I have purchased a used canon 5d Mark II. In AV mode the ISO does not seem to work.

[cwilliamson]

For some reason I get the same image in AV mode in ISO 100 as I get in ISO 6400.  Is there something I'm missing?   I usually use a Canon 70D  and have no issues....

[TCampbell]

The camera will vary the shutter speed in an attempt to get the same exposure.  

e.g. if you are in Av mode and you dial in ISO 100 and f/4 and the meter says that an exposure at ISO 100, f/4 and 1/60th sec would be a "correct" exposure, and then you then change the ISO to 6400 ((6 stops higher) then the camera should compensate by running the shutter speed 6 stops shorter (so instead of 1/60th it should go to 1/4000 sec.)

But the result should be that you see an "equivalent" exposure.  You should notice more noise at ISO 6400 (the 5D II is pretty good even at high ISO but if you inspect the shadow regions at 100% zoom you should see noticeably more noise in the ISO 6400 version.)

If you don't zoom in... the resampling on your screen (viewing images in smaller size) can hide a lot of that noise so you might think that they're the same.

If you'd like... post samples of the images along with the EXIF (shooting information) data.

[cwilliamson]

The F stop is the same though I'm was shooting at F2.8, so usually a higher ISO would let more light in right?    That's the way my other camera worked.  

[BurnUnit]

Actually, it sounds like the camera is working properly. AV mode is aperture priority... You choose the aperture and the camera adjusts the shutter speed to maintain the correct exposure. Depending on the ISO you've chosen and the lighting conditions the shutter speed will vary within the camera's range of available speeds.

[cwilliamson]

Ok thanks, I'll play somemore.  Thanks so much!!  🙂

[RobertTheFat]

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@cwilliamson wrote:

Ok thanks, I'll play somemore.  Thanks so much!!  🙂

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I'm as much of a tinkerer as the next guy, and I admit that I often learn best by trial and error. But the fact remains that "playing" can be an inefficient substitute for reading the manual.

[TCampbell]

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@cwilliamson wrote:

The F stop is the same though I'm was shooting at F2.8, so usually a higher ISO would let more light in right?    That's the way my other camera worked.  

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While it's true that a larger aperture allows more light to pass through the lens opening in a given amount of time... in Av mode (Aperture value mode aka "aperture priority" mode) the camera still seeks to balance out the exposure values to get a "correct" exposure.

To make an exposure, you can adjust the ISO and the Aperture size and the Shutter speed.  All three, when combined, allow in some amount of "total" light to make the exposure.  So if you increase the aperture size to allow in twice as much light, the camera (in Av mode) will reduce the total amount of time that the shutter remains open -- it'll cut it in half.  

This means you'll end up getting twice as much light... but for only half as long.  The result is that the same "total" amount of light will have been collected.

Here's a video that may be helpful:

If you were to use MANUAL exposure mode and changed ONLY the aperture value then you'd see an obvious difference in the image you captured.  But when you use either Av or Tv mode (or Program mode) the camera automatically attempts to get a correct exposure.  So if you increase one value, the camera will automatically decrease a compensating value to get the same total amount of light.  

This allows you to be creative with your photos.  For example... if you shoot a waterfall using a large aperture but a fast shutter speed then you can freeze every drop of water in your image... but if you shoot the same waterfall using a tiny aperture value but a long shutter exposure then you'll get a blurred waterfall -- helping to "imply a sense of motion" into an otherwise "still" photograph.

The other effect you get by changing the specific exposure values but getting the same total amount of light is to alter the depth of field -- the range at which subjects seem to be reasonably well focused.  Focus your camera for a subject ... say 25' away... and you'll find that if you use a high aperture (f-stop) such as f/16 (which is a small opening), objects which are 20' away... or even 30' away (even though the camera is focused to 25') actually seem to be focused rather well.  But take that same shot at f/2.8 and you'll notice the 25' distance is nicely focused but the 20' and 30' distances are defeinitely not in focus (and often times putting the background or close foreground deliberately out of focus with a tack-sharp subject can be a beautiful creative photo.)

[ebiggs1]

" Is there something I'm missing?"

Yes there is.  It is EV or 'exposure value'.  The EV you are making the camera do is the same as long as you have fixed the aperture.  It would be exactly the same thing if you had chosen Tv.  You have told the 1D Mk II to use a certian EV and it is trying to do just that.  Your 70D works the same exact way, all camers do.