I recently picked up an old aerial lense, cheap. I actually bought it for the filter. It came with a very large #8 yellow glass filter. The lense is more of a novelty to me. I was wondering though how I might figure out the fstop of the lense? It's a plain barrel lense with no iris, and there are almost no markings on the lense. It focuses on the wall at about 45". The only markings are scratched on the side. They read: B.F. 8.9/6 F.F 22.2/3 I'm not sure what this all means? Specs are: length: 11 inches front element: 6" across rear element: 5" across circumference: 17 1/2" (at the center) Weight: about 12lbs.

Any ideas or comments are welcome!

-- Colin Seaman (cseaman@igs.net), October 03, 2000

Answers

To find the approximate maximum aperture of a lens measure the focal length of the lens by projecting a sharp infinity image of the horizon as seen through your room window onto an internal wall and measure the distance from the centre of the lens to the wall. Then measure the diameter of the lens in the same units. Divide the focal length by the diameter to give the maximum aperture. In this case it would appear to by 45/6 = f7.5.

-- John at JCR Cameras (jcr@jcr-cameras.com), October 04, 2000.

Well I found a window that will allow me to focus at infinity, and it turns out to be a much shorter lense, about 25", but has huge coverage. I guess my next question would be, can I control my fstop, and how would I do it?

-- Colin Seaman (cseaman@igs.net), October 04, 2000.

Edmunds Scientific sells iris's in various sizes and mounts, not sure how that would work out and not inexpensive either.

-- photo123 (willy226@yahoo.com), October 04, 2000.

Sounds like it might be a Kodak 25" f/5.6 aerial Ektar. Mine came with a slot cut in the middle, so I made up Waterhouse stops.

BTW, the lens is slightly radioactive

-- John Lehman (al7jj@yahoo.com), October 04, 2000.

"B.F." probably stands for "Back Focus" and "F.F." for Front Focus. The Back Focus is the distance between the back of the rear piece of glass and the focal plane (when focused on infinity). The Front Focus is the distance from the front piece of glass to the front focal plane, which is where an image would form if you used the lens backwards on a subject at infinity. As alternative interpretation for "F.F." would be Flange Focal Distance, which is the distance from the mounting flange to the normal focal plane in the back. The numbers would be the values of these quantities. Perhaps "22 2/3" is 22 and 2/3 inches, which fits in with your estimate of the focal length as 25 inches.

You can test these interpretations by measuring these distances when focused on a distant object.

If this lens is an Aero-Ektar, then the level of radioactivity when taken apart is not trivial. I have made measurements of a 6 inch Aero-Ektar and am researching the older literature. The radioactivity probably isn't excessively dangerous, particularly if the lens is not taken apart. There is no point in taking unnecessary risks, so I would store the lens at a distance from people. Just based on it being big isn't that much evidence that it is an Aero-Ektar. The focal length 24 inch focal length model is pretty rare.

-- Michael Briggs (MichaelBriggs@earthlink.net), October 04, 2000.