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Back Focus Calculator


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Not sure if this is the right forum for this topic, but it seems appropriate.

I came across a website today that has a back focus calculator for the many permutations of cameras, scopes and filters etc. Just fill in the equipment you have in the dropdown boxes, and the calculator does the rest. It wll give you the result, and whether you need to add or subtract spacers to your OTA.


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Posted by: @Wyvern

I'm sure that will come in useful for many folks. Thanks for posting.

Thanks. I used it to work out the back focus on my new scope. I currently have a back focus of 56mm om my ED80. My new scope has a back focus of 75. You would think spacers to the equivalent of 19mm would do the job, but this calculator takes the thickness of filters into account also. The end result I need a spacer of 19.67 mm. I think 20mm will be fine, I doubt .33mm will make that much difference.

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