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I took a number of ambient light portraits on Friday and I'm a little disappointed by the amount of motion blur (not camera shake) on parts of the subject. Altering the lighting was not an option.

I was using a 50mm f/1.8 lens and shooting at f/1.8 in aperture priority mode. My Canon EOS 5D Mk II was frequently setting the shutter speed to 1/40 or 1/50 of a second, and this was quite often not fast enough to stop blurring of the subject. Even the gentle swinging of an earring was causing blurring at 1/40, and small movements in the head or hands would make an otherwise nice shot disappointing.

My question is: how do you force the camera to use the desired aperture, and also specify a shutter speed of at least (say) 1/125 of a second, forcing the camera to use a higher ISO (if necessary) in exchange for a faster shutter speed?

[In case it's of interest, a typical exposure from this scene was f/1.8 for 1/40 at ISO 200.]

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With the Canon EOS 5D mark II, the best way to accomplish what you want is to set the exposure mode to Tv, the shutter speed to the desired setting (i.e. 1/125 sec.), and the ISO to Auto. As long as the light is fairly dim, the camera will first open the aperture to the lens' maximum and then start raising the ISO. This method will only work if you are happy using the lens' maximum aperture.

There is no way on the 5DII to set a minimum shutter speed when in Av exposure mode. If you select Auto ISO and any aperture, the camera will lower the shutter speed to around 1/focal length before it starts raising the ISO. Using P exposure mode will get basically the same results as Av mode with the aperture wide open in dim light.

Experimenting in Av mode with my 5DII by gradually reducing the amount of light entering the lens:

  • With an EF 50mm f/1.4 lens the shutter speed has to drop to 1/40 sec. before the ISO starts going up. Once the ISO reaches 3200, then the shutter speed is lengthened even more.
  • With the EF 24-105mm f/4 L IS lens at 105mm the shutter speed stops going lower at 1/125 sec until the ISO is up to 3200 and then the shutter speed will drop as far as necessary (over 1 sec with a lens cap on).
  • With the EF 24-105mm f/4 L IS lens at 24mm the shutter speed will drop to 1/20 sec. before the ISO starts to go up. Once maxed at ISO 3200, the shutter will then drop even slower.

In M exposure mode, when ISO is set to Auto it stays at ISO 400 and uses the Tv and Av you have manually set, regardless of the amount of light entering the lens. If you are using an E-TTL flash in M mode, the flash power will be automatically adjusted to properly expose the scene.

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Useful observations, and thank you for shaming me by pointing out how easily I could have tested this for myself. –  user1515834 Jun 27 '13 at 13:22
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The Canon 5D MkIII and the Canon 6D for example have a "Min. shutter spd." option that is used during P and Av shooting modes. It can be found in the Menu under "ISO speed settings". The ISO speed is automatically shifted so that the value is not lower than the specified shutter speed. You can specify these options on either camera: Auto, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 0"5, 1".

Screenshot from the Canon 5D MkIII Manual Page 128: min shutter speed

I am not sure if the Canon 5D MkII has this option or not, but I couldn't find it in the Manual so I am guessing not. Another option would be to use "Auto ISO". In Av shooting mode on the 5D MkII it will automatically select anywhere from ISO 100-3200. If you want to force it to select a higher shutter speed, I would recommend using Tv mode with Auto ISO(assuming you can't select the Minimum shutter speed in Av on your model).

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The first Canon body with the option to set a minimum Tv when in Av mode seems to be the 5DIII. The 1D X, announced before but released after the 5DIII, also has this feature. Neither the 1DIV, the 5DII, nor the 7D do. –  Michael Clark Jun 25 '13 at 7:45
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The 5DMKII has this feature with Magic Lantern provided its the latest version updated to shoot raw video

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Does the added functionality also apply when shooting still photos? –  Michael Clark Aug 14 '13 at 7:43
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