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I am using a Canon 5D MKIII. I take a lot of low-light fotos and want to achieve low DOF while keeping the photos sharp with fast-moving subjects (musicians), using a 5mm 1.4 lens. So far, I always did this with TV mode and automatic ISO. I stopped down 1 step to get photos that have the same brightness as what I actually see.

Now the issue there is that sometimes, there is enough light so that the camera switches to a higher aperture and I lose the low DOF.

I thought if I switch to manual mode and fix the settings at 160th/F1.4, I can avoid that. But then, the automatic ISO brightens up the photo again and takes the images at a unnecessary high ISO.

How can I avoid that? How can I fix the settings in speed and aperture but tell the camera not to try to make the picture brighter than reality by choosing a too high ISO?

I do not want to use manual ISO on top of the other settings fixed since the light changes quite quickly depending on my position and I am not fast enough to change the ISO.

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With the latest firmware version the EOS 1D X now has the ability to use Exposure Compensation when Auto ISO is selected in M mode. I'm not sure if this capability has been added to the 5D3 as well. If not it may be included in a future firmware release. –  Michael Clark Jun 7 at 19:01
    
So your problem is you want to fix your aperture? Why not just use Av mode which does exactly that, and set your auto iso to only kick in at 1/160? –  ElendilTheTall Jun 8 at 14:16
    
@ElendilTheTall How would I do that? –  uncovery Jun 8 at 23:37
    
I've no idea, I don't use Canon. On Nikons it is a menu option. –  ElendilTheTall Jun 9 at 7:15

2 Answers 2

Exposure compensation in manual mode with Auto ISO is not possible with the 5d3, or for that matter, any other Canon camera except the 1DX (according to http://photo.net/canon-eos-digital-camera-forum/00bWQo ). This is called a "known annoyance" of Canon cameras there, I guess I would agree.

However, you could go the route of the Magic Lantern firmware extension, they seem to support this: http://www.magiclantern.fm/forum/index.php?topic=10573.0 I haven't worked with this yet though and don't know how this is used within ML / how the usability is for daily usage.

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Unless it had been included in a recent firmware update for the EOS 5D Mark III, the only Canon model capable of applying Exposure Compensation to Auto ISO in M exposure mode is the EOS 1D X. That capability was not included in the original firmware but was added by a relatively recent update.

Have you tried using the different metering modes available? If you are using evaluative or center weighted averaging the camera is brightening the exposure because it is trying to expose the entire frame, including the dark areas away from your primary subject, properly. Partial and spot metering will only use the center of the frame to calculate exposure. You may need to frame wide and then crop later to get the composition you desire but this method also has the advantage that focus performance in low light is usually better near the center than at the edges of the frame.

Although it isn't the ideal many Canon shooters (including myself) desire, here's what I do in similar situations.

1) I shoot in Av exposure mode, put the ISO to the highest setting I am comfortable with (based on the intended use of the images) and keep an eye on the Tv in the viewfinder as I shoot. If the Tv gets too slow I use the Quick Control Dial (the large dial on the back of the camera) to adjust Exposure Compensation

2) I shoot in M exposure mode, select my Tv, Av, and ISO and live with the occasional over/underexposed frame. RAW files are very forgiving with regard to exposure: I often increase or decrease the exposure by up to two stops in post production.

Face it, the situation described in the question is one of the most challenging a photographer will deal with. It is a little unrealistic to expect every frame to be perfectly exposed. Even when using a camera with EC applied in M mode with Auto ISO selected there will be times when the lighting on stage changes drastically in the split second between metering and exposure.

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