First of all, thanks for a very informative post, and thanks for your diligence and objectivity in gathering and interpreting your data. You answer a lot of questions.

However, there’s one circumstance that your tests don’t really cover, and in my own tests (much less meticulously done than yours, I’ll happily admit) I have found that NEF compression does create visible artifacts (though still not very offensive ones).

Specifically, using a D200 set to ISO 800, I took photos of a sunny Arizona scene (my back yard) with lots of blue sky included (we have no shortage of that here). I then visually compared, at 100 % magnification, the photos made with NEF compression turned on and off, after converting them to tiffs via ACR using identical settings (note that I used different shots for this, but they were made within a minute or two of each other, so I don’t expect that to be a factor).

What I found is that in the blue sky areas, the noise was pretty clearly more evident in the compressed NEFs than in the uncompressed ones. The individual noise “grains” (viewed at 100%) were a bit harsher in the compressed NEF files.

My guess is that the noise in the sky constitutes a particularly difficult challenge for compressed NEFs because the camera’s blue channel, specifically, is recording values in the highlight range. Under this circumstance, the compression algorithm can magnify small differences (caused by noise) between neighboring blue photosites, since the effective spacing between the discrete levels that can be recorded in the highlight range is increased in compressed NEFs (i.e., the quantization error is higher).

The practical consequence of this observation is, no doubt, up for debate. When viewed at more reasonable, print-size magnifications (25-33%), it was hard (but still possible) for me to detect the differences in the noise artifacts in the sky areas of the photos, but I suspect most viewers wouldn’t be bothered by them. However, just to be on the safe side, I always shoot uncompressed NEFs.

You don’t mention what ISO setting you used in your tests, but I assume it was lower than 800. If you’re interested in reconstituting your test setup, it would be interesting to see if your test methods (much more rigorous than mine) could detect significant differences between compressed and uncompressed NEFs at higher ISO settings.

Anyway, thanks again for your great work!

Best regards, -Jim

This then is the idea behind ‘lossy’ NEF compression — thin the levels in highlights where they are too finely distributed relative to the inherent noisiness of light, but keep most or all of the levels in shadows where the magnitude of the fluctuations is smaller. This is why the NEF compression table is linear at the low end, and quadratic further up. The quadratic part is a reflection of the quadratic relationship between noise and signal of light, the higher the signal the more levels can be thinned out. The linear part of the compression table kicks in when the signal is low enough that the noise fluctuations are less than or of order one raw level — then no compression is possible and the output value must equal the input value.

The ‘lossy’ NEF compression is really a clever use of information theory to save space by eliminating redundant raw levels. The noise which is unavoidably present in light effectively dithers tonal transitions so that the compression is *perceptually lossless*.

A Good write up & from the point of the end user not from a pure mathematical perspective.

But if in doubt and you really need to stretch dynamic range in your landscapes then you really want as much data as possible in case the highlights are borderline & so I won’t use compression.

As for the new ” Loseless Compression ” this will need looking at.

thanks Roy

The D80 only has the compressed NEF format, and it seems, at least to me, the only real image quality difference between them. Exposure metering, another one, is really irrelevant for those coming from the pre-AE era like me, I still use my Leica M3 and, most of the time, guess the (correct) exposure, or use the wonderful Lunasix to help.

Since DSLRs are not good investments, at least for now, it is better to go with the cheaper thing (the nice metal body of the D200 will be very soon obsolete, some say later this year).