Why does my Canon EOS 700D choose a much slower shutter speed with flash in Av than in P mode?
Asked 2/20/2015
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On my Canon EOS 700D, I tested the built-in flash in both P and Av mode. In P mode, the camera chose f/5.6 and gave me a normal-looking result. Then I switched to Av, set the same aperture (f/5.6), left ISO and flash exposure compensation unchanged, and got a very similar-looking photo and histogram — but the camera selected a much longer shutter speed.
This is repeatable. Without flash, P and Av give similar exposures, so I’m trying to understand why adding flash makes the shutter speed behavior so different between P and Av (and possibly Tv). If aperture, ISO, and flash settings are the same, why isn’t the shutter speed similar too?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
9
In Av (and Tv) mode, flash is not assumed to be primary light source, so camera will choose exposure to match metered ambient light.
In P mode, however, the camera tries to ensure exposure time is quick enough for handheld shooting, and thus will happily expose for the flash-illuminated subject, ignoring the lack of ambient light.
To put it in flash terms, Av/Tv modes consider the flash to be a fill light; in P mode, the key light.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
11y ago
0
Generated from our catalog & community — verify before relying on it.
Because with flash, P and Av are solving different exposure problems.
On Canon DSLRs, Av/Tv typically treat flash as fill light. The camera meters the ambient light and chooses a shutter speed to expose the background, often using slow sync. The flash then lights the subject.
In P mode, the camera more often treats the flash as the main light and chooses a safer, hand-holdable shutter speed instead of trying to fully expose the ambient scene.
Also, shutter speed affects ambient exposure much more than flash exposure. A flash burst is extremely brief, so once it fires during the exposure, leaving the shutter open longer does not add more flash light — it mainly adds more ambient light.
So with the same aperture and ISO:
- P mode: faster shutter, flash does most of the work
- Av mode: slower shutter, ambient is exposed more, flash fills the subject
That’s why the photos can look similar overall while the shutter speeds differ dramatically.
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UniqueBot
AI11y ago
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