If it did there would be no real need for the aperture ring because it wouldn't be accurate at all. From experience this only happens at the wide-open setting due to the movement of the focusing group in the barrel. When stopped down one stop it no longer varies the amount of light because this was caused by the angle of the focusing group as it approached the front of the lens and changed its subtended angle in relation to the front of the lens opening. Once the diaphragm is stopped down its travel doesn't change its relationship to the aperture opening caused by this so it no longer varies. Kent Gittings
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Michael Nosal Sent: Wednesday, December 05, 2001 4:25 PM To: [EMAIL PROTECTED] Subject: Re: Variable aperture zoom question At 03:36 PM 12/5/01 EST, Mafud wrote: >In a message dated 12/5/01 12:03:46 PM Eastern Standard Time, >[EMAIL PROTECTED] writes: > >> "The replies to my original query seem to agree with what I was told - that a >> variable aperture zoom lens will vary the amount of light admitted as you >> zoom in and out, no matter what I set the aperture ring to." >> >Not true. The variable aperture function happens only with PK/A lens in the >"A" position. Your question specifically stated: "when ~I~ set the aperture". >**Otherwise, you could not choose your apertures for lighting or depth of >field. > 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. Depth of field is *usually* desired in relative amounts ("more depth of field" or "less depth of field" than it is in absolute amounts ("I want 3.5m of acceptable focus") so having an effective aperture change of about 1 stop probably won't screw up your desired depth of field. But it will mess up your exposures. >Remember: other than setting depth of field, the aperture has little to do >with flash photography (assuming you lights/strobes are capable of producing >f/22 [f/32 for medium, f/45 for large format] lighting). > Aperture has everything to do with flash photography. I cannot adjust the shutterspeed more than 1 stop. Too slow and I get camera shake, too fast and it won't sync with the flashes. 1/60th, or 1/100th are my choices (on a ZX-7). Adjusting studio strobes can be tricky - sometimes your strobes only adjust 1 stop up or down, sometimes you can adjust in .1 stop increments. Sometimes you cannot physically move the lights to adjust their intensity. Film speed is also fixed - 100 or 160. That leaves adjusting the aperture to match your lighting. Except that a variable zoom lens can give a perfect exposure at 50mm, overexposed at 35mm and underexposed at 80mm, even though the aperture ring is set to f8 the entire time. >Though you didn't say so, on PENTAX camera bodies with the "AE" function, >~if~ your shutter is set on manual, and your lens is on "A," you're in >shutter priority mode. But it seems you're describing manual studio >operation, both shutter and aperture being ~set by you~. If so, and since you >didn't mention using a light meter, the presumption is you're using the >camera meter. In such a studio situation, what you describe is full manual >operation. >That is: you've manually set the camera to "X" speed (or slower), then set >the aperture for depth of field. In that situation, zooming only frames your >subject (again, depending on the power of your lighting). This is metered with a handheld meter, full manual mode on the camera. Now, it *MIGHT* be possible, for the camera to adjust for this effect. If I set the aperture on the BODY, and I use a FA lens, the camera could detect the focal length of the zoom, and compensate automatically when closing down the aperture. That is, if the lens does truly let less light in at the tele end than it does at the wide end, then the camera could close down the aperture a little bit less, to maintain consistent exposure. Anybody know of any systems that do this? --Mike - 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 .