluctuating smooth level). No amount of post blurring is going to fix this.

-h

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