Alan Chan wrote: >>If you are getting one, just make sure the focus is >>unchanged through the whole zoom range. If it did, >>it meant the lens had received some improper service.
Illinois Bill asks: >My lens has NEVER maintained focus throughout a >zoom. Are you sure it's supposed to do so? It may be vari-focus. The 1983 operation manual for this lens states "When the focal length of a zoom lens is changed by zooming, a slight shift of focus is unavoidable. To obtain accurate focus with zoom lenses, first set the lens at the desired focal length, then focus on the subject before actual shooting." This manual covers many of the classic SMC "A" zooms such as 24-50mm f/4, 28-135mm f/4, 35-70mm f/4, 35-105mm f/3.5, 70-210mm f/4, etc. In contrast, the version of the "K" lenses manual I have states "SMC Pentax zoom lenses maintain the focus setting even while zooming. It is a good practice to focus at the maximum focal length, i.e. with the largest possible image, and then zoom back to the desired focal length. This ensures maximum focusing accuracy." Lenses such as SMC P 45-125mm f/4 and 85-210mm f/3.5 are included among others. Mark Rofini
