Calculating film exposure with old brass lens

Anton Lukoszevieze

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Hello all,

I bought a rack and pinion focus brass lens for £6 (i think that is a bargain..), which is very very clean and clear. No names or any identifying marks on it at all. There is no iris. How do I calculate exposures times? Do I assume it is F1? F2 or F4.5? Any ideas?
It only focuses very close. Hope this is not too dumb but i have never used a len such as this..............is it a portrait lens? It is quite soft focus. Is it a lantern lens?

Anton

here it is
 
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removed account4

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what i have done, and it isn't perfect ---
cast an image ( like outside from a window which would be at infinity )
onto a piece of paper on a wall -rear cell towards the wall as if the wall
was film in the back of your camera ...
measure the distance between the rear cell and the paper, that will be your focal length.
then measure the diameter of the opening of your iris-less lens, and divide
the opening number into the focal lenght number.
as i said, it isn't perfect, and you won't be measuring the "node" (whatever that is .. )
but it is a ok guestimate ...

have fun!

john
 

Ole

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The node is somewhere near the center of the lens. If you measure to the rear end of the lens barrel, you will get a far too short focal length.

This is most likely a lantern lens, and these tend to be less than 130mm. The lens barrel is often around 80% of the focal length, another hint.

If the lens only focusses very close, you have too much extension.

The best way to estimate the focal length is to compare the size of an image with that formed by a lens with known focal length.
 

Christopher D. Keth

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(1/focal length)=(1/image distance)+(1/object distance) will give you the exact focal length. It's accurate enough to measure the distance from the midway point between front and rear lens elements.

I would measure the entrance pupil like this. Focus the lens at infinity and lock everything off. Remove the groundglass and put a piece of card there with a tiny hole in it, like a pinhole. Shine a bright light so it shines through the pinhole and projects a beam of light out the front of the lens. Measuring the diameter of this beam as close as possible to the front element will give you the entrance pupil of the lens wide-open.

The f-stop at any given iris setting will be the focal length divided by the entrance pupil.
 
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jimgalli

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Measure the diameter of the front glass in mm. ie. 50mm. Measure the distance from film plane (ground glass) to center of lens when it is focused. ie. 250mm Divide the 50 into the 250. = 5. You are at f5 for the exposure.

Another method. Focus, take the GG out of the way. Take the lens off your Nikon FM. Hold the nikon where the film plane used to be. You will see parts of your image on the Nikon glass and the Nikon will meter "TTL" what it is seeing.
 

HerrBremerhaven

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If it is a Petzval type design in the optics, many of those were f3.4 or f3.6. When I was testing my Holmes, Booth & Haydens I used Fuji Instant and tried f3.4 and f3.6, with the former giving better exposures. However, since this was a lens cap and wristwatch shutter timing, I could be a little off. I did shoot some transparency film recently with the same lens, so those sheets hopefully will give me a better idea of aperture/exposure.

I do like the Nikon method Jim Galli mentions. Funny that I have several Nikon bodies sitting around, and I have not thought of trying that. Now I might need to figure out an adapter to mount my brass lens on an F mount bayonet.

Ciao!

Gordon Moat
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bennoj

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I do believe there are adapters on the famous auction site to mount various 35mm mounts (including Nikon) to your 4x5.

(not associated with any such auctions)
 

Phillip P. Dimor

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Actually, I had a similiar problem w/ a petzval. I guessed that wide-open it was 5.6 and from the results, i'm pretty close. The petzval lens that i'm using is too short for 4x5, there is a bit of fall off. However, the center part of that image is really really sharp. Looks like a nice lens!
 

TheFlyingCamera

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Your Petzval was probably intended for 1/6 or 1/8 plate size, more likely 1/8 if you're getting vignetting on 4x5.
 
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