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John Downie
24-Sep-2003, 08:36
Hello,

I recently acquired a lens set and would like to know more about it. It consists of a brass main body with front and rear threads and an aperture iris that goes from 1 to 6. It has a total of 8 threaded cells, one which appears to be an extension the others are lens elements marked 25, 35, 45, 55, 65, 75 and 85. It is labelled Trousse "Parisienne" No 829.

Any ideas? The focal lengths are puzzling, but I assume there is some matrix of combinations yielding a (large) variety of final focal lengths, as well as f stops.

Any insights are appreciated.

John

N Dhananjay
24-Sep-2003, 09:17
Just a guess - maybe the marked dimensions are focal lengths of the individual cells in cms. It might be worth popping one on and seeing if it roughly in the ball park - keep in mind that using single elements typically displaces the nodal points, so measuring bellows extension might not be a good way to go but comparing the relative image sizes (and if possible to some other marked lens) should give you a pretty good idea.

When you combine cells, the combined focal length is typically got by using the formula 1/F = 1/f1 + 1/f2 - d/f1*f2

where F is the combined focal length, f1 and f2 are the focal lengths of the two individual cells and d is the inter-cell distance.

Cheers, DJ

Ernest Purdum
24-Sep-2003, 10:38
The apertures are going to be a problem. These sets came with a table translating the 1 to 6 markings of the diaphragm into actual f stops of the various combinations. Like Waterhouse stops, these tables nearly always get separated from the lens. A procedure that is laborious, but works, is to assemble a combination, focus it to infinity, then point it at a blank wall. Take readings off your groundglass, using your darkcloth if necessaary to prevent extraneous light from influencing the reading. Now switch to a known lens, again set to infinity. If the lighting hasn't changed, stop down the known lens until the same readings are obtained and record them. Now you can make your own chart.

Chad Jarvis
24-Sep-2003, 19:35
Sounds remarkably similar to an Emil Busch Vademecum set (mine goes from 15 to 75cm and has aperture marks from 1 to 5). My guess is that it's a meniscus set (Petzval, right DJ?). Does the barrel have a drop in slot by any chance?

John Downie
24-Sep-2003, 19:40
No drop in slot - do you have a table with your set for focl lengths and apertures?

lee nadel
25-Sep-2003, 06:04
hi i have a busch set with no chart afriend helped me establish f-stops 1 st find your focal lenght then measure the iris size that will give you a f-stop that will correspond to the numbers on your lens , maybe some one with the formular can help ?

N Dhananjay
25-Sep-2003, 07:30
Don't know about the Vade Mecums - could be single menescus types or maybe rapid rectilinear types depending on vintage, I guess. For figuring out f-stops, the simplest solution would be to measure the iris size and you will have a millimeter scale. Divie that by the focal length being used for the f-stop. Cheers, DJ

Ernest Purdum
25-Sep-2003, 07:48
Unfortunately, the formula is based not on the actual iris size, but the effective or apparent iris size. The actual iris size is only the same as the effective size for single, not compound, lenses. To measure the effective aperture, a rather uncommon and expensive gadget called a traveling micrometer is used. It looks at the lens from the front through a little telescope, is lined up with one size of the iris, then shifted over to the other side and the amount of travel measured. People have made gadgets that do this with sufficient accuracy, but its quite a bit of work. The formula is simple enough. Focal length divided by effective aperture equals f stop.

CP Goerz
25-Sep-2003, 12:34
Here is an F stop and focal length guide from a Busch Vademecum set, it will probably work for you as most sets were somewhat universal in nature........

John Downie
26-Sep-2003, 08:55
Thank you so much, CP.

Does anyone have the formula to go to the combined focal lengths?

J

Richard Knoppow
27-Sep-2003, 06:53
You will need to measure the actual focal lengths and find the principle points. This is not difficult. You can use a view camera as an optical bench. To measure the focal length focus the lens on infinity and mark the position of the lens. The exact point doesn't matter, just that you need a reference that you can come back to. After setting the lens exactly at infinity focus it for exact 1:1 image to object reproduction. The extension of the lens from its infinity focus point is the exact focal length. A double check is that the distance of object to image at 1:1 is exactly four focal lengths. If there is no convient target far enough away to approximate infinity you can autocollimate the lens. You need a flat mirror which will cover the front of the lens. Put the mirror over the lens and set up a small light behind the lens. If you use a view camera you can just put the end of a pencil flashlight against the ground glass near, but exactly at, the center. The mirror will project the image of the light back to the ground glass. Focus it for best sharpness. the lens is then focused exactly at infinity. Once you have the focal length you can find the rear or second principle point by measuring one focal length from the image back toward the lens with the lens focused in infinity. This is by definition the location of this point. To find the other principle point turn the lens around and focus it for infinity again. Its focal length doesn't change with direction. Measure one focal length from image toward lens again and you have the front or first principle point. You need to know the location of the principle points to calculate the focal length of the combinations (of course you can just measure them the same way). The D or distance between lenses given in the formula is the distance between the rear principle point of the front lens and the front principle point of the rear lens, not the physical distance of the cells. For thick convex meniscus lenses one principle point will lie about in line with the front apex of the lens and the other a bit in front of it, just outside the lens. The exact positions will depend on the bending of the lens. For a single lens mounted behind the stop the entrance pupil is the stop itself so the f/stop is simply the lens focal length devided by the physical diameter of the aperture. When the lens is in front of the stop, or where combined lenses are used the stop is changed by the magnification of the stop by the lens. This changes the effective size of the stop and the position of the entrance pupil. To measure the _effective_ size of the stop arrange a small light source behind the lens at the exact infinity focal point (which you have already found). Place a translucent screen over the lens (writing paper will do) and measure the diameter of the circle of light projected onto it. This is the effective size of the stop. You do not need to know the position of the entrance pupil unless you are going to do panoramic work. The entrance pupil is the correct point about which to rotate the camera for panorams. Its poisition can be calculated but its easier to measure it by setting up a camera to focus on some convient part of the lens, say the rim of the cell, at al close distance. Now look at the aperture though the camera and move it until the aperture is in focus. The distance the camera moved is the distance from the point of reference to the entrance pupil. The entrance pupil can be in front or in back of the aperture depending on the sign of the power of the lens in front of it. Again, where a lens is used behind the stop the stop becomes the entrance pupil. These measurements can be tedious but are simple and accurate enough to determine the effective focal length and stop calibration. Note, BTW, that the speed of a single cell depends on its position relative to the stop. Since these are all positive lenses (they must be to form a real image) they will be a little faster when in front of the stop than when behind it. However, in general, the correction of aberrations is better when the lens is used behind the stop.

John Downie
27-Sep-2003, 10:29
Thanks Richard - I was hoping I could take a more mathematical approach - i.e. get the formulas used in the table above that says f(focal length a) (+ or x) f(focal length b) = f(combined focal lenght). Otherwise I can extrapolate and interpolate, I suppose.

David A. Goldfarb
1-Oct-2003, 10:08
A note of historical interest--Berenice Abbott describes Atget's lenses as "a 'trousse' of rectilinear lenses." I wouldn't be surprised if they were of the same make. How many producers of rectilinear casket sets could there have been in Paris, after all?

pierre506
14-Jun-2014, 18:32
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