Mark III: Safety Shift custom setting

[Gregory]

I was very happy to learn that the Mark III has a Safety Shift custom setting (C.Fn I -8) that allows the camera to automatically increase the ISO if the exposure would be insufficient at the current settings. this would be great to have when photographing birds and not having time to switch the ISO setting when I realise that the shutter speed is too low at the current Aperture and ISO. I am a little dubious of the wording in the manual though:

"When the subject's brightness changes erratically and the correct autoexposure cannot be obtained, the camera will change the ISO speed within 100 - 3200 automatically to obtain a correct exposure."[/li][/list]I'm not sure what this means. what exactly does 'erratically' mean? how do they determine the 'correct' exposure?

has anyone tried the Safety Shift feature? I'd like to hear what your experience with this feature has been.

regards,
Gregory

[Nill Toulme]

Don't know about the "erratically" part, but my greater concern about the ISO safety shift is that it appears (from reading the docs; I don't have the camera yet) that in Av mode (which is where I would want to use it) it would depend on setting a minimum shutter speed via the other CF that provides that function.  Trouble is, it looks as though the highest shutter speed you can set as a minimum in that CF is 1/60.  

For my uses (primarily sports in fading daylight), I would want to be able to shoot wide open in Av mode, set a minimum shutter speed of 1/400, and have the ISO ramp up automatically as the light falls.

Nill
~~
www.toulme.net

[Gregory]

I see what you mean. I for bird photography would probably prefer ≥ 1/125 or even 1/160.

in your case, you might be able to get around the problem by reversing your setup. use Tv instead of Av and set the maximum aperture (C.Fn I -13) to your preferred aperture. the maximum aperture values range from f/1.0 to f/64.

regards,
Gregory

[fennario]

For my uses (primarily sports in fading daylight), I would want to be able to shoot wide open in Av mode, set a minimum shutter speed of 1/400, and have the ISO ramp up automatically as the light falls.

Nill
~~
www.toulme.net
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I've found that it works best in Tv mode.  Eg. with 400 5.6 I will set shutter speed at 1/1000.  The camera will float the aperture to get proper exposure, however, if 5.6 is still too slow, then the camera will boost ISO until the proper exposure is achieved.

That said, it doesn't work so well in AV or P, as per your observations. As such, a true ISOv would be much better.