I'm using my 600D in Av mode and can't seem to fathom the FEL function.

Backtracking a bit, with the flash closed, I can point the camera to a light area, press the "*" button, point to a dark area and fire. The camera now bases the exposure time on the values for the light area. This is all good and as expected.

If I now open the flash and do the same, the behaviour seems different. With the flash open, when I press the "*" button, the camera test fires the flash and presumably calculates the flash power based on that. I now point to the dark area with the FEL locked in and fire. However, in this mode, even thought the flash power is presumably locked, I can't seem to lock in the exposure. The exposure time seems to be calculated automatically when I press the shutter resulting in an over-exposed shot.

Is there a way to lock in both the flash power and exposure time?


Is there a way to lock in both the flash power and exposure time?

Not easily with the 600D in Av exposure mode.

Canon cameras (and most other DSLRs) are programmed to interpret that [flash + Av exposure mode] means the user wishes to use slow shutter sync, often referred to as dragging the shutter. The camera assumes you want the ambient lighting to be primary and the flash to be used as fill. The ambient lighting is regulated by the shutter speed, the flash illumination is regulated by the TTL routine and the user selected Flash Exposure Compensation (FEC).

You can restrict the shutter speed when using Av exposure mode to either 1/200 second fixed or 1/200-1/60 second auto via the Custom Functions in the third Setup (yellow) menu tab---> Custom Functions (C.Fn) ---> C.FN I: Exposure ---> C.Fn-3 Flash sync. speed in Av (Aperture Priority AE) mode. The default setting is Auto which allows a range from 1/200 second (your camera's flash sync speed) to as long as 30 seconds. It's all on page 252 of your EOS 600D Instruction Manual.

Some of the upper-tier EOS camera models with more physical control buttons and customizable mapping of those buttons allow the user to designate different buttons for Exposure Lock (EL) and Flash Exposure Lock (FEL), but none of the xx0D/Rebel or xx00D/Rebel models have such capability.

If you're shooting in low light and want to use the lens at its widest aperture setting you can shoot in P exposure mode. The shutter time will be restricted to 1/60 second at the slowest. If 1/60 second and the lens' widest aperture will still underexpose the ambient light then the flash power will be adjusted to properly expouse the subject (designated by the AF system).

Beyond that to precisely control the shutter time you need to use M exposure mode. E-TTL will still set the flash power automatically based on the user selected FEC and will attempt to properly expose the subject. How the ambient and flash are balanced will be left totally to the aperture, shutter time, and ISO you select.

  • An excellent explanation; thank you very much. So it looks like my best option is manual mode but I just wanted to be sure I wasn't missing a trick. – Geoff R Feb 5 '17 at 21:11

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