Actually, a constant f-stop zoom is a variable aperture zoom. The aperture has to vary over the zoom range to keep the f-stop constant. Variable f-stop lenses may have a fixed aperture (that is the aperture stays the same across the entire zoom range), or a stepped aperture (one that stays the same over part of the zoom range) which allows a smaller f-stop variation across the entire zoom range but is still cheaper than a fixed f-stop lens. Basically a fixed f-stop zoom lens is bigger, heavier, and more expensive than a variable f-stop lens. Once again as I have pointed out before, aperture and f-stop are not really interchangeable terms.
Ciao, Graywolf http://pages.prodigy.net/graywolfphoto ---------------------------------------------------------------- ----- Original Message ----- From: Chris Brogden <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, April 09, 2002 1:06 PM Subject: RE: Using external lightmeter with a Zoom lens... > 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 . - 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 .
