One negative point I see, is that (maybe due to lack of an AA filter, maybe because of the higher resolution) the fine contrasty lines in the Pentax's picture are prone to anti-aliassing and Moiré-effect. You can see it in the cables hanging over the railing on the second picture.
Hi Joachim,
Camera sensors without an AA-filter will be more sensitive to producing aliasing artifacts than sensors with such a filter. But even the latter can show some aliasing artifacts, although they're less likely to occur, or less obvious.
However, it is important to separate a few effects because some can be remedied easier than others.
- Moiré is caused when 2 regular patterns with a different spatial frequency are mixed. This can happen when the subject has a highly periodic and regular structure, and it is mixed with the highly regular pattern of the sensor array. Slight changes in orientation/position can make a difference in how the interference patterns look. A slight defocus, or narrow aperture diffraction, will blur (reduce the amplitude) particularly in the finer detail pattern and can reduce the modulation of the mixed pattern enough to make the pattern unnoticeable.
- Aliasing is caused by subjects with spatial frequencies that exceed (are smaller than) the highest sampling frequency, they are imaged as larger features (aliases) than the real thing. They need not be in a repetitive pattern, although they may be easier to spot when they are (because they will deviate from the expected pattern). A typical example is a brick road or wall, shot at an angle. As the detail gets smaller with distance, the smaller detail will grow in apparent size due to aliasing where we expect it to just become smaller and ultimately unresolved. There is no real remedy after the shot is taken, it should be prevented (AA-filter, defocus, diffraction) by eliminating the higher frequencies that the sensor cannot resolve.
The aliasing can also manifest itself as jagged edges or lines (stairstepping or jaggies), because a very sharp edge or line is either registering on one pixel, or the next one, without smooth transition.
- False color artifacts are caused by the different sampling densities of Red and Blue, versus Green, in our Bayer CFA filtered sensors. This means that all of the above phenomenae occur at different detail levels for Red/Blue versus Green. Upon demosaicing this will confuse the Raw converter, which has no knowledge of the real scene. This can lead to the brightly colored waves that can be seen on the mostly vertical cable strands. Fortunately, the better Raw converters are relatively good at suppressing those bright colors (most noticeable against neutral colored surfaces) by local desaturation.
All of those side effects are less prominently visible when a proper AA-filter (Optical Low-Pass Filter, or OLPF) is used, but such filters are very expensive at large sizes, and for non-retrofocus lens designs the effect varies visibly with the angle of the light, IOW the image corners will be affected differently compared to the center. This is why AA-filters are usually omitted from Medium Format (say 48x36mm, or 44x33 mm as in the 645D) sensors, or relatively large sensors in range finder cameras where the lenses have very short exit-pupil to sensor distances (and thus more oblique edge and corner rays).
What of that Moiré? Can you fix it? Presumably you'd do that on a layer and not have the filter alter anything else, but what software would you personally choose? Capture One?
The Pentax 645D's PEF Raw format is not handled by Capture One, so Nicolas had to abandon his favorite Raw converter. Lightroom 4 or Photoshop's ACR 7 offer a reasonable alternative, although each converter has its particular strong and weak points. Fortunately, false color artifacts can be relatively successfully removed. The other optical phenomenae are inherent to a non-OLP fitered sensor array, but due to the large number of sensels, the output magnification is smaller, hence some of the artifacts less noticeable. We're pixel peeping here at an image that would measure 73x54 inches (185x138 cm) at an average display resolution of 100 PPI ...
It would haver concern me to have Moiré on a model's dress, (at those times when present) and having a fine repeating texture. Thank goodness it is not ever likely to occur on a thigh or face!
Yes, fashion/fabrics are a difficult subject to shoot with an non-AA-filtered sensor. Often these subjects are shot tethered, so the issues could be spotted during the shooting session on a larger display, and by repositioning or refocusing or narrower DOF or diffraction blur one can try to subdue the detrimental effects. The rest is handled by the post-production retouching crew.
Cheers,
Bart