Safety shift is a custom setting designed to avoid over or underexposed images when using shutter or aperture priority modes with a manually set ISO. When active it can override the selected shutter speed in Tv mode, aperture value in Av modes or ISO in Tv, Av and P modes.
Shutter priority and safety shift
When the camera is set to shutter priority, and the photographer selects a specific ISO value and shutter speed then the only way for the camera to control exposure is by changing the aperture value.
Imagine if you were taking photos of birds in flight and had selected 1/2000s and ISO 400. In good light you might find the aperture chosen is f/5.6, but if the sky turns darker and the light drops the camera will try to open the aperture more. Depending on the lens this may or may not be possible. When safety shift is active the camera can either override the selected shutter speed, or the selected ISO to achieve a correct exposure.
When safety sift is not activated – the camera default – the aperture value on the LCD and in the viewfinder will flash as a warning of an incorrect exposure.
Aperture priority and safety shift
When the camera is set to aperture priority, and the photography selects a specific ISO and aperture values then the only way for the camera to control exposure is by changing the shutter speed.
Imagine if you were taking portraits outdoors in bright sunlight and had selected ISO 400 and f/1.4. It is possible that the camera will not be able to avoid overexposure even by selecting the fastest available shutter speed. When safety shift is active the camera can override the selected aperture or selected ISO to achieve a correct exposure.
When safety shift is not activated – the camera default – the shutter speed on the LCD and in the viewfinder will blink as a warning of incorrect exposure.
Tv/Av or ISO shift?
Safety shift is found on many digital EOS cameras, but only selected cameras introduced in the last 10-15 years have also had the ISO shift option.
Shutter priority mode is ideal when a photographer wants to control the movement of the subject with a specific shutter speed, so if the camera overrides that choice to get a correct exposure it probably means the picture is not as intended, but correctly exposed. In reality it’s better to allow safety shift to change ISO if needed.
Aperture priority mode is used when control of depth of field is critical, so again if the camera overrides the aperture the depth of field changes while the photo is correctly exposed. It is possibly better to use ISO shift here too.
ISO shift also impacts program mode
One last point is that if you choose to allow safety shift to change ISO values, this also makes safety shift work in program mode to override the chosen ISO value to try and avoid camera shake or subject blur from a slow shutter speed.
Advent Calendar of Tips
This year I’m writing a quick tip each day up until the 25th of December, here’s some of the others I have already posted.