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Got another lens today E Suter No. 5 Rapid Portrait
On 10-10-20 14:11 , Cheesehead wrote:
Big brass beast. No aperture or focal length markings. Rear glass diameter is right at 3 inches. Uses Waterhouse stops, but none included. Can you make your own stop plates out of metal or plastic? I'd assume you could start with a nominal diameter and measure a constant light source with an incident meter on the ground glass and then work up a bunch of plates for any stop you may want. -- gmail originated posts filtered due to spam. |
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Got another lens today E Suter No. 5 Rapid Portrait
On 10/20/2010 8:46 PM Alan Browne spake thus:
On 10-10-20 14:11 , Cheesehead wrote: Big brass beast. No aperture or focal length markings. Rear glass diameter is right at 3 inches. Uses Waterhouse stops, but none included. Can you make your own stop plates out of metal or plastic? I'd assume you could start with a nominal diameter and measure a constant light source with an incident meter on the ground glass and then work up a bunch of plates for any stop you may want. I think Richard Knoppow can tell us how to actually determine the correct hole size for a given stop, by calculation using such things as the entrance pupil size, focal length (which I guess is still unknown in this case), etc. Though you could probably get inside the ballpark with your method ... -- The fashion in killing has an insouciant, flirty style this spring, with the flaunting of well-defined muscle, wrapped in flags. - Comment from an article on Antiwar.com (http://antiwar.com) |
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Got another lens today E Suter No. 5 Rapid Portrait
"Alan Browne" wrote in message ... On 10-10-20 14:11 , Cheesehead wrote: Big brass beast. No aperture or focal length markings. Rear glass diameter is right at 3 inches. Uses Waterhouse stops, but none included. Can you make your own stop plates out of metal or plastic? I'd assume you could start with a nominal diameter and measure a constant light source with an incident meter on the ground glass and then work up a bunch of plates for any stop you may want. Its easy to make Waterhouse stops. The original material was probably brass blackened chemically. The first step is to measure the focal length of the lens. This can be done using a view camera as an optical bench. The key is that the focal length of a lens is the difference of the position of the lens when focused exactly for infinity and the distance when its focused for an exactly same size image. It is also exactly one fourth the distance from image to object when set for an exactly same size image. To focus at infinity you can autocollimate the lens. To do this you need a flat mirror to fit over the front of the lens. A first surface mirror is ideal but a flat shaving mirror will do. The second thing you need is a pen light or similar small light source. Ideally it should be mounted behind a card with a small hole in it but can be simply pressed against the ground glass of the view camera. In either case place the mirror over the lens and adjust it so that the reflected light going back through the lens falls close to but not on the source. Then adjust for sharpest focus. Make a reference mark on the camera bed to show the lens position. The lens is now focused at exactly infinity. If you now set up for an exact same size image (unity magnification) and mark that position the difference between the two focus points is the exact focal length. You can doube check this by measuring the distance from image to object and deviding by four, they should be the same. Now that you knwo the focal length you can find the f/stop size. The f stop is defined as the size of the focal length devided by the size of the _entrance pupil_. The entrance pupil is the iamge of the stop as seen from the source or object side of the lens. Now, to measure it use the same arrangement that was used to autocollimate the lens to reset it for exactly infinity focus. Now, place a translucent screen over the front of the lens. A ground glass is ideal but even tracing paper will work fine. Now place the small lamp at the focal plane of the lens and measure the diameter of the circle of light projected onto the screen. _That_ is the entrance pupil and can be measured. The entrance pupil is the physical size of the stop magnified by the glass in front of it. Depending on the design of the lens it can be smaller or larger than the physical stop. To make Waterhouse stops you need only to make a test stop of known size and measure the size of the entrance pupil it produces. The ratio of physical stop to entrance pupil will be the same for all stops _at infinity_. All lenses except for some special purpose ones have their stops calibrated at infinity focus. The _effective stop_ at other distances can be easily calculated but is not photographically significant where the distance of the object from the lens is more than about five focal length. Once you know the effective magnification of the lens you can produce a series of stops for it. The stops should be fairly thin and the edges of the hole should be smooth. The exact shape of the card will depend on the requirements of the lens mount. It may need some sort of flange to keep light from getting in around the slit. Even though Waterhouse stops were common in the days before adjustable iris stops were invented they continued to be used for many years for special purposes. In particular for the stop used in process lenses used for producing half-tone printing plates. It was found that the shape of the stop was important to the shape of the dots. It was common to use square stops and for color work lozenge shaped stops placed at the optimum angles for the various colors. Most process lenses had slits for using either Waterhouse stops or filters even though they also had regular iris stops. I can't remember what I knew of Suter. Its not mentioned in the index of Rudolf Kingslake's book on lens history but I seem to remember reading some history of it. -- -- Richard Knoppow Los Angeles, CA, USA |
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