The Canon EOS system bodies have no means to do this. The EOS lens interface is fully electronic and Canon EOS EF and EF-S series lenses are stopped down via electrical signals from the camera.
Others can be programmed with a focal length and aperture using the camera to do the programming. Some can be focus calibrated. Make sure you know what you are getting if you buy one.
In addition you don't get any sort of automatic iris operation. Stop Down Metering In most SLRs and DSLR s, focusing is done at full aperture, and if you set the aperture to, say, f11, it remains fully open until just before exposure, then it.
This gives a brighter viewfinder image and makes focusing easier and more accurate. For almost all manual focus lenses, this stopping down during exposure is accomplished mechanically via a lever which is moved as the cameras mirror flips up.
Since a mechanically adapted manual focus lens has no intrinsic electrical coupling to the body, the body doesn't know it's there and so with most current EOS bodies the LCD display will readout an aperture of "00".
With this in mind, you can be more creative with your shots. For example, you can photograph a beach landscape keeping the aperture high (for example f/22) so everything is in focus, yet at the same time set a slow shutter speed to create dreamy.
It is possible to get accurate focus, but you need a properly aligned viewfinder screen and good eyesight. A viewfinder magnifier is often helpful. Alternatively, most current DSLR s have a live view capability, which displays the actual image from the sensor on the rear.
The camera will then display that aperture, but you can safely ignore it. The camera will measure the amount of light actually coming through the lens and in aperture priority will calculate the appropriate shutter speed.
Either that or do not lock the adapter completely in the EOS mount. If you don't fully rotate it so that it "clicks" into place it won't trip the switch. Of course the adapter is them not fully locked onto the camera, so you need.
It wants to see an EOS lens on there. If you have a manual focus lens on an adapter with a focus confirmation chip built in, things should be OK. The lens then tells the multiplier and camera that it's OK and it's a compatible.