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Re: Bellows fabric

Posted: 15 Nov 2017, 11:56
by andrewch59
Ray, stand in a dark room, with window or door that receives light, point the rear of the lens toward the light coming through the window. Hold a piece of plain paper up to the front of the lens and when an image appears sharp on the paper, measure the distance between the paper and the waterhouse slot or iris (axis). Ensure that you have the focus on something outside in the distance(infinity)

Re: Bellows fabric

Posted: 15 Nov 2017, 12:04
by andrewch59
I guess I should add that the aperture should be wide open, and you can turn the lens round the other way and recalculate to find the back focal length(which some would deem more important).
To find the F stop, measure the smallest aperture of the lens wide open, normally where the iris or waterhouse stop fitting is and divide by the focal length.

Re: Bellows fabric

Posted: 15 Nov 2017, 14:59
by Ray Heath
Thanks Andrew, I actually knew all that but when you wrote calculate I thought maybe I'd missed something.

Why would back focal length be important?

Re: Bellows fabric

Posted: 15 Nov 2017, 18:02
by andrewch59
I know some use it to calculate bellows factor, and realistically its the back end that determines the size of the image.

Re: Bellows fabric

Posted: 15 Nov 2017, 18:03
by andrewch59
Ray, everyone puts their lenses on ebay these days.

Re: Bellows fabric

Posted: 15 Nov 2017, 20:44
by Ray Heath
Thanks Andrew.