Thank you John, when I wrote my response I did not include any facts as I felt that would have been inappropriate <g>.
Take two examples: FA28-70/4, and FA80-320/4.5-5.6 (I have these so can speak with confidence). When the 28-70 is set to 70, the iris is fully open giving f/4 (the opening is 17.5mm diameter). Zoom the lens to 28mm and the iris gradually closes, noticeably as you zoom between 35mm and 28mm. When you do this you become aware that the lens could (in theory) be a variable aperture f/2 ish to f/4. With the 80-320, at 80mm the iris diameter is 80/4.5 = 17.8mm. If you applied this iris to the 320mm end the effective aperture would be f/18, so the lens's aperture mechanism is connected to the zoom mechanism to close the iris as you zoom wider. The maximum aperture at the wide end is limited by optical performance rather than geometry. In any case, with the 80-320 set to minimum aperture (f/32 at 80mm), the viewfinder indicates a change to f/38 as the lens is zoomed to 320mm. Regards Jim Brooks > ------------------------------ > > Date: Thu, 06 Dec 2001 20:09:05 -0600 > From: John Mustarde <[EMAIL PROTECTED]> > Subject: Re: Variable aperture zoom question (on topic shurely) > > On Thu, 06 Dec 2001 10:13:26 GMT, you wrote: > >>"[EMAIL PROTECTED]" wrote: >> >>"I'll say this and no more: when you ~manually~ set the aperture, the >>"variable aperture" becomes a "preset" (by you) aperture. Nothing you do >>while zooming will (can) change the aperture until (you) change it to a >>different setting. **And it does not matter whether you set it wide open or >>close it down all the way. The aperture CANNOT (does not) change until you >>change it-period." >> >> >>Er... simply, **this** (is) ~wrong~ -period. >>Regards >>Jim Brooks > > How right you are, Jim. Selecting F8 at the aperture ring will give > F11 at the tele end of a variable aperture zoom, assuming the zoom > varies by a whole stop. > > Of course, one might not notice this difference in the viewfinder of a > Super Program or other camera that reads in full stops, if the > variable aperture zoom varies by only one stop; but one would > certainly notice the exposure difference in the resulting slides if > one used the same selected Tv and aperture ring Av at the tele end as > at the wide end. > > I don't know if the aperture blades actually close during zooming (I > think not) so someone could argue that "the aperture really doesn't > change" but in fact, as the word aperture is commonly used in an > exposure and DOF connotation by photographers, the aperture actually > does change, even if it is only the "relative" aperture as stated in > F-stops rather than the physical diameter of the iris opening. > > In other words, selecting F8 at the aperture ring will give F11 at the > tele end of a variable aperture zoom, assuming the zoom is one which > the manufacturer says has a nominal one-stop variation such as the > very common f4-f5.6. > > I don't think angels dancing on the end of a pin can confuse the > difference between F8 and F11, but I'm sure someone will try to say > that F8 ~actually is~ F11 in *all* circumstances, and if I were only a > ~good~ *professional* photographer I would know that by heart, or else > I should be out taking photos instead of testing variable aperture > zoom aperture behavior. > > Here's how I did the test: I took out a couple of cameras, variable > aperture zooms, and fixed aperture zooms. Then I set the aperture to > F8 on every zoom. Then I set each lens to its widest setting. Then I > took a reading using center weight of a light, evenly illuminated, > wide wall, using each lens on each camera. Then I set each lens to its > longest setting, and repeated the tests. > > In every case, the proper exposure for the fixed aperture zoom was the > same whether at wide or long end; the proper exposure for the variable > aperture lens was one stop different at the long end than at the wide > end. > > So, in my small sample test, the variable aperture zoom varies the > effective aperture when zooming from wide to tele, despite the fact > that the desired aperture is fixed on the aperture ring. > > And the PZ1p and MZ-7 actually display this difference in the > viewfinder. Selecting F8 at the aperture ring shows a value of F8 in > the viewfinder when zoomed to the wide end; when zoomed to the long > end, the value in the viewfinder gradually migrates to F11. > > In other words, as I zoomed to the long end I could see the exposure > bar graph descending and the aperture readout changing from f8 to f9.5 > and finally to f11. > > Lenses tested: > > variable aperture zooms: F 70-210/4-5.6; FA 28-105/4-5.6 (original > Power Zoom version); Vivitar 28-105/2.8-3.8 (manual focus); > > Fixed aperture zooms: F 24-50/4; 35-70/2.8 (old original AF for the > ME-F). > > Cameras: PZ1p; MZ-7 > > I'd like someone with a constant aperture zoom that goes to 100mm or > longer try this same test, and let me know the results. And someone > else with a variable aperture zoom with a manufacturer's stated > nominal variation of more than one f-stop range (say, > 100-500/f4.5-7.3) conduct a small test to see if the effective > aperture varies by the same amount as the stated range of variability > of the zoom. > > In conclusion, if anyone thinks their variable aperture zoom fixed at > F8 on the aperture ring will shoot at F8 when zoomed to tele, they > should keep their camera on autopilot, or meter again and adjust > shutter speed downward at the tele end, or just leave the shutterspeed > alone and trust the wide exposure latitude of print film, lest they > suffer from underexposed negs. > > - -- > John Mustarde > - - - 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 .
