Exposure control in a raw developer: misnomer?

Started Jan 30, 2013 | Discussions thread
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Great Bustard Forum Pro • Posts: 35,671
Absolutely correct -- again!

dosdan wrote:

For many, "exposure" = "brightness". Here's another conundrum for you: does f/2.8 1/100 ISO 1600 on a 5D have the same exposure as f/2.8 1/100 ISO 1600 on a 6D? After all, the noise is most certainly not the same.

The answer is yes, they are the same exposure. QE is not part of it. It's the photons falling on the unit area of the sensor that are the same, not the photo-electronics produced. Since only scene luminance, shutter speed and f-number are involved with determining the amount of light/unit area arriving, for the same scene luminance, f/5.6 & 1/100s results in the same exposure on APS-C AA-less, FF Foveon & MF cameras. (Assuming the same lens transmittance.)

So while I use the phrase "photons captured" (OK if considering different settings on the same camera), exposure is really about "photons arriving" rather than"photons converted".

Winner!  Yes, you are absolutely correct.  Exposure is simply the density of light falling on the sensor while the shutter is open, not the total amount of light used for the photo:

Total Light Collected = Exposure x Effective Sensor Area x QE

where the QE (Quantum Efficiency) is the proportion of light falling on the sensor that is recorded (e.g. if QE = 50%, then half the light falling on the sensor is recorded).

However, in terms of the noise in the photo, we must also include the read noise (the amount of additional noise added by the sensor and supporting hardware).  So, we can phrase the situation thusly:

Photos made from the same total amount of light will have the same noise if the sensors are equally efficient.

So, what we have is:

  • Exposure
  • Total Light
  • Total Light Collected
  • Brightness

which are all related, but mean different things, and are discussed in detail here:


In the two exposures with different shot noise figures, I was assuming the same camera.

In the case of using the same camera, then the same exposure results in the same total light which results, the sensors are equally efficient, which means the same total light is recorded and the read noise is also the same, thus the same noise.

In other words, for a given format, we can use the words "exposure" and "total light" interchangeably, and for the same format and sensor efficiency, the noise differential between two systems is purely a function of the exposure.

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