In a message dated 12/6/01 12:49:19 PM Eastern Standard Time, [EMAIL PROTECTED] writes: > Well that is correct except at the widest setting. If you have a 28-80/4-5.6 > lens and set the aperture manually at F4 and zoom from 28 to 80mm you will > end up at F5.6 even if the ring says F4."
You're saying that a lens at f/4 ~migrates~ to f/5.6 in what is a no eletronic situation. The "A" setting closes (opens?) a circuit. Of the "A" the les is a manul lens. You must then explin how this "phantom" migration happens. > setting the aperture ring is not involved in > the actual diaphragm function." Yup. > The insides of the lens barrel is what determines the aperture in that case. > It's actually easy to see the effect. If you sit at 28mm and change the ring > from 4 to 5.6 the shutter value will change." Goes without saying. > and switch it between 4 and 5.6 the shutter > value will not vary at all." Only if the one-stop movement does not interfere with the amount of light, though theoretically it should. > that's because the amount of light is not changing because the barrel of the > lens housing is already stopping the amount of light to around F5.6 anyway." > OK, twisted, but OK. > > constant aperture zooms that are opened up at the short end." Zoomed to 80mm? That's the LONG end. > design of a constant aperture zoom is nothing more than > opening up the front element and lens barrel so that the diaphragm is not > vignetted by the barrel at the long end." OK > Kent Gittings > _____________________________________________ > -----Original Message----- > > Subject: Re: Variable aperture zoom question > > In a message dated 12/5/01 4:19:36 PM Eastern Standard Time, > [EMAIL PROTECTED] writes: > > That's precisely the point - that because the zoom lens is variable > > aperture, I cannot be guaranteed that the amount of light admitted through > > the lens will be constant over the zoom range of the lens. > IF you use a PK/A variable aperture lens with the lens in any position other than "A", the aperture WILL NOT change, withthe exception of using TTL flash in the aperture AE mode. The way you decribe it, ~you're~ setting the aperture yourself. It won't change no matter how you zoom (with the one TTL flash exception noted). > 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. > Specifically: when an "A" lens is not on "A," the lens becomes either > semi-manual? (aperture only) or full manual (aperture and shutter on manual > 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. > > Mafud > [EMAIL PROTECTED] > [EMAIL PROTECTED] - 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 .
