[pure-silver] Re: bellows contraction factor
- From: Georges Giralt <georges.giralt@xxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sat, 21 Apr 2007 06:26:29 +0200
Shannon Stoney a écrit :
Hi, just wondering if there might be something called bellows
contraction factor. When I use my new wide angle lens, which of course
necessarily is closer to the film plane in a view camera, my negatives
look overly dense, even when I expose and develop the same way that I do
with the 150mm lens (for a 4x5 camera). I had the lens checked to make
sure the shutter was timed right, and anyway I am timing it myself most
of the time in seconds rather than fractions thereof. Could it be that
contracting the bellows causes more light to hit the film, just as
extending the bellows causes less light to hit the film plane?
--shannon
Hi Shannon !
The lightmeter and aperture on a given lens are set as to give proper
exposure when the lens is set at infinity setting. Whatever distance the
lens is from the film plane. This distance depends upon focal length
(and construction) of the lens. So you've to increase exposure when
focusing to get closer than distant object because the lens give light
to a greater surface of film (it is a virtual film but it is large).
So if your wide angle is set at its normal infinite distance, you get
normal exposure. Even if the rear lens element is barely touching the
film (as in the Zeiss Biogon design)
May I suggest you check your bellow for pinholes ? It may be that you
have some sort of fogging appearing only when the bellow is contracted
(wrong assembly, material deterioration, light leak betwenn the lens and
its panel,...)
Last but not least, you've had the wide angle lens checked. So I
consider it fine. What about the other lenses ? Are their shutter
accurate ?
Hope this helps
--
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