I wish I had the cameras in question - if I did, I would just measure for you and find out! But in all likelihood the focal length for each is somewhere right around the 100mm or 105mm figure many have been suggesting. My experience with medium format boxes and other simple roll film cameras is that this is typical for 6x9, and that relatively few of the genre have focal lengths significantly shorter. Unless these cameras are exceptions in that regard, calculating FOV and object dimensions based on the 105mm figure will most likely be within 5% or so of the true answer. Edit: This is also assuming that the photos are uncropped images (as I think Shutterfinger pointed out). If you have reason to believe that they are cropped, then there is little you can say with certainty.
For reference, your first method of measuring the focal length of your Tengor was correct (as Tony said). There are a couple reasons why the second method will give you a different result, but I will leave that explanation to complicate another discussion. For whatever it is worth, I happen to have measured my own 56/2 Tengor a while back and came up with a focal length of 102mm, with an uncertainty of -3mm to +5mm (no clue at this point how I derived those uncertainty values though!).
Good luck with our quest. Sounds like an interesting project!
Last edited by Denverdad; 06-04-2014 at 02:15 AM. Click to view previous post history.
Your are correct at pointing out that these are not contact prints, but rather enlargements printed on one of the standardised photo-paper of the day (approx. 17 cm x 24 cm).
Originally Posted by shutterfinger
I will employ several methods of analysing the photographs, making certain assumptions along the way and will then cross-reference the results to see what can be concluded.
In addition, one of the photographs does show a part of what looks like a telegraph pole. I'll try to work out fl from the data assigned to this particular object.
Anyway, I appreciate your feedback very much.
Thank you for your positive feedback and encouragement, Denverdad. I don't think much cropping has been done, perhaps 2 mm on the sides. I base this assumption on the fact that I have other photographs in my collection which were made from 6 x 9 cm negatives in the smaller standardized prints by a professional photographer under my supervision. No cropping occurred during enlargement process and yet the ratio is almost the same as in the prints that I'm currently analyzing.
Originally Posted by Denverdad
It is very interesting to hear about your measurements of 56/2 Tengor's focal length. I'm now quite curious to learn more about the cause of your uncertainty regarding the margin of error of -2 to +5 mm? I am asking this because I too had difficulties in measuring my Tengor. To be honest, I haven't used proper measuring tools, just an ordinary ruler. I'll have to repeat the exercise using dividers, small scale rulers, etc.
I'll report about the results here.
The original Brownie was a 2.25 by 2.25 inch frame and there are drawings of it in US patent 725,034 of year 1900.
The plan (Fig 3) and elevation (Fig 4) sections are detailed but not dimensioned, I think there is a high probability the aspect ratio scaling is preserved.
Both frame dimensions (Item 24) were set to 2.25 inch on the monitor, and the distance from the film to the front of the meniscus (Item 6) was measured . It is about 3.875 inch (1.72 times 2.25 inch)
That is about 98.3mm. If the lens is focused at 15 ft, by 1/f=1/v-1/u
the focal length (of the lens) would be approximately 96.2 mm
(4 inch is 101.6 mm, and the above estimation could be in error by up to 6 mm or so)
I was not able to find section drawings of the 2 Brownie or the 6-20.
So I'm not the only one who measures dimensions off of a monitor? It probably looks a little funny to see me holding a ruler up to the screen, but hey it works!
Originally Posted by wombat2go
But I think your method of getting the dimension from the patent drawing is perfeclty fine and should be pretty accurate too. For reference, I think Jim Jones has it right as to where the principle plane is located for a simple meniscus lens (i.e., where from to measure the focal length). This Wikipedia page about cardinal points and principle planes in optics also has a drawing for a meniscus lens. The exact location may depend on the focal length, curvature of the two sides, glass index etc., but I think if you just assume the vertex of the convex side you should be within a millimeter, maybe two at most.
As for uncertainty of my focal length measurement with the Tengor, it is partly due to lack of knowledge of the principle plane locations on that lens since it is actually a cemented doublet meniscus, not a single element lens. And then too, there is just plain old measurement uncertainty due to the fact that it is difficult to get measuring implements into the camera where they need to be to make the measurements.
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