Building a smaller wire finder - what's the math?

[dante]

I understand the rule that if you want to build a wire finder, the easiest way is to put a film-sized frame the same distance from the eyepiece as the focal length. But if I want the front frame to be smaller, what is the relationship between distance to the eyepiece and frame size?

For example, if I want to build a frame finder for Horseman 6x12 size (56x112) and a 75mm lens, I would normally place a 56x112 frame 75mm (~3") from the peep sight in the back, with the peep site centered at 28mm high for ∞. That's not really practical from a durability/ergonomics standpoint.

Say though I want to get that front frame down to 28x56 (half the size), with a peep sight 14mm high, which is far more manageable, would I place that front frame 37.5mm from the peep sight (half the distance)?

Thanks!

 

[E. von Hoegh]

Yes. As long as the proportions are the same, all is good.