Why does the ISO change with the shooting mode when the aperture and shutter speed are constant and using a flash?

Question

I have a canon 60d, Canon EF-S 17-55mm f/2.8 IS USM lens, and a Canon Speedlite 430EX II TTL flash. I like to use the manual modes, such as Av and Tv but I am having some difficulties when using the flash. I notice that when I use the flash in auto mode I get much better ISO performance. For example this morning I performed a few test shots of my remote control for the TV. In Auto mode the camera shot 1/60 at 2.8 using 1600 ISO. When I put the camera in Av mode I was only able to get 1/6 at 2.8 using 1600 ISO. If i wanted around 1/60 shutter I would have to put the ISO up to 1/6400. I am unsure why this is true. Does anyone have any ideas or suggestions?

Thank you.

Answer 1

Canon uses a stupid, stupid, stupid, way of calculating how intense the flash should be in Av or Tv mode. Basically, the camera meters for ambient light and decides to use the minimum amount of flash required to light the scene, i.e., the flash always works in fill mode. Your flash intensity settings are not honoured in these modes. This leads to very low shutter speeds when using the flash with Av or Tv modes.

When using the flash, I prefer to just use the M mode and shoot at the flash sync speed (1/200 or 1/250 sec, depending on the camera) and adjust the flash intensity accordingly. This habit comes from being a photographer who has very shaky hands. You can of course use a smaller amount of flash and slow down the shutter speed if you need a particular effect or just need the flash to light the frame more brightly.

PS: There is usually a menu item buried deep inside the custom functions of the camera that allows you to specify that the camera always shoot at the flash sync speed in Av and Tv modes. On my 30D, it is called "Flash sync. speed in Av mode" and is custom function number 3.

Answer 2

In AV all the camera is trying to do is get the exposure correct (18% grey across your metering area) whereas Auto mode is trying to take a nice picture. Either your metering area is different (because you specified it or that's part of the auto mode magic) or part of auto mode has identified what the subject is (perhaps based on focus points) and how to better light it.

Either way, you have too many variables in your test. ETTL is not a constant, but varies based on the camera magic at the time you press the shutter. Try setting your flash to manual and repeat your tests and see if you can reproduce the auto mode settings in Av that way.

Also, Av has stepless shutter speeds which means that you can get weird shutter speeds of 1/192nd of a second and things that you can't specify manually because you can only go in 1/2 or 1/3 stop increments. You can get close, but it won't be exact.