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I have a camera (Bolta Photavit IV) with a simple reverse Galilean finder that is missing the rear element (this is a common problem, they fall out).

If I have a complete finder and disassemble it, how can I find the focal length of its rear element?

Donor cameras with the rear element in their finders are uncommon.

The lens is small so accurate dimensional measurement or profiling would be difficult.

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    \$\begingroup\$ I'm not too familiar with optics, but have you looked into any methods of empirically measuring the focal length of a lens? I remember from high school physics that there were some simple experiments you can do, not sure how it applies to meniscus lenses. Perhaps you've found similar questions on the web? If so, could you please include that in your question such that nobody suggest something you've tried and which did not work? Thanks! \$\endgroup\$ Mar 27, 2022 at 22:08
  • \$\begingroup\$ @SaaruLindestøkke I encountered the idea just yesterday when the incomplete camera arrived. Seeking expertise from this community was the first thing I have tried. In part because this site seems like a good place to have a definitive answer. \$\endgroup\$ Mar 27, 2022 at 22:53
  • \$\begingroup\$ While I'm glad to hear that you think of this community when you need photography expertise, I think it's expected of question askers that they do research prior to writing a question. In my experience doing research, and including that in you question, helps structure improve the question as well as the answer quality. Personally I'm fine with research being very minimal (i.e. a single websearch) as long as it is included in the question. \$\endgroup\$ Mar 28, 2022 at 16:04
  • \$\begingroup\$ @SaaruLindestøkke So nothing constructive, informative, or insightful? Just giving me grief to create drama instead. Sorry I offended you by asking a question out of your league. \$\endgroup\$ Mar 28, 2022 at 18:03
  • \$\begingroup\$ I'm sorry that I cannot provide you with an answer to your question and that you experience this as "drama". I constantly encounter questions "out of my league", that's no issue for me. However, I try to contribute to this site such that it becomes/stays a useful repository of knowledge for all, present and future. Part of that is asking (both new and established) question askers if they can include their research, even if minimal. It's the first point on the how to ask a good question after all. \$\endgroup\$ Mar 28, 2022 at 19:27

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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.

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  • \$\begingroup\$ "Adjust card to lens distance to achieve the smallest bright circle of the sun's image." – Wear eye protection and take care not to set yourself on fire. \$\endgroup\$
    – xiota
    Mar 28, 2022 at 11:14
  • \$\begingroup\$ @ Xiota --You are an alarmist - the tiny diameter lens associated with old camera viewfinders will not harm your eyes in this manner plus fire is not likely. Why interfere with a teaching moment? \$\endgroup\$ Mar 28, 2022 at 14:04
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Here are a few ways measure the focal length of an element:

  • Measure the angles that a laser beam is deflected by different points on the lens. Calculate where parallel rays would converge.

  • Use a lens with known focal length to produce parallel rays of light from an image. (This can be done by looking through the front of a camera lens at a slide on the film plane with the lens set to infinity.) Set the unknown element parallel to the lens. Focus the image onto ground glass. Measure the distance from the ground glass to the center of the element.

  • Put the element in a helicoid attached to a digital camera. Focus on an object that's sufficiently far away. Measure the distance from the center of the element to the sensor.

The last two methods won't work with diverging lenses because diverging rays won't focus.

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