# Finding focal length of old lens

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• 08-22-2009, 03:03 PM
Marcus
Finding focal length of old lens
Hi,

Hopefully somebody can help me with this.......

I have a 1/2 plate camera called 'The International' made by Lancaster & Son of Birmingham, of about 1886 vintage. The brass lens is wonderful & has excellent optics!

I would just like to know what the actual focal length is and/or how to work this out, e.g.- 6 inch??

Written on the lens is - "J. Lancaster & son, Birmingham, 1/1 pl patent, Rectigraf". The F stops are F/1 - F32

The lens is 2 inches diameter, by 3 inches in (total) length.

Thanks,

Marcus
• 08-22-2009, 03:18 PM
Ian Grant
Find the Sun :D Not easy in the UK :)

Focus the sun on some paper, measure the distance from approX the middle of the lens (glass elements ) to the paper, that's the Focal length :)

Ian
• 08-22-2009, 03:24 PM
Marcus
The lens has lenses at both ends with aperture ring in middle/centre. Does this change the calculations? Or do you mean the actual centre of the length of the lens?

Marcus
• 08-22-2009, 03:25 PM
KenFretz
Take the camera outside and focus on a distant object (you are trying to focus it at infinity). The distance from the front of the lens board to the inside of the ground glass should be the focal length of the lens.

Ken
• 08-22-2009, 03:32 PM
Marcus
Quote:

Originally Posted by KenFretz
Take the camera outside and focus on a distant object (you are trying to focus it at infinity). The distance from the front of the lens board to the inside of the ground glass should be the focal length of the lens.

Ken

So, when I have the camera open with full bellows extended (it being a draw-back camera), basically it is the length of the full bellows extention from lens board to ground glass?

Marcus
• 08-22-2009, 03:45 PM
timbo10ca
Quote:

Originally Posted by Marcus
So, when I have the camera open with full bellows extended (it being a draw-back camera), basically it is the length of the full bellows extention from lens board to ground glass?

Marcus

No- focussed at infinity will require minimum bellows draw- then measure as Ken said. If you had the bellows fully extended you would be focussing at something near, i.e. close to if not at or beyond 1:1 (macro/close-up)
• 08-22-2009, 03:47 PM
Mahler_one
Go to JBrunner's site...he has a wonderful explication of how to find the focal length of any lens.
• 08-22-2009, 03:52 PM
John Kasaian
I'd take the lens in a darkened room with a window opposite a plain, preferrably white wall. Point the lens (make sure it is wide open) towards the window and when you can focus an image (upside down) of whatever is outside clearly on the wall, measure the distance between the lens and the wall. This will also give you an idea of the image circle the lens can cover.

It works for me! :)
• 08-22-2009, 03:57 PM
Marcus
Quote:

Originally Posted by timbo10ca
No- focussed at infinity will require minimum bellows draw- then measure as Ken said. If you had the bellows fully extended you would be focussing at something near, i.e. close to if not at or beyond 1:1 (macro/close-up)

Ah, I get it now......!! Thanks. Will try it tomorrow when the big yellow disc in the sky returns!

Marcus
• 08-22-2009, 03:58 PM
Marcus
Quote:

Originally Posted by Mahler_one
Go to JBrunner's site...he has a wonderful explication of how to find the focal length of any lens.