d7k moire!!

Started Feb 13, 2012 | Discussions thread
hoof Senior Member • Posts: 1,904
Re: Any camera that captures moire is poorly designed

blue_cheese wrote:

Based on my study of image processing, in order for moire to occur, the low pass filter before the sensor would have to be weaker than should and let through frequencies that are higher than the density of the sampling lattice of the sensor. If such a thing occured then the high frequency would re-sample into a lower frequency depending on the sampling lattice density and pattern. This is called aliasing as a high image freqency incorrectly gets captured as a lower frequency. Once the image is captured with aliased freqencies, there is nothing that can be done to correct this without damaging the image as removing the aliased freqency will also remove parts of the image that could be the correct frequency for other parts of the image. Best demonstrated in just universally blurring the whole picture until aliasing goes away. More sophisticated methods exist that do edge detection anti aliasing, they simply limit the area that they damage to specific areas, but the image is damaged.

There is ZERO value to intentionally allowing for frequency aliasing, an AA filter must exisit before the image is captured as the sensor effectively re-samples the light with a coarser sampling matix than that of the light coming in.

Given the the AA filter on the D7K is a constant (does not vary in strenght) and that it makes all the engineering sense in the world for it to be at least as strong as the sensor density, then moire should never occur in the captured image, if you see moire it is because of display/print/post processing, otherwise quite simply the AA filter design would be poor for the camera. This is why I dont get the point of a camera such as the D800E, removing the AA filter achieves nothing positive, you "percieve" your image as sharper because thin fine lines that would have been burred by the AA filter to look like smooth lines now look sharper due to jagginess/aliasing, this sort of thing would never be acceptable if everyone used 6MP cameras, but at 32MP it seems people dont mind jagginess at 100%, when the image is output for consumption on a display or print, it gets downsampled automatically by the post process/viewing software which simply MUST apply an AA filter before resizing, that is a fact with no two ways about it, leaving only the worst of aliasing visible as "moire". No image would look good if even the most basic viewer did not do some level of AA before shrinking the picture, and all pictures would have to be displayed at 100% as in most cases the display is lower DPI than the image meaning its sampling lattice is coarser/larger.

I totally agree.

I feel bad for future D800E owners who don't understand the point of AA filters and bought the E version thinking they're going to gain in the "sharpness" category, and can deal with moire in PP.

You can't get around basic physics and signal behaviors, I'm afraid.

And to head off some people's misconceptions, the AA filtering done prior to downsampling in software WILL NOT fix moire in the original image, even for the downsampled image. All it does is eliminate moire introduced due to the downsampling. Any moire in the original image will survive the downsampling.

That said, there are cases where no AA filter will be useful, just like removing the UV/IR filtering is useful if you wish to photograph in infrared or ultra-violet. But just like the UV/IR effect, there are consequences with doing this that affect everyday photography. For a specialist camera for specific shooting situations, the D800E makes sense. For a general-purpose camera, the D800E is just goint to disappoint, IMO.

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