One of the downsides of the Canon EOS system is that there is no way to manually set the aperture when the lens is off the camera as the lens iris is electronically controlled. However there is a workaround that we can use, it is a couple of extra steps, but it will do the job:
1) Put the Canon EOS lens on a Canon EOS body and turn the camera on
2) Put the camera into Manual Mode and set the aperture to what you want to shoot at (ie, f/8)
3) Set the shutter speed to something super long, 30 seconds for instance
4) Take a picture, and during the 30 second countdown, remove the lens from the camera
* This will not harm the lens or the camera, the moment you put the lens back on a Canon EOS camera, the aperture will return to normal
This should lock the lens temporarily to what you set it at, f/8 for instance. So when you put the lens on the adapter you will be preset to f/8. One problem though is that this will make the viewfinder dark and hard to see or focus, so its best to do it right before you are ready to shoot. The other problem is that anytime you want to shoot at anything other than f/8, you will need to go through those steps again, so it’s not a fast or elegant solution, but it will work.