Quote Originally Posted by David A. Goldfarb
A pre-set aperture isn't fixed. It just doesn't have an auto-diaphragm. Usually there is one ring to set the aperture, and then one ring without click stops to check the focus wide open, then you can return it to the "pre-set" aperture chosen on the other ring.

In practice, for birds, you're usually going to be wide open. You need all the speed you can get.
Hi Wayne,

In the case of the Leica f6.8 400mm and 560mm lenses, you are working at the aperture set on the lens. There is no linkage with a lever on the camera (though there is a lever on the mentioned R Cameras that will work with the auto diagram Leica lenses) or on the lens. Works fine. As David says you are normally working wide open or near wide open. The aperture ring is far forward on the lens and the aperture information can not be read in camera (shutter info is however and with other lenses with auto diagram aperture and shutter speed info is available). However, the camera will still meter through the lens (with choice of spot and averaging metering) and meter readings can be set either manually or via Aperture prefered with all the Leica R cameras mentioned except the R 6 and R 6.2. The lens does stop down to f32 if needed (as I recall), but not normally recommended.