I have a very hard time believing that you can adjust a zoom lens by changing the back focus distance by 5 mm or whatever. Lens focus tends to change a little when zooming and a little when stopping down the aperture. So a fixed number probably isn't always going to work.
The sensor in the camera, be it film or digital, does not move w.r.t. the lens when zooming. So zoom lenses have to be designed to focus the image at exactly the point (plane) where the sensor lies, regardless of zoom length.
So, yes, a tiny back-focus adjustment can and does work. That's how Canon themselves do it when shipping the lot back for service -- it's just an offset value that gets programmed into the lens and/or camera firmware.
The issue here is whether or not they continue to make this adjustment user-accessible or not.
Cheers
Previous Topic
Index
