Auto v Manual Focus Exposure Differences

[David]

Hi All - Apologies if this topic has been covered in another post or tutorial, but having searched I cannot find anything exactly relevant. I have been testing my Canon 40D with a Canon 50mm 1.4 lens to see if I can get better focus using manual focus rather than auto focus. I have the camera on a tripod approximately 4 feet from the subject (a calibrated ruler against a "white" background). I am using primarily AV mode, and to further reduce possible camera movement I've set the mirror to lock up and use a remote cable. I take a shot with auto focus and a single point setting on the target area, and then carefully change to manual focus and take another shot. The images ought to be the same at this point, I would have thought. However, that taken with manual focus is approximately 1/3 stop darker than that with autofocus. The lighting has not changed, nor the camera position. I have checked at different apertures, with different lenses, with different white balance, with different focus via manual focus and Live View, but the result is always the same: the manual focus image is always about 1/3 stop darker. In addition, some small differences appear in the histogram.

Any comments as to why this should be??

PS I can't do better at focusing than the autofocus system on this test.

David

[McQ]

Have you checked the EXIF information in the two photos to see whether the camera is changing metering modes? The camera might automatically switch to center-weighted or some other mode when it detects manual focus, for example. The metering center point is often linked to the AF point, so if there's no longer an AF point I could see how the exposure might change...

[David]

Hi McQ - Thanks for the comment, I've been following it up. After countless further shots using different metering settings I've come to the conclusion that not only are focusing and metering involved in this, but also the dynamic range of the test shot. Originally, my test image had a modest dynamic range across the centre of the histogram. I noticed that the file sizes of the manual focusing shots were always slightly less than those of the autofocus shots (eg 9.2 MB cf 9.8 MB). I changed the test image to one where there was a completely full dynamic range and the problem disappeared. The file sizes were then about 13 MB. There must be some subtle interplay between focus, metering and range involved, but I'm not sure it is worth pursuing further.

Thanks again for your help.

David