5dIII review at DxOMark

[neuroanatomist]

I don't get the D800's high score for Sports (Low Light ISO) performance. I didn't see where they offered graphs or anything to back this up. I would have thought the 5D III would have been the victor here.

I'm going to repost something I put up on CR (modified slightly):

Honestly, everyone should take a step back and consider what the DxOMark scores mean - and don't mean. They are quite transparent about their scoring metrics and methods, and there's no reason to suspect there's collusion or favoritism occurring. At the same time, it's important to remember that the sensor score is just that - a score of the sensor itself, not a 'camera score'.

Furthermore, keep in mind that all of their overall scores are based on reducing the image to 8 MP - thus, the greater the starting resolution the more downsampling, which means lower apparent noise. That lower apparent noise means apparently better ISO performance and apparently more dynamic range (lower apparent noise lowers the 'floor' for the DR estimate). If you like, you can substitute 'artifically inflated' for 'apparently'.

They have a page describing the mathematics of the normalization to 8 MP, and on that page, there's the following statement:

What should be remembered is that doubling the resolution adds:
3dB to the normalized SNR
0.5 bit to the normalized DR
0.5 bit to the normalized TR
1.5 bit to the normalized CS.

So, compared to 8 MP the D800 is 4.5-fold higher (just over two doublings), whereas the 5DIII is 2.75-fold higher (just over one doubling). What that means, mathematically, is that the D800 has approximately one extra doubling of resolution relative to DxOMark's 8 MP normalized value - that accounts for all of the differences in the scores for both ISO and Color Depth, and part of the difference in dynamic range.

Note that DxOMark does provide the non-normalized data, they just don't use those data to calculate the overall scores, the rationale being that normalizing to 8 MP allows appropriate comparisons. In one sense, it does - if you're going to print 8x10" images all the time, then their scores actually apply pretty well.

Let me give a specific example for color sensitivity, which is the basis for DxOMark's Portrait Score. That's one area where the D800 with 25.3 bits 'beats' the 5DIII with 24-bits. In the comparison without normalizing to 8 MP, the D800's advantage pretty much entirely disappears.

Rick, that piece about reducing to 8 MP is what makes the D800 'better' on their Sports (Low-light ISO) score - the higher MP of the D800 means relatively greater effective noise reduction when reducing from 36 MP to 8 MP, compared to going from 22 MP to 8 MP.

This is why I quit reading DxO: http://www.dxomark.com/index.php/Cam...(brand2)/Nikon

I don't see how these measurements give one sensor a score of a 95 and the other a score of an 81.

Here's another example: http://www.dxomark.com/index.php/Cam...brand3)/Pentax

Notice that the Pentax K5 has scored an 82 overall; however, it is inferior to the 5d Mark III in ever category except Dynamic Range at low Iso settings. Thus, am I to infer that Dynamic Range at low Iso settings is the only measurement that matters when rating a sensor?

Again, it's about understanding what the measurements mean and how they're derived. It looks like the Landsacpe (Dynamic Range) and Portrait (Color Depth) have a much greater impact on the overall score than Sports (Low-Light ISO), and that actually is the case - for a reason. The overall score is an 'average' of the three use-case scores, but two of those three, Portrait (units are bits) and Landscape (units are Ev) are on log scales. ISO is a linear scale, so for example, comparing the ISO (Sports) values for the K5 and the 5DIII, the 5DIII 'score' is almost double that of the K5 (2293 vs. 1162), but when you log transform that difference, the difference is less than one stop (Ev).

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To summarize, one key point about DxOMark's scores is that they are all normalized to an 8 MP image, and this strongly impacts the overall and use-case scores, giving an advantage to sensors with higher MP counts. Likewise, since DR and Color Depth are measured on a log scale, they have a relatively greater weight in the overall score than low-light ISO.

Try the following: click on the first link above (5DIII vs. D800), then in the comparison click the Measurements tab. I'd recommend skipping the ISO Sensitivity plot (it has nothing directly to do with the Sports/Low Light ISO score, despite the name of the test - it's really looking at ISO accuracy relative to the actual International Organization for Standardization criteria). But...look at SNR (signal-to-noise ratio), Dynamic Range (basis for the Landscape score), Tonal Range, and Color Sensitivity (basis for the Portrait score), and for all of them, look at the Screen plots - those are the data that are not normalized to 8 MP (vs. the Print plots, which are normalized and used to generate the overall and use case scores). When you do that, you'll see the following:

• SNR - 5DIII wins (when normalized they tie)
• Dynamic Range - 5DIII loses up to ISO 1000 but wins at higher ISOs (when normalized, the 5DIII loses up to ISO 1600, then they near-tie)
• Tonal Range - 5DIII wins (when normalized they tie)
• Color Sensitivity - they tie (when normalized, D800 wins)

So, for all of the above measures, the higher MP count of the D800 gives it an advantage when downsampling the images to 8 MP.

Bottom line is that I think DxOMark's measurements are more useful than their scores, but even their scores are useful - as long as you understand how those scores are generated, and the inherent limitations and caveats about them. The Overall Score is something the have to have (how can you have a ranking site and not actually actually assign ranks). It's like looking the Gross Domestic Product by country, and concluding that the USA is the best country in the world simply because it has the highest GDP. Or, to use a photographically relevant analogy borrowed from Bob Atkins, it's represeting the Mona Lisa by it's average color value.