... I get the don't over expose point but if you are significantly under with your target exposure how do you pick the right setting ? Set the EV to - something, like 1, 2 or 3 stops ?
I tend to pick ISO for a shoot and that's it
You just adjust until you get the results you want. Your eye is the discriminator. You can work pretty much as you do now, except that when light values become such that you are underexposing by a couple of stops the LCD review display will show you a very dark image and the histogram won't be useful. There's obviously a range in which this technique is viable ... I think 3-4 EV underexposure is getting close to the limits of the sensor for good results.
The thing to test to your satisfaction is whether whatever raw process you are using does a better job in boosting the ISO than the in-camera ISO setting does. If it does, then use the technique. If the camera proves better, or you want the LCD review to be viable, use the camera setting. Always be mindful of the limits of the sensor AND the raw processor that you use.
PS why are the M9 noise graphs non linear and will this non linearity be matched by the LR exposure increase process ?
No idea. I never looked at any noise graphs. I'm not sure whether the question makes sense.
PPS won't lifting exposure in LR also bring an emphasis on the mid tones, it would be interesting to try a grey scale for the test
The Exposure (EV compensation) adjustment is a linear bias offset of all values in the image. The Contrast, Highlights, Shadows, Whites, Blacks, and Clarity adjustments change the shape of the curve within the black point and white point settings: these are non-linear adjustments.
I didn't shoot a gray scale, just used the two test images I already showed to test this.
Making the same exposure at the ISO 1250 setting and again at the ISO 160 setting, and then applying +3 EV via the Exposure slider to the ISO 160 exposure, nets the exact same result, with output histograms that are so close as to be identical (they're not exact as I didn't have the camera on a tripod so the scene is shifted slightly).
G