Yes. That indeed was what I was trying to say. When you get a zoom lens that doesn't maintain a constant aperture from extreme to extreme, it's because they wanted to produce a cheaper, consumer zoom and, in order to do that, the glass and barrel were made smaller, making it necessary to limit the expansion of the aperture iris diaphragm at the tele end. There fore, there was room for constant aperture (f-number) only up to a limit. In other words, f/8 at the wide end had plenty of room to expand to maintain f/8 at the tele end but f/3.5 at the wide end ran out of expansion room at the tele end and would only be (ratio-wise) f/4.8 or whatever.
Len (Who was noticed by the boss as spending too much time on non-company business) ---- -----Original Message----- From: T Rittenhouse [mailto:[EMAIL PROTECTED]] Sent: Tuesday, April 09, 2002 1:22 PM To: [EMAIL PROTECTED] Subject: Re: Using external lightmeter with a Zoom lens... 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 . - 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 .
