


Hi Tony,
The matter complicates even further, it seems. Namely, from which point on the lens should I measure the distance to the film plane? If I use the point on the outer side of the lens (i.e. the part outside of camera), I get roughly 100 mm. However, if I measure from the inner side of the lens (i.e. inside of camera) then I get about 85 mm!
Which reference point should be used?

A diagram in Rudolf Kingslake's Lenses in Photography suggests that the focal length of a meniscus lens with the concave side and aperture away from the film is measured from slightly behind the lens.

Originally Posted by Alex_L
Hi Kent,
So, which is it then, 105 or 65 mm?
Which method of measuring the focal length is a correct one?
These are simple miniscus lenses, no magnification or anything.Questions of where on the lens to measure are complicating things. This is a oneelement lens system, the lens is about 1 mm thick, at least on mine. I can't see whether it makes any difference where you start.
No, you would not measure from where the lens diaphram (fstop thingy) is located. Focal length is measure from the center of the optical system, wherever that is. In this case, it is the lens.
I have here before me an Agfa Synchro Box, allowing for the thickness of the back, it is about 105mm (10.5 cm) from the rear to the lens.
On a 6 by 9 format, that's a moderate wide angle, about 35mm in a 35mm film camera equivalent, exactly the same as on my Baby Crown Graphic.
Last edited by summicron1; 06022014 at 05:42 PM. Click to view previous post history.

Summicron1,
You missed the point. Ruler is useless if one doesn't have an actual camera to measure it! I don't have the cameras of the models I specified (i.e.  No 2 Brownie Junior,  Six20 Brownie Junior and  Six20 Brownie Model 4).
I do have Zeis Ikon Box Tengor, though, which is not the same thing.
I don't think you are right when you say that 'on a 6 by 9 [cm] format' the focal length is '35 mm in a 35 mm film camera equivalent'. The equivalent for a 6 x 9 cm format 105 mm focal length is actually 45 mm focal length in a 35 mm film!

About Lens by Eastman Kodak Co., 1921 http://www.cameraeccentric.com/html/info/kodak_5.html on page 14 shows a #0 Brownie to have a depth of field from 10 1/2 feet to infinity, a #2 Brownie 14 ft. to infinity, a #2A, #2C, and #3 Brownie 15 ft. to infinity. On page 31 depth of field table it states "fixed focus cameras are focused on 25 feet.
Now using an older version of fCalc in DOF mode a focused distance of 25 feet that gives a DOF of 10.5 feet to inf @f8 is 52.5mm; 14 ft to inf @f8 is 70mm; 15 ft. to inf is 75mm on 6cm x 9cm format.
http://www.browniecamera.com/ technical data shows the #2 and #3 box cameras to use f11 to f22 lens and when focused at 25 feet one gets some non common focal lengths to fit the close focus distances to infinity DOF focus range listed for the Brownie cameras.
Last edited by shutterfinger; 06032014 at 12:41 AM. Click to view previous post history.

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Originally Posted by pdeeh
surely 10.5cm (note cm, not mm)
Surely, you are correct sir!
Matt
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative  The New Ansel Adams Photography Series / Book 2

Originally Posted by shutterfinger
About Lens by Eastman Kodak Co., 1921 http://www.cameraeccentric.com/html/info/kodak_5.html on page 14 shows a #0 Brownie to have a depth of field from 10 1/2 feet to infinity, a #2 Brownie 14 ft. to infinity, a #2A, #2C, and #3 Brownie 15 ft. to infinity. On page 31 depth of field table it states "fixed focus cameras are focused on 25 feet.
Now using an older version of fCalc in DOF mode a focused distance of 25 feet that gives a DOF of 10.5 feet to inf @f8 is 52.5mm; 14 ft to inf @f8 is 70mm; 15 ft. to inf is 75mm on 6cm x 9cm format.
http://www.browniecamera.com/ technical data shows the #2 and #3 box cameras to use f11 to f22 lens and when focused at 25 feet one gets some non common focal lengths to fit the close focus distances to infinity DOF focus range listed for the Brownie cameras.
Thank you Shutterfinger for your useful input!
If we assume the models I referred to are foused at 25 ft, and that they are set at f/11, then can we conclude that the lens focal length of such a camera is somewhere in the range between 70 and 75 mm on 6 x 9 cm film (perhaps 72 mm)?
The focal length of 72 mm on a 6 x 9 cm format would correspond to a 31 mm on a 35 mm film.
Until you referred me to the publication by Eastman Kodak from 1921, I was almost certain that the models of camera I am considering in my investigation had 105 mm focal length lens. However, I now have doubts.
See for example this link: http://www.mhs.ox.ac.uk/cameras/item36.htm
That particular model had a 105 mm fl!

If there are any objects/people in the photos that are a known height then their size could be used to calculate other dimensions in the photo which will aid in the focal length determination.
The camera you linked to shows a 105mm f18 lens, another non standard lens measurement.
At f11 a 83mm lens has a dof of 14 ft. to 110 ft., angle of view 56.93° vertical, 39.74° horizontal, 66.18° diagonal.
At f16 a 98mm lens has a dof of 14 ft. to 115 ft., angle of view 49.33° vertical, 34.04° horizontal, 57.79° diagonal.
At f22 a 117mm lens has a dof of 14 ft. to 111 ft., angle of view 42.08° vertical, 28.76 horizontal, 49.62° diagonal.
Stated values in the published texts are likely the base standard from which deviations were used to achieve a desired prospective.
A 105mm f18 focused at 25 feet has a dof of 14.14 ft. to 107.35 ft, angle of view 46.4° vertical, 31.89° horizontal, 54.5° diagonal.
The dof and angle of view in the photos are your best clues outside of finding the actual tech data for the camera used.
Also see: http://www.brownie.camera/
Last edited by shutterfinger; 06032014 at 03:19 AM. Click to view previous post history.

Thanks again for the further information you supplied, Shutterfinger.
You suggested I should use people or objects of known size depicted in the photograph to work through the focal length. But that's just it! There are no people or known objects in the photograph, just a distant background landscape and an expanse of water.
In fact, I'm trying to find out the focal length in order to determine distances and sizes contained in the photograph (a reverse process to the one you proposed). However, I never knew it would be so difficult to get a hold of such technical data for Brownies as the focal length.
For what is worth, The photographs' dimensions are 16.7 cm x 23.7 cm.

The "other" key information you need is
Is this a contact print? Contact print=original negative size. Your measurements say NO.
If not a contact print what is the degree of enlargement to fit the existing print?
What is the crop factor, if any?
http://en.wikipedia.org/wiki/Film_format
You may have to buy a few old Brownies and optically measure their lens to find a suitable answer.

