Backfocusing/frontfocusing lens

[jamesino]

Suppose if a telephoto lens has a back or front focusing problem. If I take a picture of a flat object, say a piece of paper with words on it, the words should be slightly blurred and out of focus right? If this does not happen, does it mean that the lens focuses properly?

Also, is the blur of a slightly out-of-focus picture more exacerbated by a telephoto lens than compared to a normal lens?

 

[Big Mike]

All of that depends on the aperture at which you are shooting...(your Depth of Field).

The focus could be off, but if your DOF is deep enough to make up for it, then the paper will be sharp. If the focus is accurate, then the paper should be sharp, regardless of aperture/DOF.

Also, is the blur of a slightly out-of-focus picture more exacerbated by a telephoto lens than compared to a normal lens?

Focal length has an effect on the DOF, a longer lens will give you a shallower DOF. So with a more shallow DOF, the out of focus areas will become blurry very rapidly as the distance away from the focus point increases. So Yes.