I've been reading various articles, posts, etc on the web that deal with Leica's new M10. Many of those suggest that the improvement in dynamic range from the Leica M240 to the M10 is of the order of 1.5 to 2 stops. But I find that difficult to agree with.
I would guess the improvement in dynamic range between the M10 and the M240, just by eyeballing the published images, to be closer to 0.5 stops than 1.5-2.
Which raises an interesting question - why the huge discrepancy, when everybody is looking at the same images?
What is Dynamic Range?
To someone like me, with seven years of engineering school, the definition of dynamic range is deceptively simple - for an image it's (in non-mathematical terms) the difference between the lightest and darkest levels in which detail is discernible. Usually, dynamic range is measured in stops, or EVs.
So the problem is???????????
Actually, there are two problems. Firstly, there's a major problem of definition, and then there's a minor problem of measurement.
For simplicity, I'll deal with the minor problem first:
The minor problem of measurement
The minor problem of measurement is a fairly simple one - what exactly does "detail is discernible" actually mean?
By way of example, does that mean you could recognize a black cat against a black background? Or a white cat against a black background? Well, as it happens, there is a standard, although (regrettably) it has nothing to do with cats, black or white. For engineers, the most commonly used definition is that "detail is discernible" means that the signal to noise ratio is one. Now you should note that a cat at a signal to noise ratio of one doesn't actually look much like a cat. Or anything. From the folks at
Point Grey:
Now there are a whole host of additional complications - for example:
- Color images have red, green and blue channels that saturate at different levels - so does detail vanish when one channel is saturated, or all channels?
- Images in cameras are digitized, and digitization creates it own noise. Are you including that or not? Also, are you including amplifier noise, etc?
- Human perceptions of signal to noise ratios aren't too good. Notably, you can use dithering (basically, adding carefully controlled amounts of noise) to make a picture look better. Technically, this increases the perceived signal to noise ratio. So sometimes, when you see a signal to noise ratio quoted, it's "perceptual".
On the last point about dithering, one of the reasons, although not the most important, why I called Adobe's lossy DNG compression an
"engineering abomination" a few years ago was that it clips images to 8 bits, and then uses dithering to make the images look better.
So there are good reasons why you could have different people measure the dynamic range of a camera, and they could come to somewhat different answers. By way of example, I'll contrast two different sources of dynamic range measurements for cameras:
- DxOMark. DxOMark is well known in the photographic community - basically, loved and hated by equal measure. I have my issues with what DxOMark does - mainly that they're not very transparent about how they get to their measurements, and some measurements don't make a whole lot of sense. But they do apply a consistent set of measurements that at least provide a common basis for comparison that you won't get from the camera manufacturers. Or most camera sites either.
- Sensorgen.info. Sensorgen.info is the imaging nerd's site - it has information that people that work with image sensors want to see, such as quantum efficiency and saturation capacity.
Here's a table of what both sites say about the dynamic range of some cameras:
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Firstly, you'll note that DxOMark is consistently 0.6 to 1 stops higher than Sensorgen. Next, those with a technical background will notice something odd - the DxOMark result for the D810 is 14.8. But, for a camera with a 14-bit A/D such as the D810, that's impossible, at least if you're using the standard definition of dynamic range, because the maximum dynamic range achievable would be 14. My guess is that one reason for the difference between the two sites is probably that DxOMark is trying to remove the effects of A/D resolution, and just look at the sensor as an analog device. They may also be trying to compensate for the presence of other sources of noise, e.g., in the amplifiers. That would remove the quantization noise element, and give a consistently higher reading for dynamic range across the board. But if anyone knows exactly how DxOMark do their measurements, I'd like to hear from them.
But what is reasonably consistent is the difference between cameras. So, e.g., DxOMark view the M240 as 1.6 stops better than an M9 versus Sensorgen at 1.4 stops. For what it's worth, my experience from when I've had the opportunity to test cameras in an optics lab is that Sensorgen is closer to the readings that I get. Of course, I'm measuring the whole package, not trying to isolate just the sensor.
So, bottom line - there are legitimate technical reasons why not everyone will agree about exactly what the dynamic range of a given camera is. But those differences tend to be more about what the absolute number is, rather than the relative performance of cameras. So while DxOMark and Sensorgen disagree about whether the dynamic range of an M240 is 12.5 or 13.3, they both agree that it's dynamic range is at least a full stop better than the M9, and at least half a stop worse than a Sony A7R.
The Big Problem of Definition
But the differences in measurement don't explain why my guess of the M10's dynamic range relative to the M240 was so far from many individuals with a great deal of experience of Leica's M cameras. Some additional research led me to believe that many of those individuals were concluding that the M10 was usable at 1.5-2 stops higher ISO than the M240. "Usable" here being defined as being able to produce a print that was commercial usable without extensive post-processing. Based in large part on the 1.5-2 stops higher usable ISO, they were then concluding that the M10 had 1.5-2 stops of additional dynamic range. Unfortunately, that's not a conclusion that you can safely draw.
The reason why an increase in usable ISO doesn't necessarily imply the same increase in dynamic range goes back to the definition of how dynamic range is measured, which is based on whether detail is discernible within noise. But how "usable" an image is in the view of a photographer takes a lot more than just noise into account - it's an assessment of the entirety of image quality. In the specific case of the M240 vs. the M10 the culprit is probably banding. Now banding does contribute to noise, but perhaps less than many might think. Banding is usually lighter and darker bands across the image, usually numbers of pixels wide. But within those bands, detail is still discernible. It's actually only on the transition between bands, the edges, that banding makes discerning detail more difficult, and contributes to noise. So the effect of banding on a noise measurement, and hence on dynamic range, is less than you might guess. But banding, even light banding, can quickly make an image unusable.
This then is the problem of definition - there's a general assumption that improvements in dynamic range and improvements in high ISO performance are effectively the same thing. That's not the case - for a "perfectly designed camera", a camera where the only source of image quality imperfection was simple sensor noise, that would be true. But in real cameras, quite frequently there are other sources of defects in image quality. As soon as those other sources come into play, then the direct equivalence between dynamic range and improved high ISO performance is no longer valid.
Conclusion
So, in summary, here's the explanation: It's probably quite reasonable to say the the M10 has a 1.5-2 stop advantage over the M240 at higher ISO. But that doesn't mean that the M10's dynamic range is 1.5-2 stops better than the M240, simply because the M240's practical usability to a photographer is mostly limited at higher ISO by banding, not by simple noise performance.
So what should you take from all this? Given the issues with dynamic range as applied to cameras above, my best recommendation is that, unless you are actually in an optics lab, just avoiding mentioning the phrase "dynamic range" is probably the best way to go.
Note: this post has been updated since it was first published.
Also Note: There's an update
here, with the actual measured M10 dynamic range.
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