help me butcher a camera lens


TPF Noob!
Feb 1, 2006
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in the middle of north carolina
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somebody help me out here please. I just made a lens change and need some advice. First of all I'm no math whiz so I'm looking for an answer not a formula... I will however take a formula. I might have to work it backwards though. Here is the problem.

I salvaged a shutter off an old kodak camera. The glass is set up for a 100 mm lens. I am in the process of attaching the shutter to a barrel lens I have made. Now here come the problem.

The shutter at 100 mm had a top aperture of 32.. that shutter and aperture is now on a lens of 135mm. Someone tell me what the new aperture will be. From my pinhole calculator Im guessing f45... Does that sound about right.
f32 for a 100mm fl lens is an aperture with a physical diameter of 100/32, or 3.125mm.

Using that aperture [3.125mm] with a 135mm lens will give an f stop of 135/3.125, or f43.2.

You can easily work out the other f stops now.
I could be thinking backwards here, but I would think the number would go down. The shutter itself will only open to a certain width no matter what lens it's on. Going from 100mm to 135mm you might lose half a stop, or maybe a quarter (i really dunno the math). At the widest the shutter can physically open, it will be letting less light in on a 135mm lens than on a 100mm, so even though physically it's the same width, it's letting less light in, and therefore the effective aperture is lower. I.e. you'd need to open the aperture wider on a 135mm than you would on a 100mm in order to let the same amount of light in.

Seriously, someone correct me if I'm wrong.
Okay so maybe i'm wrong. Oh well.

Edit: yeah, i was thinking totally backwards.Torus is right. Even after all these years I still slip up sometimes and think wider aperture=larger number. Doh.
I got exactly the same number with my pinhole calculator running it backwards. So I evened it off to f45.... If the shutter still works I'll test it as soon as I build a 4x5 back for a polaroid I bought in a junk shop saturday. It should be interesting.
That sounds like a nightmare in the making. The math wiz I am 'grats to those that figured it out and to those that didn't ...don't worry you're not alone :)

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