What is the true sensitivity of a digital sensor?

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MinAZ
MinAZ Veteran Member • Posts: 4,439
What is the true sensitivity of a digital sensor?

This started on another thread, but I never had the chance to ask, and now I am dying to know...

Assuming the following are true:

- Cameras are generally ISO-invariant (yes I know, this is an assumption)

- Over the range of ISO-invariance

- You have an external light meter

- ISO measures the amount of in-camera post-processing applied to the RAW file rather than the actual sensitivity of the camera sensor which does not change due to changing the ISO setting

- you care only about exposing the image for the sensor, and you do not care about how "bright" the image appears until after post-processing

And you know the following things:

- Amount of light in the environment/illuminating your subject (measure by a light meter)

- Your shutter speed

- Your aperture

How do you know what the true sensitivity of the camera's sensor is?

Presumably, you would need that information to determine the correct exposure for a scene... if you did not know the sensitivity of your recording medium, there would be no way to judge how bright you need to illuminated the subject, right?

So is there a true-ISO (or range of true-ISOs) for a particular sensor? Is it measure some other way (but to make sense we would probably want to convert it to ISO-equivalence anyway since that is how every light meter works)?

And without it, why are we even bothering with the values output by a external light-meter which presumably does not know what the sensitivity of a camera's sensor is? The camera will adjust brightness to show that an image is "properly exposed" based on ISO settings but the reality is that it could have been just as properly exposed at some other combination of setting which might actually have been a more accurate exposure for the camera.

If camera manufacturers could ignore ISO conventions, would that mean they could more accurately tell you what the optimum settings for you camera should be (and leave the actual "printing" of the image to post-processing)?

Would we be better off if each camera could tell you what its sensitivity is and how much light it needs to expose an image, and then guide you to that exposure, instead of having to brighten/darken an image so that it conforms to the ISO standard?

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