Equivalence and dynamic range

Started Dec 27, 2017 | Discussions thread
alanr0 Senior Member • Posts: 2,094
ISO setting is not part of equivalence

Iuvenis wrote:

I can see that read noise and megapixel counts introduce a complication, but I am not sure that it is as significant an issue in the real world. For example, if you use the 12 megapixel A7s rather than the 42 megapixel A7rii as the full frame example on the dynamic range chart I linked to in my original post, the effect will not be much different.

We are used to the idea that cameras of the same sensor size will have similar low light performance, even though they have different pixel counts. I don't see much empirical evidence to support the notion that increased pixel counts reduce dynamic range, and plenty to counter it (eg. the superb dynamic range of the a7riii and D850). On the other hand, lower pixel counts do not seem to increase dynamic range at normal settings.

For that reason, is it not a viable rule of thumb to say that dynamic range is equivalent between sensor sizes at equivalent settings?

Put it another way. The larger sensor is receiving 4 times as much light as the smaller sensor at the same settings, and the same amount of light at equivalent settings. The light falling on the part of the sensor containing the shadow areas will also be the same in each case, so the dynamic range will be similar in each case.

How do you define dynamic range? Are concerned only with shadow noise, or is highlight clipping a consideration?

Do you require that ISO is set for appropriate out-of-camera brightness, or is it simply a convenient means to manage read noise and saturation? I see no reason to constrain ISO when defining "equivalent settings".

My understanding is that "equivalent" exposure have the same:

  • Exposure time
  • Physical aperture
  • Field of view and perspective

As a result, they capture the same total light (total number of photons), and the same light in a defined fraction of the image field of view. ISO has no direct influence.

From this point of view, an f/4 exposure at 1/1000 s at ISO 1600 is equivalent to an f/4 exposure at 1/1000 s at ISO 400 in the same camera. With a near-isoless camera such as the Nikon D850, there is only 1/4 stop difference in read noise (1.4 e- compared with 1.7 e-), but shooting at ISO 400 gives an additional 2 stops of highlight headroom, and very nearly 2 stops of PDR (9.81 c.f. 7.85).

More generally, cameras using similar technology tend to saturate with broadly similar numbers of photons per unit area (rather than similar photons per pixel). Available read noise values below 2-3 electrons per pixel mean that the read noise penalty for shooting at reduced ISO can be minimal. All but the deepest shadow noise is dominated by photon noise, rather than read noise.

With freedom to choose an appropriate ISO, large sensor cameras can offer proportionately higher dynamic range than equivalent exposures in small sensor cameras.

For wide dynamic range, pushing shadows and pulling highlights from an 8-bit JPEG is not the best way to go. If you do need out of camera JPEG images with acceptable brightness, choose a camera which offers a choice of tone curves which protect highlights and/or shadow details.

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Alan Robinson

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