# Math: how to calculate what focal length is needed to fill the frame.



## PicaPica (Mar 17, 2014)

how can i calculate for FF and APS-C what focal length i need to fill the frame with a certain object?

for the cases when i a) have the angular diameter of the object or b) the size of the object and it´s distance from my camera.


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## neuroanatomist (Mar 17, 2014)

*Re: Math: how to calculate what focal lentgh is needed to fill a frame.*

For (b), you can use the calculator here: http://www.cambridgeincolour.com/tutorials/camera-lenses.htm


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## PicaPica (Mar 17, 2014)

*Re: Math: how to calculate what focal lentgh is needed to fill a frame.*

thanks .. i think i found the answer for a) at wikipedia

http://en.wikipedia.org/wiki/Angular_diameter

after rearranging the equation i get:

focal lenght = sensor size / tan (angular diameter)

so for the moon as example:

24mm / tan (0.5) = 2750mm


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