Originally posted by adwb Am I correct in assuming that it is pointless trying to adjust for a possible front or back error on a zoom lens?
Yes and no.
Please have a look at my
AF adjustment hints. I hope you'll find them useful.
Just as lenses (prime or zoom) are often impossible to optimise for all distances, zoom lenses are often impossible to optimise for all zoom settings.
That's why the new Sigma dock allows you to set AF adjustment values depending on both subject distance and zoom setting.
With just a one-value body AF adjustment, you'll have to find a compromise.
Originally posted by adwb I have a Sigma 24-70mm F2.8 IF EX DG HSM which depending on the apature varies in softness at any given focal length and therefore is impossible I assume to adjust.
Well, that the sharpness depends on the aperture is normal.
Excellent lenses peak at around f/4 but most need to be stopped down nearer to f/8 to reach their maximum sharpness.