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Assuming the camera is in "auto-iso" mode with a range of say 100-6400, and set to shutter priority with a shutter speed of 1/500, when the meter detects that the scene is "too dark", will it increase ISO first, or go to maximum aperture before increasing ISO? And is there any way to influence this behaviour?

I stuck with film longer than most, so "Auto ISO" is still a little unnerving for me. My impression is that the camera is a bit hasty in jumping to ISO 6400, but I have no scientific basis for this claim.

Camera is a 7D Mk2, in case it makes any difference. I don't think I'm talking about the "safety shift" feature, which AIUI will reduce or increase the set shutter speed in Tv mode.

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  • What happens when making pictures with your camera? Jan 23 at 19:14
  • well, I thought it was bumping ISO before going for max aperture, but I don't have any proper evidence for this and the answers below suggest that this is incorrect anyway In England in January it isn't really practical to test in "strong light" Jan 23 at 22:53
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    In low light, prioritize a slow shutter speed to make test exposures. Then look at the EXIF data without worrying about sharpness, composition, etc. Photography is an experimental activity. Jan 24 at 0:15

2 Answers 2

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With every camera I've used (Canon, Nikon, etc, etc), what it wants first is the minimum/desired ISO you set and the SS you set, while adjusting the aperture. So it is adjusting down to the desired/minimum ISO first.

But unless the light is extremely strong, the minimum ISO setting is set quite high, or the SS is extremely long, the camera generally won't be able to reach the minimum ISO... so in shutter priority the camera will seem to adjust the aperture to maximum first; especially with slower lenses. That's why I don't use the shutter priority mode (to me, the aperture setting is generally more important than ISO). And no, you cannot really change this behavior.

Instead, most cameras (like your 7Dii) are designed to work this way better when in aperture priority mode (Av). In this mode the camera also wants to use the ISO setting you selected first, but there are also more options to control the behavior/sequence. In the menu's associated with auto ISO there are minimum/maximum ISO settings and a minimum shutter speed setting (but no aperture options). And when both are set the camera will typically first use the minimum SS menu value along with your chosen aperture, and it varies the ISO (because it can't get to minimum). If the ISO does reach the minimum setting the SS then increases above the minimum value set. And if the ISO reaches the maximum setting the camera will then reduce the SS below the value set. Basically it makes the same choices I would make, in the same sequence and at the same time. And I can always override it by simply changing my chosen aperture or the desired/minimum ISO (changing the minimum SS setting requires going into the menu). Often it may be better to set the minimum ISO to a higher value such as 400 or 800; or to set your minimum SS to more of an "optimal" speed rather than the actual minimum you think you can manage/tolerate (both have about the same effect).

Another good option is manual mode with auto ISO. I'll use this when I care about a specific aperture and SS equally; like when forcing a long SS for panning.

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  • Thanks for detailed explanation, context is athletics and my lens isn't very fast, compounded by a particularly dull day today. It also occurs to me that since the max aperture isn't constant for my telephoto zoom, I'm not 100% sure whether it is at the max or not when looking at the EXIF data afterwards. Jan 23 at 23:00
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    If EXIF reports that the ISO is not at the minimum you set, then the aperture is at the maximum for that zoom position. Jan 24 at 13:24
  • @GavinCampbell You can cross reference the focal length value reported in the EXIF info with the Av and know if the Av is at max or not for that FL. For most Canon variable aperture zoom lenses, Bryan Carnathan's reviews at The-Digital-Picture include the FL range for each Av in 1/3 stop increments. Or you can just test the lens yourself in Manual exposure mode by setting the Av to the lens' max Av at widest FL and then zooming until the camera reports the next higher f-number in the viewfinder.
    – Michael C
    Jan 26 at 3:19
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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).
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  • This is what I wanted to hear, I think; I'd rather it increased the ISO and put up with the shallow DOF than have everything in focus and photos that look like the Zapruder tapes Jan 23 at 22:56
  • obv I meant the opposite; I'd rather it increased the aperture to the max and kept the ISO as low as it can Jan 23 at 23:17

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