Aliasing by the NEX

[David Kilpatrick]

Yes, it happens, as it also did in the Alpha 100 the moment the CZ 16-80mm was fitted...

http://forums.dpreview.com/forums/readf ... d=35967259

this thread has a very misleading post in it which suggests that aliasing of sharply imaged diagonals in NEX images might be caused by a poor lens, and would be removed by a higher quality.

In fact, the situation is the exact reverse. Aliasing can be reduced by putting the lens slightly out of focus, using a strong AA filter, or having a softer lens to start with. It is made worse by high quality lenses, because it happens when the lens resolution exceeds the Nyquist frequency of the sensor (linear resolution/1.5). That's why the 18-70mm on the Alpha 100 never showed moire or aliasing (they are a result of the same physics) but the 16-80mm CZ when it appeared, did.

The NEX with the 18-55mm definitely shows moire and aliasing artefacts - very fine ones - and quite strong patterns can appear in videos on things like fabrics, escalator ribs, rooftiles etc.

Thje reason is, I suspect, that a weak AA filter is used with a slightly greater than normal gap between the filter and sensor. This has the same low-pass effect as a stronger filter used closer to the silicon, but any sensor dust is less sharply imaged. Because of the sensor dust issues with the NEX, I think they have adopted this solution and that high contrast edges, at certain lens apertures (non-diffraction limited, but stopped down from wide open) may show aliasing; and patterns or textures may show moire.

David

[Vidgamer]

Here's my aliasing discovery, found while testing the Minolta 28/2.8. On the left is a photo processed from RAW (with sharpening added). On the right is the in-camera JPEG with +1 sharpening.



I think a lot of the problem comes down to the RAW conversion. Use a nice external RAW converter on the computer, and the worst of it never appears. Even adding sharpening only adds a bit of a staircase, but the artifacting in the in-camera JPEG is fairly pronounced in a couple of areas.

It's not something to lose sleep over, but more evidence to me that if you want to maximize quality, use RAW.