Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
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Re: Understanding ISO
In reply to Great Bustard, Jul 19, 2014

Great Bustard wrote:

The most important factors about the sensor that manufacturers do not tell us are:

  • QE (Quantum Efficiency -- the proportion of light falling on the sensor that is recorded)
  • Read Noise (the additional electronic noise added by the sensor and supporting hardware)
  • CFA (Color Filter Array)
  • Microlens Efficiency

Which of these are likely to see significant improvements in the fairly near term (next few years)?

Sensorgen has used DxOMark data to get the QE and read noise for a lot of sensors. However, differences in the dyes in the CFA can result in a greater QE by letting more light in with the result of less luminance noise at the expense of greater color noise (there may also be issues with metamerism, although that's more complex than merely the transmissivity of the dyes).

For example, I compared the Canon 6D at ISO 6400 to the Olympus EM5 at ISO 1600:

Here are sensorgen’s figures for the 6D and EM5:

  • 6D: QE = 50%, read noise @ ISO 6400 = 2 electrons / pixel = 9 electrons / µphoto
  • EM5: QE = 53%, read noise @ ISO 1600 = 2.6 electrons / pixel = 10.4 electrons / µphoto

That's about as identical as it gets, yet the EM5 was cleaner when they should be all but indistinguishable. Assuming that the exposures for the DPR's test scene were exactly two stops apart, I’m thinking the reason for the EM5’s cleaner appearance is due to less color noise. So, I did own conversions and processing to both photos.

As it turned out, they did end up looking all but identical in terms of noise (well, I had to lower the red gamma and overall gamma on the 6D photo a bit to get the colors nearly the same, too).

So, I don't know if that means that color noise isn't a big deal, or that the color noise in this particular example was close enough to where NR (noise reduction) resulted in no significant loss of detail.

This reminds me of something else, and leads me off into a bit of a detour...

Back when one of the lengthy ISO/exposure disputes was raging, I decided to test the (to me at the time) counterintuitive notion that given fairly ISOless sensors (like my then Sony A57), images at ISO 100 brightened in RAW processing would be very similar to the same images shot at ISO 1600.

Indeed this proved to be largely true, and I was duly impressed. However, there was some additional shadow color noise in the 'pushed' ISO 100 images.

(Is this the result of slightly-less-than-perfect "ISOlessness" of the sensor, or artifacts of RAW development, or what? It did seem that some RAW processors did better than others at this, but I don't know what kind of noise reduction they might have been doing.)

As a result of these crude tests, plus info I found on the Net, I kept my ISO between 100 (max dynamic range?) and 400 (min noise?) on that camera.

The A77 II is new enough that I haven't run all the tests I should, but judging from what I see on DxOmark, the ISO 100-800 range may behave similarly.


Then there's the issue of microlens efficiency (DxOMark's F-stop blues, discussed in more detail here), which can have a decided impact on how much light falling on the sensor makes it into the pixel at wide apertures.

For astrophotographers, or any other photographers that use long exposures, thermal noise is a big deal, so that's another factor to consider which I did not list above.

I suspect that given our existing battery life constraints, active cooling for DSLR sensors isn't in most photographers' future.

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