# Thread: Optical centre of this type of Lens?

1. ## Optical centre of this type of Lens?

Hi,
This is a convertible Protar:

B&amp;L Zeiss Protar by Ari4000, on Flickr

I am trying to determine focal lengths and apertures.
Where would the centre of the lens most likely be for my measurements?

Thank you

2. ## Re: Optical centre of this type of Lens?

I would suggest that you find it by focusing multiple magnifications and doing some math. Say you focus at infinity (M=0) and 1:1 (M=1), the change in extension will be equal to the focal length.

If you don't have enough extension to hit 1:1, you can do it with any particular known magnification and slightly more maths.

3. ## Re: Optical centre of this type of Lens?

Thanks for the quick reply, but I'm afraid I might not have been clear.

The f-stops on the lens are old-style; converting them to modern f-stop equivalents has not been successful, at least my film bears the out.
Furthermore, the aperture is not consistent from, say, f16 to f22 (it should be half as wide at f22 as at f16; it isn't).
So I'm trying to get the actual f-stops by dividing the focal length by the diameter of the aperture.
In order to determine the proper and exact focal length, I need to know where to place one end of the measurement (i.e. the centre of the lens); the other end is the film plane.

Thanks

4. ## Re: Optical centre of this type of Lens?

Problem solved with the kind assistance of a forum member, thank you.

5. ## Re: Optical centre of this type of Lens?

Ari:

Take a photo of some widely separated distant objects. Measure the distance between them on the negative, and the angle between them for the camera's perspective. Do the math and you'll find the FL.

Focus the camera at infinity and take it into the darkroom. Mask the GG with something opaque, and put pinhole through the center of it. Take a scrap of photo paper and hold/tape it across the front of the lens. Shine a light through the pinhole, then develop the paper. The size of the resulting dark circle is the size of the entrance pupil. Do the math.

The more carefully you do any of this, the more accurate your results, but don't be afraid to just get ballpark numbers. Ask if any of this is unclear.

Charley

ps: Ooops - problem solved. I typed too slow. I'll leave the post up anyway, in case it helps anyone else.

6. ## Re: Optical centre of this type of Lens?

Hey Charley,
Thank you for those tips, I like the methodology.
I just finished explaining to the forum member who is helping me on this that I am not technically-minded like many here, and I knew nothing about any of these calculations until yesterday (no joke).
The three focal lengths written on the lens are 19, 23 and 27 inches; yet I have also been able to obtain 6.5", 9", 11", 13" and 16" by taking the glass out and reversing its orientation. Fun lens, and sharp, too.

Thanks again for those great ideas; I've bookmarked them

7. ## Re: Optical centre of this type of Lens?

I have a protar set that has four screw-in sets of elements that can be combined in a variety of ways to get different focal lengths. I really quite like it and use it on my 5 x 7 Agfa/Ansco. Sufficiently sharp, and quite pleasant image.

http://www.flickr.com/photos/89514126@N05/11078994004/

8. ## Re: Optical centre of this type of Lens?

That's what mine does as well, Jim; I have 16 focal lengths on this lens, ranging from about 7 inches up to 32 inches.

9. ## Re: Optical centre of this type of Lens?

Finally remembered how to post a photo inline

Mission by Kirigakuresaizoh, on Flickr

10. ## Re: Optical centre of this type of Lens?

And here's the lens set.

Protarsatz by Kirigakuresaizoh, on Flickr

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