Mike: > Suppose I have a variable aperture zoom lens, say 28-80mm > f3.5-f5.6 If I set the zoom at 28mm, and set the aperture wide > open, it should be f3.5. If I then zoom to 80mm, the effective > aperture goes down to f5.6, right? And if I zoom to 50mm, the > aperture will be somewhere in between, say f4 maybe.
Right. > Now suppose I set my zoom to 50mm @ f8. > If I zoom out to 28mm, will the effective aperture stay at f8? > If I zoom in to 80mm, will the effective aperture stay at f8? Nope, in both cases (although my understanding is that the relative differences in aperture at different focal lengths diminish as one moves away from "wide open"). > In other words, how can I tell if I'll really get f8 when I set > it to f8? You can't, exactly (although at f/8 you'd probably be close enough if you were using print film). That's one reason why many of us Pentax dinosaurs prefer sticking to the ol' constant aperture zooms. The fact that, in general (with some exceptions, of course), most (typically older, often MF) constant aperture zooms are better built than most (typically newer, often AF) variable aperture zooms, is another reason for the dinosaurs amongst us. But I digress... In any event, if you're using TTL metering (which most of us use most of the time), then the exposure recommendation should automatically compensate for the variable aperture problem. Fred - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
