Sunday, March 17, 2013

Nikon D7100: More on the Lack of the Anti-Aliasing Filter


D7100 units are trickling out slowly this week across the world. For all the pestering that I've been doing with my local retailers, it doesn't seem as though we'll be getting units on the ground until later this week.

I just wanted to touch briefly on the topic of the removal of the anti-aliasing filter, of which I will go into greater detail when I get a production unit in my hands. However, if you scan through the early impressions, you can find a few people who are puzzled that the removal of the optical low-pass filter doesn't seem to give as big of a sharpness kick as expected. Part of the reason is that some of the early samples showing up on the web have not been as rigorously setup as they need to be, but the other reason is because the photographic community has been passing along tall tales about filterless cameras. Don't belief the hype? Well, I wouldn't go so far, but consider this:

If you go through the various test charts performed by DPReview et al, you can see the effect that the anti-aliasing filter has on image quality. Typically, if you look at the number of lines per picture height (LPH) that a camera can resolve, it falls short of the maximum number of pixel rows (Nyquist frequency) by about 10-15% depending on the camera. That's not to say that the filter is exactly robbing the camera of this much resolution, but that you can go this far before you're not getting any more "real" sampled data. If you compare the DPReview D800 and D800e resolutions charts, then yes, removing the filter pushes the number of lines resolved much closer to the theoretical maximum; however, even though the lines are clearly delineated, you can make out false colour towards the higher end of the spatial frequencies.

So all told, you would expect that a Nikon D7100 would resolve approximately 10% more in terms of linear resolution compared to the D5200, D3200 and other similar 24mp APS-C cameras. How much is that you might ask? Let's put it this way... 10% is roughly the linear resolution gain that a D90 has over a D80 (or a D300 over a D200). If, when viewing images at the same size, you can pick out a D90 image from a D80 image, then that's the difference in resolving power that we are talking about. There's more to it than that, of course (there always is). We've only been talking about resolution, there's also the matter of contrast.

For that, removing the anti-aliasing filter will bump the contrast up as well. Contrast isn't the same as detail, but if there is more of it, the human eye will perceive it as extra sharpness. Just as a word of caution, the key phrase here is "at the same image size"... some of the early D7100 samples do show a marked improvement, but you guessed it... you have to pixel peep to notice the difference. However, this is the one that's going to make a real difference, as improved all around contrast is not only more pleasing to the eye, but also has implications in post processing as well.

All this becomes moot, though, if you don't hold your D7100 steady enough... the natural hand-shake of the photographer acts like a anti-aliasing filter. For that mater, so does mirror slap. Just like the extra resolution, the gain from removing the filter can easily be erased by unmindful shooting technique.

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