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What is the easiest way to measure the magnification of a viewfinder?

Must be able to do it for both electronic (EVF) and optical viewfinders. Also, I only have access to one focal-length.

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2 Answers 2

Use a second camera, preferably a P/S with a small aperture. Take a picture through the viewfinder of your main camera, then a second picture of whatever your main camera is aimed at, and from the same distance. Be careful to use the exact same focal length for both pictures.

Now you just have to compare the sizes of the two images. Take a random element visible on both pictures, measure its size in pixels in both pictures and calculate the ratio.

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Thanks but I am not getting anything sensible. How close do I take the photo through the VF? At the specified eye-point? Does the focal-length matter for that shot? When I take the shot of the object from the same position, what is the same position? Lens front? Sensor plane? Entrance-pupil? Oh and my second camera does not have a sensor plane marker :( –  Itai Jul 12 '11 at 19:40
    
Take the photo through the VF from where the eye is supposed to be placed. The focal length does not matter, but for best resolution, it's better if you can see most of the VF image. Same position means same entrance pupil, but the further the subject the less important it is to be exactly at the same position. If the subject is on the other side of the street, then a few cm error will be inconsequential. –  Edgar Bonet Jul 12 '11 at 20:32
    
Getting close. I've got 6% error still though :( Maybe I should try with the moon as target! –  Itai Jul 13 '11 at 13:56

Viewfinder size = Focal length multiplier * Viewfinder coverage * Viewfinder magnification Check this table: http://photolectic.com/reviews/dslr/dslr-viewfinder-size/coverage-comparison.html

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But the question asks how to measure viewfinder magnification, not compute viewfinder size from a known value. Or are you suggesting that one can fill in the other values and solve for magnification? –  mattdm Dec 27 '11 at 4:55
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Interesting table. But the actual formula is 2 × atan(0.433 × coverage × magnification / f_multiplier) and is irrelevant to the question. –  Edgar Bonet Dec 27 '11 at 9:53

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