[LensBeginner]

There actually is a way to use a manual lens in Av mode. Mount the lens as usual. Set the aperture ring so that the lens is set to fully stopped down. Press the lens release button and turn the lens as though removing it, but stop when the aperture blades are fully closed down. You can now set the aperture ring to the fstop of your choice and shoot in Av mode.

But it's not my fault if your lens falls off (though it shouldn't).

I'm using this trick, with a completely different motivation, when I shoot agains the sun in bright sunsets & I'm watching the scene through the optical viewfinder... staring at the sun is quite dangerous in itself even without the magnification added by telephoto lenses... darkening the viewfinder lessens the issue.