Optical centre of this type of Lens?

[Ari]

Hi,
This is a convertible Protar:
http://farm6.staticflickr.com/5500/10391437963_d5b6d3c5ff_z.jpg (http://www.flickr.com/photos/zsari/10391437963/)
B&L Zeiss Protar (http://www.flickr.com/photos/zsari/10391437963/) by Ari4000 (http://www.flickr.com/people/zsari/), 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

[polyglot]

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.

[Ari]

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

[Ari]

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

[c.d.ewen]

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.

[Ari]

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 :)

[Jim Andrada]

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/

[Ari]

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.

[Jim Andrada]

Finally remembered how to post a photo inline

http://farm6.staticflickr.com/5484/11078994004_9dfe190791_c.jpg (http://www.flickr.com/photos/89514126@N05/11078994004/)
Mission (http://www.flickr.com/photos/89514126@N05/11078994004/) by Kirigakuresaizoh (http://www.flickr.com/people/89514126@N05/), on Flickr

[Jim Andrada]

And here's the lens set.

http://farm4.staticflickr.com/3775/11457738244_dba8265411_c.jpg (http://www.flickr.com/photos/89514126@N05/11457738244/)
Protarsatz (http://www.flickr.com/photos/89514126@N05/11457738244/) by Kirigakuresaizoh (http://www.flickr.com/people/89514126@N05/), on Flickr

[Mark Sawyer]

...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.


Chiming in late... Keep in mind that if you're trying to calculate the optical center down to a millimeter or two, it will shift around with each configuration. The aperture location is B&L's best compromise for all three "official" configurations, and for practical purposes, you can just use that as the optical center for figuring focal lengths, f/stop ratios, and such.


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)...

Not if you're talking f/stops (as opposed to US stops); f/22 should have half the area as f/16, not half the width. Uh-oh, here comes that math again... :rolleyes:

[Ari]

Mark, thanks for chiming in.

I ended up using the aperture location as the centre, as you just suggested, though, with respect, I will stop short of measuring the area of the aperture. :)
A friend with more experience in these prosaic matters will help me with the f-stops.
In the meantime I took a stab at it.

I only bothered with the FLs that I am most likely to use, six of them, ranging from 200mm to 685mm.
These FLs would have been slightly shorter, but I mounted the lens to an Ilex #5, since I plan on using the lens that way.

My results seem a little odd: I got some f4 and f5 at shorter FLs; with longer FLs, such as 685mm, I only get max f-stops of f64.
I'm not sure how accurate my work is; once I have the time, I'll burn a few sheets of aerial film to see how close (not likely) I was.

Here's a snap of the lens barrel with my FLs/f-stops.
The numbers in the far left column are the FLs (mm) and the numbers in the far right column are the apertures when wide open.

Thanks

[polyglot]

Your progressions are wrong. The markings on the lens are the old american-style linear markings and are exactly one stop apart (except for the 10; if there were an 8 it would be one stop below 16) regardless of the focal length in use. Whatever the focal length of any particular lens configuration, there should be a factor of sqrt(2)=1.414 between the successive markings.

Your second-bottom row is particularly crazy, showing f/16, f/16.5, f/32; it should be f/16, f/22, f/32 or similar.

As an aside, in converting from the linear markings to modern f/numbers, 16 is f/16. The linear markings go in powers of 2 per stop (they are linear with exposure time) whereas f numbers go in powers of sqrt(2)=1.414 (they are linear with aperture size and exposure is related to the square of that).

So: 1 = f/4, 2 = f/5.6, 4 = f/8, 8 = f/11, 16 = f/16, 32 = f/22, 64 = f/32, 128 = f/45, 256 = 1/64.

[Ari]

Polyglot, thanks; I re-did my calculations after re-measuring the aperture at its smallest opening, then plugged in the 1.414
The results are attached in the photo below.