Originally posted by Sicariphus 
So you *could* set it for wide open and then control the aperture with the lens's ring... Hm. This is really a huge help, thanks so much.
Not really. If you tell your body the lens is wide open, the body will hold the lens wide open no matter what you set on the lens' aperture ring. I am guessing you are picturing the mechanics of aperture control exactly backwards of how it works.
Take your lens off and look through it from the back. It will be fully stopped down. So how does it become fully open for focusing? The body holds it wide open against the spring. So if you set the aperture on your body to wide open, the lens won't close down at all during the exposure, no matter what you set on the lens' aperture ring.
Now lets say you set the body to the lens' minimum aperture, but you set the lens to some mid-range number. When you trip the shutter, the body will totally release the lens' aperture lever. However, the aperture will stop mid-way to wherever you set the lens' aperture. But hold on.... YOU told the camera the lens would be fully stopped down, so your metering is based on this same value. But if the aperture stopped mid-way because of how the lens was set, you will have a picture overexposed by however many stops there are between fully stopped down and where you put the lens.
Leaving the lens' aperture ring at its minimum value and controlling the aperture from the body MIGHT work, I've never tried it. But the other way around will decidedly not work. My recommendation is to look at what the camera's metering says, then manually set the lens' aperture ring to the same f/stop. At worst you will slightly overexpose doing it this way. Mechanically it will not be possible to underexpose.
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