On Tue, 9 Apr 2002, Paris, Leonard wrote: > The aperture itself varies in diameter as you zoom, the f/stop which > is a ratio of diameter to focal length does not. The design of > constant aperture zooms is more limited by the diameter of the glass > at the tele end. All zooms can be constant aperture zooms if you want > to pay for them.
We've been through this before, haven't we? I thought that it was the diameter of the lens where the aperture blades are that counted, not just the diameter of the front element. Otherwise it would be impossible for Pentax to change the design of their Super-Takumar 35mm f2 screwmount lens from a 67mm thread to a 49mm one and still keep it a 35/2... and yet they did. Variable-aperture zooms also change their effective f-stop as you zoom in... that's why they're so inexpensive compared to fixed-aperture zooms. If this isn't the case, then why are f2.8-4 lenses so much less expensive than fixed f2.8 lenses? What's the difference? chris - 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 .
