PDA

View Full Version : Measuring focal length of old barrel lenses

papah
1-Aug-2009, 14:51
So, how do you do it? I have some old lenses I'd like to better understand.

-Bruce

Peter K
1-Aug-2009, 15:18
Bruce, you can calculate the focal-lenght from image size and angle of view, use a collimator telescope or Abbe's "Focometer".

But much more easy is to align the camera in such a way a glass-ruler or a transparant graph-paper is shown acurately on the ground-glass in the ratio 1:1. Than focus on a realy distant point. The difference in bellows-extension is the focal-lengt of the lens.

Peter

Paul Metcalf
1-Aug-2009, 15:19
Mount lens to camera, focus on something at infinity, and measure from the film plane (inside of ground glass) to.... that's the hard part. If it looks like an old lens that is still in its original form, then I measure to the aperture if it has one or the waterhouse stops if it has those, otherwise to the flange. Or try and find some original specs on the lens from the various antique lens folks on this forum.

Carioca
1-Aug-2009, 15:44
As Paul mentions, the difficulty with old lenses is to figure out if you have to calculate from the aperture or flange plane.

I have tried the following calculator (see link). It has come pretty close to the actual focal length of some old barrel lenses I have.

http://www.ukaoptics.com/selection.html

Sidney

Mark Sawyer
1-Aug-2009, 17:38
If you focus the camera to infinity and measure from the focal plane (ground glass) to the rear element of the lens, you get the back focal length.

The flange-to-focal-plane distance is irrelevant, as flange location is arbitrary. A lot of old barrel lenses have focusing tracks built in that let one change the lens position in relation to the flange.

What I think you want is "effective focal length" which is aperture-location-to-focal-plane. The aperture is usually at or very close to the rear nodal point. If you don't have an aperture, you're kinda screwed and just have to guess where it would be based on where it is on similar lens designs, (which should actually get you pretty close).

Personally, I think it's time we all started referring to our focal lengths in "35mm-equivalents", like the digital people do. :rolleyes:

papah
2-Aug-2009, 12:25
Thanks, all, for the information. This was prompted because I purchased a lens on the big auction site that is marked at a certain focal length, but doesn't seem to be that length.

It is a Cooke-Luxor Anastigment, 5- 3/4 inches, f/4.5. The back focus at infinity is less than an inch from rear element to image, placing the nodal point about 2 inches in front of the lens.

Is that possible?

The barrel of the rear element is slightly darker than the rest of the lens, leading me to the conclusion that someone switched rear elements. The lens was purchased from a certain aussie dealer before I read the reports on this website.

Dan Fromm
2-Aug-2009, 12:54
Cry foul. The Luxor is a garden variety Cooke Triplet. These lenses are in no sense telephotos.

ic-racer
2-Aug-2009, 12:54
Here is one on e-bay. Does the rear element look like that one? (or is this the one you have?)

http://cgi.ebay.com/T.T.&-H.-LTD.-5-3%2F4-INCH-%2FF4.5-COOKE---LUXOR-LENS--_W0QQitemZ200368911445QQcmdZViewItemQQimsxZ20090801?IMSfp=TL090801134003r20322

papah
2-Aug-2009, 13:10
Here is one on e-bay. Does the rear element look like that one? (or is this the one you have?)

http://cgi.ebay.com/T.T.&-H.-LTD.-5-3%2F4-INCH-%2FF4.5-COOKE---LUXOR-LENS--_W0QQitemZ200368911445QQcmdZViewItemQQimsxZ20090801?IMSfp=TL090801134003r20322

That's identical in markings. My rear element is shorter (seller's photo):

Shen45
2-Aug-2009, 20:31
As Paul mentions, the difficulty with old lenses is to figure out if you have to calculate from the aperture or flange plane.

I have tried the following calculator (see link). It has come pretty close to the actual focal length of some old barrel lenses I have.

http://www.ukaoptics.com/selection.html

Sidney

Wonderful link. Thank you for that.

I just tried it with a couple of known lengths and it was very close indeed. I have wondered about a couple of barrel lenses without FL marked that I have and now I know thanks to that link.

Steve

Don7x17
2-Aug-2009, 20:59
Thanks, all, for the information. This was prompted because I purchased a lens on the big auction site that is marked at a certain focal length, but doesn't seem to be that length.

It is a Cooke-Luxor Anastigment, 5- 3/4 inches, f/4.5. The back focus at infinity is less than an inch from rear element to image, placing the nodal point about 2 inches in front of the lens.

Is that possible?

The barrel of the rear element is slightly darker than the rest of the lens, leading me to the conclusion that someone switched rear elements. The lens was purchased from a certain aussie dealer before I read the reports on this website.

do a quick visual check -- look in each end - are the effective pupil sizes the same? Should be the same for this lens. If not, then you have a mismatched pair of elements. (this is a very quick check for a telephoto design - try it on any modern telephoto like a Nikor T lens....And telephoto means telephoto design, not just a longer focal length lens)

papah
2-Aug-2009, 22:20
do a quick visual check -- look in each end - are the effective pupil sizes the same? Should be the same for this lens. If not, then you have a mismatched pair of elements. (this is a very quick check for a telephoto design - try it on any modern telephoto like a Nikor T lens....And telephoto means telephoto design, not just a longer focal length lens)

Yep, they're different. Thanks.

The seller was ocean_image. Bad me..I should have known better....:(

Don7x17
2-Aug-2009, 22:34
Yep, they're different. Thanks.

The seller was ocean_image. Bad me..I should have known better....:(

Another remote possibility is that the original owner was either playing around with some lenses and either assembled them for some reason, or left them in pieces... only to become deceased and have the estate seller reassemble random elements to make lenses. Or the seller bought one or more lot(s) of pieces and made some lens combinations never seen before.

Paul Fitzgerald
2-Aug-2009, 23:25
the lens collector's vade mecum (pg 282) mentions that they had made exchangable rear cells for these very lenses.

papah
24-Sep-2009, 15:38