Differences

[dkfish]

I don't know if its my camera or a common issue. When an image is captured using aperture priority and the info is reviewed, the shutter speed and aperture are displayed. If I set my manual exposure to that same setting, the image is always darker. Is this normal or a problem with my camera? I'm currently using a Canon T1i camera and the lens used makes no difference.

[OsmosisStudios]

Depends on how your ISO is set: if it's automatic in the AV/TV modes, but manual in M, then you'll see a difference based on what you set the ISO to in manual.

[ravncat]

Os has got it, Also make sure that you are not bracketing exposure between the two shots. The other possibility (though remote) is that you've some other setting that disappears on manual - Something like Nikon's D-lighting. (though I don't think D lighting will deactivate in manual)

[dkfish]

Sorry! I should have stated that the ISO did not change when I switched from AV to M. I have a bad tendency to keep my ISO on AUTO, so I'm mindful of where it is when I get to the processing stage. There is at least a 1 stop difference when I switch. I've tried this with both lens' I have with the same results. Lately I have been controlling the white balance manually also, so that is not a factor.

[OsmosisStudios]

Show us examples with EXIF, then.

[sk66]

Manual with all three settings the same (ISO/Aperture/SS)? I doubt it, there would be no difference in the exposures.
You said the ISO is set to auto...that allows it to change; it will not "change" itself in manual mode, but it will in Aperture priority.

Also, my camera (Nikon D7000) has all sorts of settings which change themselves when I change between modes....I HATE that.