Safety shift has existed in some form as far back as the original EOS 1D. Its options have changed slightly over the years and though varying cameras but the intent is the same. The function, unfortunately, is somewhat cryptically labeled, potentially leading many to skip over enabling what can be a rather useful function.
Safety Shift Mode 1: Tv/Av
Safety shift mode one applies to all Canon cameras except the D30, D60, and Rebels (EOS ##0D and EOS 1000D). The manual describes it as follows.
1: Enable (Tv/Av)
This works in shutter-priority AE (Tv) and aperture-priority AE (Av) modes. If the subject’s brightness changes suddenly and the current shutter or aperture becomes unsuitable, the shutter or aperture is shifted automatically to obtain a suitable exposure.
From reading that, one might assume that enabling safety shift isn’t necessarily a good idea. The implication is that even in one of the priority modes the camera will be constantly overriding your shutter and aperture settings as the “exposure changes”.
The reality is that Safety Shift only kicks in when your metered exposure exceeds the limits of one of the camera’s ranges.
In Av mode, Tv/Av Safety Shift works something like this; suppose the camera is set to f/2.8 and the metered shutter speed is 1/16000th of a second, 1 stop faster than the maximum shutter speed the camera can support. Without safety shift, the image will be 1 stop over exposed. With safety shift, the camera will automatically stop the lens down 1 stop so you’re shooting at 1/8000th at f/4 and the image will be properly exposed. On the flip side, an exposure of 60s @ f/22 would prompt the camera to open the aperture 1 stop to get an exposure of 30s @ f/16.
In Tv mode, the Tv/Av Safety Shift behaves the same way only changing the shutter speed instead. For a required exposure of f/1.4 at 1/250th with a lens that can only open up to f/2.8, Safety Shift would cause the camera to drop the shutter speed to 1/60th to insure a proper exposure. Alternatively, an exposure that required an aperture smaller than the lens could produce would result in shortening the shutter speed until the exposure was correct.
What exposure is shifted?
Safety shift uses the meter to determine the exposure. However, exposure compensation settings are factored into the exposure before the safety shift is applied. Therefore, if one is shooting a high-key scene with +2 stops of exposure compensation set, the compensated exposure will be the base exposure and the Safety Shifted exposure will be equivalent to the compensated exposure.
Safety Shift Mode 2: ISO
Canon’s EOS 1 series bodies since the Mark 3s have a second Safety Shift mode to adjust the ISO instead of the aperture or shutter speed. The manual entry for mode 2 reads:
2: Enable (ISO Speed)
This works in Program AE, shutter-priority AE, and aperture priority AE modes. When the subject’s brightness changes erratically and the correct auto exposure cannot be obtained, the camera will change the ISO speed within 100-3200 automatically to obtain the correct exposure.
Like mode 1, mode 2 only alters the exposure once the shutter speed or aperture has reached the limits of what the lens or camera can do. In addition, like Tv/Av mode 2 factors in the exposure compensation that is set when correcting the exposure.
Faking Auto ISO on a EOS-1D(s) Mk. 3 or newer
Auto ISO is either handy or completely useless, depending on the implementation and photographer. Nikon’s implementation is arguably one of the best. In starts with allowing the photographer to set their preferred ISO, from there the photographer can configure a minimum shutter speed and a maximum ISO for the camera to use.
In other words, a camera could be configured so that the shutter speed won’t drop below 1/60th and the ISO won’t be set above ISO 800.
If you aren’t providing at least that much, the implementation borders on worthless.
Fortunately, the EOS 1 series bodies don’t provide an auto ISO implementation as such at all. However, mode 2 (ISO) Safety Shift combined with the built in shutter speed limitations (Custom function I-12) can be used to mimic Nikon’s auto ISO to some degree; you can’t register a lowest shutter speed faster than 1/60th and you can’t stop the camera for shifting outside of the registered ISO limits (Custom Function 1-3).
However, even with those limitations this “fake” implementation of auto ISO provides more control than Canon’s auto ISO as implemented on any other Canon camera.
In my experience, Safety Shift, especially the ISO shift on 3rd generation EOS-1 bodies, provides the latitude to be able to quickly capture a fleeting moment without having to worrying about overexposing an image because the camera couldn’t reach a high enough shutter speed or narrow enough aperture. Moreover, coupled with the ability to limit the lowest shutter speed to something approaching a generally hand-holdable speed (1/60th) you have a rather good, if slightly more complex to configure auto-ISO system.