Did you disable high speed sync without realising?

This is a cautionary tale about how experimenting with custom functions can significantly change how your pictures look. In this case I was getting very overexposed pictures in aperture priority (Av) when using a flash and high speed sync (HSS).

Setting high speed sync on your flash allows you or your camera to select a shutter speed faster than your cameras sync speed. For much of the year in the UK, high speed sync is not needed since the ambient part of the exposure can easily be handled with a shutter speed less than 1/250s or maybe 1/300s, 1/200s – depending on camera. However with the weather being very bright of late you might want to check out what you have set on your camera over the winter.

Unexpected behaviour to catch you out…

I was doing a 1-2-1 training with a photographer and I was explaining how to use flash to fill in shadows and still keep a shallow depth of field when the ambient light is bright sunlight. The photographers camera simply would not do high speed sync if it was in Av mode. With the camera in Av mode with a selected aperture of f/2.0 and ISO 200, the camera selected 1/2000s for the ambient, but adding the flash set to high speed sync the pictures were all overexposed. With the ambient exposure being 1/2000s at f/2.0 at ISO 200 I expected high speed sync to work, but it wouldn’t. When I was reviewing the pictures on the camera LCD that I could see that the shutter speed was only 1/250s. Switching the camera to manual mode and I could set 1/2000s at f/2.0 with no problems and the high speed sync exposure worked as expected, but it just wouldn’t select faster shutter speeds in Av. The reason was custom function, C.Fn I-7 which determines the shutter speed in Av with flash.


C.Fn I-7

Limits shutter speed in Av mode with flash AND disables high speed sync

The C.Fn was set to limit the range of shutter speeds in Av with flash and was set to 1: 1/250-1/60sec. auto. The photographer had set this so that when in Av their shutter speed would be sufficient to avoid camera shake since normally in Av mode the shutter speed with flash runs from the sync speed to 30s – yes thirty seconds. However the C.Fn. doesn’t care about high speed sync, so when pictures were taken with the flash the camera shutter speed was 1/250s making the scene three stops overexposed by the ambient light.

Depending on the camera model the exact number of the C.Fn may change:

  • C.Fn I-15 on the EOS-1D Mark IV
  • C.Fn I-7 on the EOS 60D, EOS 7D and EOS 5D Mark II
  • C.Fn I-3 on the EOS 600D


About the author

Full-time photo tutor and photographer. I love to share my knowledge and skills to make photos, videos and teach others. I write books and articles for photo magazines and I always have at least one Speedlite flash in my camera bag