Exposure control in a raw developer: misnomer?

Started Jan 30, 2013 | Discussions thread
Great Bustard Forum Pro • Posts: 42,060
Re: Absolutely correct.

hedwards wrote:

We should probably stop calling it ISO because it's not really the same thing. It's something that's very different. ISO on a film seems, to me at least, to be the film equivalent of EV compensation as all that does is change the way that the camera decides to request more or fewer photons striking the film. Most of the time, people take the recommendation and use that when shooting their film. But, there's no rule that you can't choose to use the recommended ISO value just because the film box asks you to use that value.

Digital sensors though, are not at present user replaceable, so the misnamed ISO setting is really gain. The sensitivity is roughly equivalent to film of that speed, so the F-stop and shutter speed should be the same to get the same exposure, but it's a completely different way of achieving that.

Basically, the sensitivity of film depends on its speed rating, whereas the "sensitivity" of a sensor is fixed throughout the ISO (gain) range, except inasmuch as some sensors have less read noise at higher ISO settings.

As far as f2.8 at 1/100 and 100 ISO and F2.8 at 1/100 and 1600 ISO, they're the same exposure as far as I'm concerned.

For a given scene, yes, as the same density of light falls on the sensor.

The one at 1600 ISO will be brighter, but the compositional elements will all be the same.

Yes, but due to the limited bit depth of the image file, more of the ISO 1600 photo will be blown, resulting in less detail in the highlights (but more detail in the shadows if using a non-ISOless sensor where the higher ISO setting results in less read noise).

But, I can see why one would disagree with that, I think there are valid reasons to consider it to be a part of the exposure as well.

If "exposure" and "output brightness" are synonymous, sure.

I just don't generally make a decision about it, I mechanically choose the lowest ISO I can and usually prefer that over changing my F-stop or shutter speed. The DoF is the same in both cases, although, the circle of confusion is presumably larger in the latter case, they are what I would consider to be the same exposure. Unlike film which sometimes has pleasant looking grain at higher ISO sensitivities, I've never found noise in images to be particularly pleasant to my eye. In both cases it represents a loss of resolution and generally one uses the the least sensitive film one can use.

The photographer looking to maximize IQ balances DOF/sharpness, motion blur / camera shake, blown highlights / plugged shadows, and noise.  The more light you have, the more options you have.

PS, if anybody works for Sony or Canon reads this thread, user replaceable sensors would be awesome.

Yeah, but I don't think they can replace just the sensor -- you'd have to replace all the supporting hardware as well.

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