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## Main Question or Discussion Point

Hi,

Have a query on the back focal plane diameter of an objective.

I have got an equation back focal diameter = 320mm X NA / M

where NA is the numerical aperture and M is the magnification.

Just wondering is there any explanation for it?

Thanks

Have a query on the back focal plane diameter of an objective.

I have got an equation back focal diameter = 320mm X NA / M

where NA is the numerical aperture and M is the magnification.

Just wondering is there any explanation for it?

Thanks