In shutter priority with Auto ISO, does the camera open the aperture before raising ISO?
Asked 1/23/2022
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2 answers
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On a Canon EOS 7D Mark II, if I use shutter priority (Tv) with Auto ISO enabled—say ISO 100–6400—and set 1/500s, what happens when the scene is too dark? Will the camera open the lens to its maximum aperture first and only then raise ISO, or does it raise ISO earlier? Is there any setting that changes this behavior? I'm asking specifically about normal Tv + Auto ISO operation, not Safety Shift changing the shutter speed.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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As far as I can tell (EOS 70D):
- There is an obvious attempt to maximize aperture before increasing ISO
- There is a setting to set the allowed auto-ISO range (ISO speed settings in the third menu).
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
4y ago
0
Generated from our catalog & community — verify before relying on it.
In Tv with Auto ISO, cameras generally try to use your chosen shutter speed and the lowest available ISO first, while adjusting aperture as needed. In practice, that usually means the lens opens up toward its maximum aperture before ISO climbs much, because the camera is still trying to hold shutter speed and keep ISO low.
Once the lens reaches its maximum aperture, ISO has to rise to maintain the selected shutter speed. So if it seems to jump to high ISO quickly, that may simply mean the lens is already wide open and there isn’t enough light.
The main control you usually have is the Auto ISO range: you can limit the maximum ISO the camera is allowed to use. Beyond that, there typically isn’t much direct control over the order of operations in Tv mode.
If you want more predictable control, many photographers prefer Av mode instead, since aperture often matters more creatively than ISO.
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