How can I measure the focal length of a small finder lens element?
Asked 3/27/2022
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I have a Bolta Photavit IV with a reverse Galilean viewfinder that is missing the rear element. This seems to be a common failure. If I can disassemble a complete finder, how can I empirically determine the focal length of that rear element so I can identify or replace it? The element is very small, so measuring its curvature directly would be difficult.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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Remove the lens you are to test from its assembly. Take the lens, millimeter ruler, and white paper card outside in bright sun. Hold the lens between card and sun. Adjust card to lens distance to achieve the smallest bright circle of the sun's image. Measure the card to lens distance, this will be the focal length.
Alternate -- Procure a small transparent millimeter ruler. Affix the lens on a small wood block using clay. Illuminate from behind the transparent ruler. Behind the lens. Affix a white (projection screen) card on a small wood block. The idea is to place the lens so that it projects an image of the ruler on the white screen. Adjust lens to ruler and lens to screen distance to obtain a life-size image of the ruler on the screen. Use a duplicate ruler to make this measurement. When life-size (unity) has been established, measure distance rules to image of ruler. Divide this distance by 4. You have discovered the focal length of this test lens.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
4y ago
0
Generated from our catalog & community — verify before relying on it.
If the finder element is a converging lens, the simplest method is to remove it and measure its focal length directly.
In bright sunlight, hold the lens over a white card and move the card until the sun forms the smallest, sharpest bright spot. The distance from the lens to the card is approximately the focal length.
Another practical method is to project an image of a millimeter ruler onto a white screen. Adjust the setup until the projected ruler is life-size (1:1). At 1:1 magnification, the lens-to-subject distance and lens-to-image distance are each about twice the focal length, so the focal length is roughly half either distance.
You can also place the lens in front of a camera or optical setup, focus on a very distant object, and measure from the lens to the image plane; that distance is approximately the focal length.
Important: these image-forming methods work for converging lenses. If the missing rear finder element is actually a diverging lens, it will not form a real image on a card by itself, so you would need a more elaborate optical comparison method using another known lens or a collimated beam.
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