K screen with wide angle lenses

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Just noticed something strange with my AK screen for my F-1N. Maybe this is common knowledge, and I know that the K screen is for long lenses. But I was surprised when I put my FD28mm f2.8 on the camera and saw HUGE vignetting-I thought that the lens was faulty until I checked my original E screen. The K is fine for my 50mm f1.4, but why it does strange things to the 28mm I don't know?

-- Steve Phillipps (steve@redvixen.freeserve.co.uk), February 08, 2002


I did the same thing and thought I'd busted something :o).

The F-1N screens contain a condenser lens to help brighten the image. (see http://www.mir.com.my/rb/photography/hardwares/classics/canonf1n/meter ing/screens/screens.htm )

I understand that the condenser lens focal length should be matched to that of the taking lens because to get the maximum light into your eye the light rays coming off the screen should be as close to parallel as possible (or at least as if they were from an object 1 meter away).

For the standard screen most of the light rays coming off are scattered by the groundglass in the screen and a fair number are lost. Because the light rays are scattered the focal length of the taking lens doesn't matter as much. For the bright screens the groundglass diffuses the rays less and most of the light rays maintain the paths given them by the taking lens which the condenser then makes more parallel. For shorter lenses the angles of the light rays are such that the condenser of the K screen can't direct them to where your eye can collect them.

Does this make sense?



-- Duane K (dkucheran@creo.com), February 08, 2002.

Yeah, thanks, Duane. I'm sure it makes perfect sense and I'm sure you're absolutely right-just don't ask me to explain what you've said! Respect. Steve

-- Steve Phillipps (steve@redvixen.freeserve.co.uk), February 09, 2002.

I sure hope I've got it right too. I re-read it just now - Maybe in a week it won't anymore :o)

-- Duane k (dkucheran@creo.com), February 09, 2002.

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