Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 42,053
Re: Understanding ISO

John Sheehy wrote:

Great Bustard wrote:

  • Photon Noise (noise from the light itself)
  • Read Noise (additional electronic noise from the sensor and supporting hardware)

So far as photon noise goes, modern sensors have a QE of right around 50%. What this means is that half of the light that makes it through the CFA (Color Filter Array) onto the pixel is recorded. Thus, for a Bayer CFA, there's only a stop more to go to 100% efficiency.

... in the green channel. The blue and red channels can improve even further. For future sensor geometries, red capture could conceivably increase by

I've always wondered what the difference in QE is between the different color channels -- do you know?

In other words, what we want is lower read noise, not "higher ISO". The thing is, though, a read noise of 2-3 electrons per pixel is already pretty low (considering that the pixels on FF DSLRs, for example, are able to produce signals of around 100,000 electrons), so why are high ISO photos noisy? The answer is simple: they get very little light, and less light means more noise.

2-3 electrons is not a big issue at base ISO, with big pixels, but for small pixels and and high ISOs it is tremendous read noise.

This is why I said below:

For sure, the read noise represents a kind of floor beyond which the noise in the photo quickly diminishes.

And of course, real world read noise does not look like gaussian noise created by a computer; it is highly correlated, visually, even if not significantly so, as a statistic.

I'd be interested in more info on that.  I've heard others say that read noise is pretty much gaussian (assuming we don't count banding as noise, since it is systematic).

In other words, it's not like sensors can keep on getting more and more efficient and low light photography will get less and less noisy. For sure, the read noise represents a kind of floor beyond which the noise in the photo quickly diminishes. But even with zero read noise, we are limited by the light itself.

That limit is much farther away, and is only limited in regard to how much we want to magnify the capture in display. Pure photon captures can work very well with very, very tiny signals if you do not need to display them large. They have no contrast issues in near-blacks, and no linearity problems down there, either.

The thing is, though, while lower read noise will certainly push the noise floor further out, the photo will still be rather noisy due to low signal.  For example, while we may be able to shift the "noise floor cliff" from ISO 25600 to 102400 with lower read noise, we're never going to make ISO 25600 look as good as ISO 3200.

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