Why does Canon Av/Tv choose slow shutter speeds with flash, even when aperture stays the same?
Asked 12/28/2012
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I’m using a Canon 60D with an EF-S 17-55mm f/2.8 and a Speedlite 430EX II in E-TTL. When I compare the camera’s full Auto mode to Av mode indoors, the results are very different. For example, Auto gave me about 1/60 sec at f/2.8 and ISO 1600, while Av chose about 1/6 sec at f/2.8 and ISO 1600. To get 1/60 sec in Av, I would need to raise ISO a lot.
Why does the camera behave this way with flash? If aperture and ISO are the same, why doesn’t Av choose a similar shutter speed? Is this normal Canon flash behavior, and is there a better mode to use if I want more predictable indoor flash exposures?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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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.
Originally by user1293. Source · Licensed CC BY-SA 4.0
user1293
13y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—this is normal behavior. On Canon bodies, Av/Tv with E-TTL flash often meter the ambient light first and treat the flash mostly as fill. In a dim room, Av may choose a slow shutter speed to expose the background, then add only enough flash for the subject. That’s why you can see something like 1/6 sec in Av instead of the 1/60 sec you saw in full Auto.
Full Auto is not trying to meter the scene the same way; it is trying to make a generally usable picture, so it may pick a more hand-holdable shutter speed and use flash differently.
Also, E-TTL flash output is not fixed—it changes shot to shot based on the camera’s metering and subject evaluation.
If you want predictable indoor flash results, use Manual exposure mode on the camera. Set a shutter speed near your camera’s flash sync speed (or any speed you want below it), choose your aperture and ISO, and let E-TTL handle flash power—or set the flash to manual for fully repeatable results.
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