Quote:
Originally Posted by hallsworthb
If I take a photograph at (say) ISO 400; F5.6; at 1/250th second and I look at the histogram, then I can change any of them to a maximum of +/- "clicks", giving a spread of 5 or 6. ideally, and without killing the highlights, that will move the histogram to the right, to preserve as much information / retrievable image as possible.
The decisions I make about which to move include speed of subject; depth of field; noise; etc as normal
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Yes. Although I'm not sure what you mean by "clicks". If you're talking about the notches on the adjustment wheel, they usually adjust in 1/3 of a stop so 6 "clicks" is 2 full stops. The method you've described retains as much possible highlight and shadow detail.
Don't forget that you can adjust the settings you've got above and main the same overall exposure by compensating one adjustment with another.
For example you've said ISO 400, f/5.6 1/250th. If you wanted a larger aperture to get more depth of field you could adjust either the ISO or shutter speed to compensate. Say to ISO
200, f/
4, 1/250. Or (adjusting the shutter speed) ISO 400, f/
4, 1/
500th.
Quote:
Originally Posted by hallsworthb
Finally (Sorry) If I include compensation, and am on "P", does the camera decide which to change? I assume it has criteria programmed , with ISO last to be moved.
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This will be specific to your camera and will be in the manual somewhere. The camera chooses what to change and it will have a preset range. It's called the program line.
In dim light it opens the lens up as far as possible and uses the highest "safe" ISO (as preset by Canon) to get as fast a shutter speed as possible. Once the shutter speed reaches 1/f (ie 1/4 at f/4) then the lens stops down as the shutter speed increases.
Then it tries to keep the aperture 1 stop smaller than wide open and the shutter speed faster than the reciprocal of the focal length (ie 1/200th at 200mm) and the ISO at the minimum setting it can. You may also have an adjustable auto ISO limit which you use to tell the camera it can use any ISO up to and including whatever you've set.
There are program line diagrams (although they don't account for auto ISO)
here.