Lens Interchangeability Question

[Roxie2401]

Equipment:  Canon 40D; Canon EF-S 17-85 f/4-5.6 IS USM; Canon EF 70-300 f/4-5.6 IS USM.

I recently was taking photos of a band in an outdoor amphetheater and started out with the 70-300 and later switched to the 17-85.  I had the camera set for Av using f/11 and the shutter speed was consistent at 1/200.

When I looked the photos later they didn't look quite the same relative to exposure.  Then I realized I had the camera set for Auto ISO and the 70-300 shots were using ISO of 800 and the 17-85 were ISO 400.

So, I'm curious - was the change in ISO due to the 17-85 "seeing" more available light or something else?  It wasn't the "grain" on the images that seemed different, but more the exposure - but with the same f-stop and shutter - well, this is where I could use some professional input.  What would cause the images to appear different?

Also, if I had set the camera ISO to a fixed value, would I have been able to just "swap" lenses (since they are both the same f-stop range/speed) and keep the same Av and Shutter speed or would that have changed, too?

Thanks

[KeithB]

It could be related to exposure.  I assume that you had the 17-85 set to a wider angle than the 70-300.  If so the auto exposure would see a different scene and set a different EV value.  I am not sure why it adjusted the ISO and not the shutter speed, but they should be equivalent.

You could try some test shots with the lenses set to the same focal length (70-85 mm) and see if they give the same exposure with the same scene. 

[Roxie2401]

There must be some internal logic that the camera uses that says, "try ISO, then try Shutter Speed, etc."

I'm still curious with both lenses the same speed, what caused the change - it is probably what you mentioned - the 17-85 was set to a greater focal area, catching more light.

Thanks for the input.

[Bob Atkins]

The ISO issue might be related to focal length. I don't know if the camera tries to keep shutter speeds higher at longer focal lengths in order to increase the chance of sharp images, but it might. If it does then it might tend to select a higher ISO speed for longer focal lengths.