I am worried that Canon won't do it right, however, if they do implement it. Canon's 1/3-stop ISO resolution seems to be quality-reducing trickery for the most part, so I certainly wouldn't want to see 30D- or 5D-like 1/3 stop ISOs floating around in manual exposure with auto-ISO.
John,
That's a good point. I've often wondered why Canon have not already implemented this concept of a floating ISO that adjusts itself to a manual setting of exposure and aperture. The obvious problem, which you've tried to address with points 1 & 2, is the likelihood that for many combinations of exposure and aperture, ISO 1600 is not going to be nearly high enough. The inexperienced user is going to frequently fall into the trap of getting ISO 1600 shots that are not just 1/3 or 2/3 stop underexposed, but 1, 2 or even 3 stops underexposed, and we all know that the consequences of underexposure at maximum (real) ISO are serious noise and image degradation.
Once you start messing around with additional, user settable conditions to the automatic ISO feature, there's not much point in having it. The whole purpose of automatic adjustments is to facilitate the taking of acceptably good shots instantly with the minimum of fuss. For example, let's suppose I'm in a situation with poor lighting and I want a fast shutter speed and good DoF without compromising image quality. Well, without flash I simply can't get it. I have to sacrifice
something. I've basically got 3 choices. (1) Fast shutter speed, good DoF but underexposure. (2) Fast shutter speed, shallow DoF, accurate exposure. (3) Slow shutter speed, good DoF, accurate exposure but blurred shot.
The solution is bracketing. I can bracket for either 'time value' or 'aperture value' (TV or AV). If my lens doesn't have a wide enough maximum aperture for the autobracketing intervals I've set, I'll simply get a repitition of the same exposure at the same maximum aperture.
What might be useful is autobracketing of ISO in relation to a fixed aperture and time value, but on reflection, I'm not sure just how useful. Lets see what happens in the above example of poor lighting conditions. Since I'm a fairly experienced user, I'll set the ISO to 1600. Generally, a sharp image with shallow DoF is preferrable to a blurred shot with extensive DoF, so I'll set the camera to TV mode and a shutter speed to whatever I think will freeze the action. Depending on the shutter speed, the results might vary from all 3 shots being identically underexposed at the same maximum aperture, to all 3 shots reflecting accurately the EV interval I've set.
If were able to bracket for ISO, I could expect the same result, ie. the 3 shots would vary from being all equally underexposed at ISO 1600, to all accurately reflecting the EV interval set. There
could be an advantage there if one of the shots was correctly exposed 'to the right' at, say ISO 800 instead of overexposed at ISO 1600 with blown highlights. This option of (sometimes) being able to
not use a higher ISO than is necessary, I would consider useful.
Another useful feature which could flow on from a 'floating' ISO is the facility of autobracketing DoF whilst maintaining a fixed 'accurate' exposure (as opposed to a fixed shutter speed or time value). At present, with camera in Tv mode, I can only bracket for DoF concomitant with varying exposure, which means the shot with the ideal DoF might be either under or overexposed. With a floating ISO that changes with changing aperture to maintain a constant 'exposure', I could get a real choice of DoFs easily and quickly. This is something Canon should look into.