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MF Digital, myths or facts? - OLP (AA) filtering

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Article Index
MF Digital, myths or facts?
Collecting more photons
Comparing the top of the line MF back with the top of the line DSLR
Is Capture One better on IQ180 files?
A larger format is less demanding of the lens
The Really Right Stuff
OLP (AA) filtering
The 16 bit issue, truth or myth?
Do MFD have better DR?
MFDBs have better color, truth or myth?
Flexibility
Summary and conclusions
Are large pixels better?
Read also
Experts's comments
Thanks!
All Pages


OLP filtering

(This section is under construction)
In general, it could be said that MF backs dont have an optical low pass filter (OLP) also known as an AA (Anti Aliasing) filter and DSLRs do have OLP filtering.There are exceptions to this "rule". For instance:
  • The Mamiya ZD had an optional OLP filter
  • The Leica M-series (M8, M9, ME, M), Nikon D800E, Kodak ?? and the Sigma SD series lack OLP filter.
What is an OLP filter? The OLP filter is intended to supress fine detail contrast that the senor cannot resolve. If the resolution of the lens exceeds the resolution of the sensor, fake resolution will arise. This fake resolution can actually enhance the visual impression of the image, but is nevertheless fake.
With bayer sensors the problem is compounded by as the artifacts often show up as color moiré.
To reduce these problems most cameras have a so called OLP filter. This filter normlly consists of two wafers of birefrigent material, normally Lithium Niobate. These crystals have different index of diffraction for the polaried components of light. So light passing to the two wafers is split into four parallell beams.
Normally the thickness of these wafers is chosen so they reduce color moiré to reasonable levels. As a consequence they also reduce find detail contrast. Much of the contrast can be regained trough adequate sharpening.
The image on the left below is the best example I have seen of aliasing. If you look carefully the line pattern disappears at the 18 mark and the lines pattern returns at 20. At the 24 mark we still have a lite pattern but it is obviously fake as we only see three line pairs. On the other hand if we don't know about the test target the image looks natural and the fake resolution can actually add to the perception of sharpness. This image comes from DPReviews test of the Sigma DP2 one of the few cameras having a "Foveon" sensor. The Image on the right coming from a traditional "Bayer filtered" camera has some color artifacts in spite of OLP-filtering.
Tim Parkin has investigated the difference ibetween the D800 and the D800E in this excellent article. He essentially found that the studio test images from the D800E could essentially take no sharpening while the D800 images aherpened well in LR/ACR with (Amount = 100, Radius = 0.6 and Detail = 0). I measured MTF on the unsharpened D800E image and the sharpened D800.
The sharpened D800 image is below:
Here we can see that the sharpened D800 image is actually sharper than the unsharpened D800E image. On the other hand a different subject could allow more sharpening on the D800E image. Sharpening is to a significant effect up to taste.
The dotted lines in the figures above show the theoretical MTF of a perfect lens at f/4.
Interestingly enough the IQ 180 image show much less color artifacts. This image was sharpened similarly to the the Nikon D800 image. The unsharpened image would have around 17% MTF at Nyquist. The dotted line here indicates the diffraction limit.
If we check the slanted edge test target for aliasing artifacts we can see that the Nikon D800 shows less color artifacts, followed by the IQ180 and the D800E being worst.
IQ180 below:
Nikon D800E below:
My take on the question? The OLP filter is quite expensive and also has a significant thickness. It would probably simply not fit in the M series Leica. On the other hand, I don't think DSLR vendors are using OLP filters for pleasure. A few DSLRs have lacked OLP filtering and had problems with aliasing artifacts Kodak DSLRs and the Nikon D70 comes to mind. According to rumor, Sony has tested the Alpha 99 without AA filter and found that there were a lot of Moré issues.
Reducing pixels pitch and stopping down also reduces aliasing. Aliasing is mostly a problem with very good lenses at best aperture. I guess that MF cameras are often used stopped down. Still, it seems that under studio conditions and shooting textiles color Moiré (the most visible form of aliasing artifacts) is quite problematic.


Last Updated on Thursday, 13 December 2012 11:56  

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