What are the true focal lengths of the 35mm, 45mm, and 55mm Rodenstock APO Grandagon lenses?
What are the true focal lengths of the 35mm, 45mm, and 55mm Rodenstock APO Grandagon lenses?
35mm, 45mm, 55mm.
Why not ask the "true" focal lengths of a 150, 135 and 210mm?
What do you really want to know about them?
I'm guessing he wants to know the distance to focus the lenses. Maybe to see if the lens will focus at infinity on his camera?
Flange focal lengths at infinity in Copal 0 are:
35mm = 43.2mm 45mm = 55.5mm 55mm = 67.6mm
Jerry,
Perhaps I can clarify what the others have said. The true focal length is the distance from the film plane to the lens when focused on infinity. It could differ slightly from the stated focal length for individual lenses but not by enough to matter. The problem is that this would apply only if a lens were an idealized point. Actual lenses have some significant physical extent, so one must choose some reference point in the lens. That point is called the rear principal point, and its actual location depends on the design of the lens. For most lenses it is pretty close to the center of the lens which in turn is pretty close to the lens board. For lenses of telephoto design, it may be far in front of the lens. For most wide angle lenses, it is some distance in back of the lens board. The reason for this is so that the lens need not be brought quite so close to the film plane when focused at infinity. It also helps with light drop off over the field. Lens manufacturers give as part of the specifications the distance that the lens board will be from the film plane when the lens is focused at infinity. That is called the rear flange focal length. Thus Bob Salomon has told you that for the Grandagon 55 mm lens, the rear flange focal length is 67.6 mm. That means the rear principal point is 67.6-55 =12.6 mm back of the rear of the lens board. That means that the lens functions for geometric optics purposes as if it were a plane element centered on a point 12.6 mm in back of the lens board.
When a lensmaker names a len's focal length, it ain't necessarily so. For example, Schneider's Super Angulon 47mm XL actually has a focal length of 48.0mm. Their current 47ml nonXL is really 47.5mm. Their SSXL 80mm is really 81.0mm. This information is on their website. (They use f' and F' as symbols for the true or "effective" focal length.)
I really do want their focal lengths, but with a little more precision than just using their names.
Sorry Jerry, that information is not in their literature. If you are need this for an OEM purpose we can put you in touch with the proper people. If that is the case please call me at 800 735 4373 x15.
Jerry,
The actual true focal length is likely to be different from any stated focal length. If you really need to know it as accurately as possible, the best thing to do is to measure it yourself. There are a variety of ways to do that, and you can probably find one that you like somewhere on the web. What I would do is to measure as carefully as possible the distances from the front of the lens board to the image plane for some number of subject distances. There are basically three variables you need to determine: the focal length, the distance from the lens board to the front nodal point and the distance from the lens board to the rear nodal point. So in principle, if you use the lens equation and some algebra, making these measurements for three subject distances should suffice, but I would do it for several more and take an average to improve accuracy.
In the case of the data on the Schneider website, isn't that just a sample test of one representative lens, which may not be accurate for a lens that you actually purchase? My impression was that lenses vary in focal length from sample to sample due mainly to variations in the refractive index of the glass, and this is why Leica and Linhof rangefinder cams are calibrated individually to each particular lens.
Actually there is a very easy way to find the effective focal length. Simply point a lens out the window at a distant object and measure the distance to the projected image that is formed on a card held behind the lens.
However if Jerry wants to have a lens of exact focal length Rodenstock does offer that service at an extra charge. They will deliver lenses (at an extra cost) that are certified as to the exact focal length and flange focal length distance. Or they will certify the focal length, flange focal length and back focal length (Schnittweite) of a lens or lenses. Price depends on the number of lenses calibrated. Price breaks are 1-4 lenses, 5 - 24 lenses, 25 - 99 lenses and 100 or more lenses.
The first calibration was done automatically on all Rodagon G enlarging lenses longer then 50mm as well as Apo Ronar lenses delivered for process cameras. As Apo Ronar lenses and the G series (except for the 50mm) are out of production this is not a commonly requested feature.
And many modern enlargers for mural printing with electronic AF can calibrate the lens in the set-up process eliminating the need for factory calibration.
And, as someone else pointed out, there is no "true" focal length. The actual focal length will be the marked focal length ± a small percentage. It is this small percentage that the factory will measure per the above.
in fact it has never been asked of us before for taking lenses for large format work.
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