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. This is because at the widest setting the aperture ring is not involved in the actual diaphragm function. 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. If you move up to around 80mm and switch it between 4 and 5.6 the shutter value will not vary at all. 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. >From a different perspective you could say that these are actually F5.6 constant aperture zooms that are opened up at the short end. Often times the 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. Kent Gittings
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of [EMAIL PROTECTED] Sent: Wednesday, December 05, 2001 7:51 PM To: [EMAIL PROTECTED] 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. > > I *thought* this phenomena applied to variable aperture zooms only when set > wide open. I'm trying to determine if this applies to *all* aperture > settings of the lens. > 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 . ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept by MIMEsweeper for the presence of computer viruses. www.mimesweeper.com ********************************************************************** - 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 .
