colin_seaman2 Posted October 3, 2000 Share Posted October 3, 2000 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 22.2/3I'm not sure what this all means?Specs are:length: 11 inchesfront element: 6" acrossrear element: 5" acrosscircumference: 17 1/2" (at the center)Weight: about 12lbs. <p> Any ideas or comments are welcome! Link to comment Share on other sites More sharing options...
john10 Posted October 4, 2000 Share Posted October 4, 2000 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. Link to comment Share on other sites More sharing options...
colin_seaman2 Posted October 4, 2000 Author Share Posted October 4, 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? Link to comment Share on other sites More sharing options...
bill_zelinski Posted October 4, 2000 Share Posted October 4, 2000 Edmunds Scientific sells iris's in various sizes and mounts, not sure how that would work out and not inexpensive either. Link to comment Share on other sites More sharing options...
john lehman, college alask Posted October 4, 2000 Share Posted October 4, 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. <p> BTW, the lens is slightly radioactive Link to comment Share on other sites More sharing options...
michael_briggs2 Posted October 4, 2000 Share Posted October 4, 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 ofglass and the focal plane (when focused on infinity). The Front Focusis the distance from the front piece of glass to the front focalplane, which is where an image would form if you used the lensbackwards on a subject at infinity. As alternative interpretationfor "F.F." would be Flange Focal Distance, which is the distance fromthe mounting flange to the normal focal plane in the back. Thenumbers would be the values of these quantities. Perhaps "22 2/3" is22 and 2/3 inches, which fits in with your estimate of the focallength as 25 inches. <p> You can test these interpretations by measuring these distances whenfocused on a distant object. <p> If this lens is an Aero-Ektar, then the level of radioactivity whentaken apart is not trivial. I have made measurements of a 6 inchAero-Ektar and am researching the older literature. Theradioactivity probably isn't excessively dangerous, particularly ifthe lens is not taken apart. There is no point in taking unnecessaryrisks, so I would store the lens at a distance from people. Justbased on it being big isn't that much evidence that it is anAero-Ektar. The focal length 24 inch focal length model is prettyrare. Link to comment Share on other sites More sharing options...
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