It's probably more 'standard' to express this as a variable ISO and have it realised through adjusting the gamma curve of the preview LUT. In this scenario the ISO represents the (shifted) mid grey point. It might also help to have a little +/- figure indicating how many stops above and below this mid grey point you have.

Take a look at the way it works on an Alexa: http://www.arri.com/camera/alexa/learn/alexa_family_faq/

Section 3: exposures.

The bonus of this approach is that you retain the full DR of the camera.

ISO settings correspond to analog on-sensor gain. I know some manufacturers claim ISO to be a post processing digital gain only that is a non destructive operation, but that's not the case with the CMV12000.

Hi sebastian - take a look at the link I showed you. In the Arri The variable ISO settings are not to do with digital or analogue gain - these both remain the same (to make sure DR is maximised at all times) rather they are to do with tone mapping.

In the instance you describe - wanting over or underexpose a scene, then the Arri approach is the best approach to take - since it allows you to maximise the dynamic range in the areas of interest. So, for example, you wish to record a 'dark' scene that contains a lot of shadow detail and want to maximise the detail recorded - in this instance you would set the Alexa to ISO 200 (for example) - causing the (scene) grey point to be shifted down the curve and prompting the DP to open up the stop (or increase the light levels) to ensure scene mid-tones are moved to the 'correct' point on the curve. Of course you lose overexposure latitude, but you don't need it (as the scene is dark) and instead the shadow tones are remapped so you end up with richer and cleaner blacks. If you have time read the following articles from Art Adams which shows how the Alexa performs at various ISOs:

http://provideocoalition.com/aadams/story/alexa_dynamic_range_its_all_in_how_you_use_it/P3

http://provideocoalition.com/aadams/story/alexa_iso_settings_the_least_you_need_to_know/

Of course, being able to adjust the ISO using analogue gain settings would also be useful, bearing in mind this would reduce the dynamic range of the chip, especially for low light situations and also for Raw recording. The question then becomes how to communicate to the user what's happening - is it a tone remapping or the application of analogue gain or both?

The variable ISO settings are not to do with digital or analogue gain - these both remain the same (to make sure DR is maximised at all times) rather they are to do with tone mapping.

So on an Alexa if you close the iris by one stop and increase the ISO by one stop you will end up with a completely different looking image as before?

Also if I understand this correctly on the Alexa if you have clipping highlights lowering the ISO will not help you a bit, right?

Doesnt that annoy the hell out of DOPs?

In T268#3955, @sebastian wrote:

So on an Alexa if you close the iris by one stop and increase the ISO by one stop you will end up with a completely different looking image as before?

Yes. The shadows will look a bit more crushed, the highlights will show more detail, but the mid-tones will look pretty much the same. As to whether or not you see this will depend upon the image

Also if I understand this correctly on the Alexa if you have clipping highlights lowering the ISO will not help you a bit, right?

Doesnt that annoy the hell out of DOPs?

No. But I'm not sure how an analogue approach will help this either. If the bin of full the bin is full, pulling the signal down afterwards will not get you any extra detail in the highlights.
"
I'd say it would only annoy the hell out of a DoP if they didn't bother to understand the tool they were using. If they did, and they were worried about losing highlight information they could, for example, stop down then raise the ISO to retain this detail and remap the tones.

If they found the whole situation confusing they could just stick with ISO 800 where they know dynamic range is split evenly above and below mid grey.

Actually I reread the title of this task. Exposure compensation is a pretty good way of describing what I'm talking about (duh! I'm pretty stupid sometimes) - so + or - a stop would be one way of expressing it, although some might prefer to see it as a change in the 'ISO' (not that digital sensors really have an ISO). But ultimately I think we're talking about the same thing, no?

Technically speaking as I understand it those are two different things.

I will create a second task for the wish to implement this Alexa like tone LUT remapping. In the AXIOM Alpha we already have these LUT capabilities so its just a matter of creating the curves matching the neutral gray shifts.

Done: T271

That sounds like a good idea - although I'm not sure how this set up would be much different - except as a more basic form of the same thing?! Could you perhaps explain the exposure compensation idea in more detail?

Thanks!

The ALEXA LUT/EI method shifts the middle values while blacks/whites are untouched in contrast to the exposure compensation method which affects the entire range with a constant factor.

Alex mentioned this as good reference from ML: http://www.magiclantern.fm/forum/index.php?topic=9597.msg99327#msg99327

and the formula from Alex: y = (1 - 1/(1+a*x)) / (1 - 1/(1+a)), x is input data in [0..1], a is exposure_gain - 1 (e.g. for 3 EV, a = 2^3-1 = 7)