I'm still confused about using expose-to-the-left. Do you do this when the required ISO setting for an image is:
- In range of the native ISO
- In the extend/extrapolated ISO's
- Only if your camera is ISO Invariant.
Pretty much every camera is ISO-invariant at
some point. For older cameras it may well be the highest non-extended ISO value, for modern cameras it may be quite a bit lower than that. The trick is to find out what that point is. Then you can use ETTL as soon as you would otherwise go over that point.
Finding out is not that difficult. What you need to do is the following: set up a shoot that is correctly exposed with your highest non-extended ISO value, so for example ISO 6400. Let's say that you'll find that 1/60 sec, F5.6 and ISO 6400 is the correct exposure for this scene. Set the shutter speed and aperture to these values manually. Now make a series of exposures where you lower the ISO speed one stop each time, so in this example you shoot the following series:
1/60 sec, F5.6, 6400 ISO (i.e. correctly exposed)
1/60 sec, F5.6, 3200 ISO (i.e. one stop underexposed)
1/60 sec, F5.6, 1600 ISO (i.e. two stops underexposed)
1/60 sec, F5.6, 800 ISO (i.e. three stops underexposed)
1/60 sec, F5.6, 400 ISO (i.e. four stops underexposed)
Load these images in Lightroom, select them all with the 6400 ISO shot as 'most selected' image and then choose 'Photo - Develop Settings - Match Total Exposures'. Lightroom will now correct the underexposed images by setting the exposure to +1, +2, +3 and +4, meaning all images will be identical as far as the exposure is concerned. Now zoom in to 1:1 and compare the four underexposed images with the first image. Look at the noise level. If your camera is ISO-invariant at say 1600 ISO, then you will see no noticeable difference between photo 1 (6400 ISO) and 3 (1600 ISO, +2 stops corrected in Lightroom), but you will see that photo 4 (800 ISO, +3 stops corrected in Lightroom) is getting worse than photo 1. The example below may be a little difficult to judge because I could not post it at full size, but if you look carefully you will probably agree that ISO-invariance of this camera is reached at 1600 ISO.
So now that I know that this camera starts to be ISO-invariant at 1600 ISO, I will use 1600 ISO (and underexpose) in all cases where I would normally need to use a higher value to get the correct exposure.