A list member recently contacted me off-list asking whether it was possible to use a certain Pentax DA lens on Canon cameras. I don't want to reveal the list member, but I'm sure they won't mind me posting my answer as a general piece of info, I thought it might be of interest, and perhaps de-mistify things...
>Would it be possible to put a Pentax DA 16-45 on a Canon camera with an >adaptor? It is not feasible. Notice I didn't say that it's not possible, just not feasible. The reason why is that the Pentax DA 16-45mm lens has no aperture ring, so if it is mounted, via an adapter, onto a Canon EOS, there is simply no way to change the aperture. Simple as that. So the answer is that it is very possible to mount the DA 16-45 onto a Canon with an adapter, but unless someone take the lens apart and designs a way of manually changing aperture, then you will be stuck with the maximum aperture of f/4, and no way of stopping down. Electronic control via the camera is not possible, or maybe it is, but at many hundreds of times the cost of the lens. An expert could probably spend weeks designing and rebuilding the lens to allow EOS control - but that's daft, right ? ;-) >What about metering is that affected at all? When I put my K15mm or my A*85mm onto my 1D, it basically behaves like an old fashioned stop-down manual lens from the days of old. I switch the camera to manual (M), I put the lens on, and I focus on a scene, then I decide what aperture I want to shoot at, say f/8, so I stop the lens down to f8, and a half press of the shutter button and the camera literally looks at the amount of light coming through and tells me to set the shutter speed (say) to 1/250th sec. I do this buy using the main dial (small revolving dial on top near shutter release), moving it until 1/250th sec is shown in the viewfinder, and then I fire the shutter. The only other way to use one of these lenses with the Canon is this: instead of selecting manual (M), I select aperture priority automatic (AV). Now, when I use the K mount lens, I first of all focus on the scene, then I select the aperture I want to use (say f/8 again), and as I turn the aperture ring, and at the same time have a half-press on the shutter, the camera is changing the shutter speed to match, automatically. So as I come to rest the aperture ring at f/8, the camera has already set the shutter speed at the correct 1/250th sec, and away I go. It's as simple as that. Obviously, the camera has no electrical connection with the lens, so (for instance) it does not know what the focal length of the lens is, nor the aperture being used. All it knows is what the amount of light is coming through the lens. And if it knows that the ISO is set to (say) 200, and it knows the amount of light, then it can figure out the shutter speed required to expose the scene correctly. An external light meter can do this. A lot of photographers can do this by sight. So now you can see why it is vital when mounting a Pentax or Nikon or Contax lens on a Canon camera that the lens has an aperture ring to manually adjust the amount of light coming through. Cheers, Cotty ___/\__ || (O) | People, Places, Pastiche ||=====| http://www.cottysnaps.com _____________________________
