Right, he shouldn't use Program Mode.

The camera can't set the aperture--but it also knows it can't (it checks for the lens
electrical connection). To find out what Program mode will do in that situation,
one would have to look at the firmware source code (fat chance) or call Canon.
But why try to use a broken mode?

Aperture priority mode might work, provided he sets the aperture manually
and selects the same aperture in the camera. But basically, he'll be shooting in
manual mode--like every photographer who lived before the 1970s.

It's called a "manual transmission". That's what that stick between the seats
is for.

I have an old camera i got years ago - a Minolta when i first got interested in slr cameras because they took so much better pictures than my old Kodak!! i learned a little bit about f stops and shutter speed but didnt keep up with photography, I now have a Canon EOS 5i and am slowly learning about using it - my main interest is flowers and landscape cuz i dont get out much. But my question is can i use a wide angle lens and a 135mm lens on my Canon camera with some kind of adapter to fit it onto my Canon? And will it take good pictures. I have a 135mm for my Canon but would like a wide angle lens, but dont want to buy one. What i really want is a macro lens but that is on the back burner. I just wondered how the other lens would work on my Canon. I dont know the brand of the lenses but they are not Minolta lenses, will check later for the brand. Thanks for any info.
Betsy

It works as far as metering when set to P it uses the light coming in and sets the shutter. I didn't say that it set both but that it meters in the P setting.
Where many Nikon cameras just to tally shut down all metering if they don't have a largest lens.
This is why even a T5i would work great on even odd accessories like old bellows, telescopes and other even non Canon accessories and still meter while many Nikon models just shut down all metering. So the comment that there is no metering with the T5i is incorrect and is a Nikon failure should not be extrapolated to the Canon line of cameras in a blind blanket statement as was done.

Sorry, this makes no sense. Setting camera for P mode does not accomplish anything, unless you set the aperture on the lens (which you can't do with Canon AF lenses), and also set it on the body (which you can't do since camera does not "see" the lens).
I'm looking at my Canon 40D with a Fotodiox adapter and Canon FD lens on it.
In P mode and Av mode there is no shutter speed control.
Tv and M modes are the only way to control aperture (on the lens) and shutter speed.
Thus, non-electrical-contact adapter with an FD lens is MANUAL MODE only.

Right, he shouldn't use Program Mode.

The camera can't set the aperture--but it also knows it can't (it checks for the lens
electrical connection). To find out what Program mode will do in that situation,
one would have to look at the firmware source code (fat chance) or call Canon.
But why try to use a broken mode?

Aperture priority mode might work, provided he sets the aperture manually
and selects the same aperture in the camera. But basically, he'll be shooting in
manual mode--like every photographer who lived before the 1970s.

It's called a "manual transmission". That's what that stick between the seats
is for.

First off, if the lens is 28 or 35 mm focal length, it won't be a wide angle on your Rebel 5i. Because of the crop factor, 1.6x, it will give you the effect of a 45 or 55mm lens. This is "normal", not wide angle. Plus, the adapter would also increase the crop factor somewhat.

Program mode will accomplish nothing. Neither will aperture priority. You need to use Manual mode, setting the aperture on the lens where desired and doing stop-down metering, adjusting shutter speed to give correct exposure. You can use different equivalent f/stop-shutter speed combinations to get more or less depth of field, slower or faster speed for stopping action, if needed......