Optical formula for focussing flange distance > F

**Nick_3536** · 24-Mar-2005, 16:14

I'm trying to figure out how badly I'll need/want a recessed board. I'm wondering if the formula below holds if the flange distance is longer then the lens focal length? Or if maybe I should use the flange focal distance for the focal length? Or should I be looking for a new formula.

1/u + 1/v = 1/f

**Michael S. Briggs** · 24-Mar-2005, 18:16

If the focal length is "f" and the Flange Focal Distance is "FFD", define the difference "D" to be D = f - FFD. D may be positive or negative -- for most lenses D will be positive. Now measure the distance from the film or ground glass to the flange (i.e., lensboard face, shutter back), "d". Then the image distance "v" is v = d + D. This value of v may be used in the equation to determine the distance "u" to the object in focus.

A simple case: the lens is focused on infinity. Then d = FFD, so v = d + D = FFD + (f - FFD) = f, and 1/u + 1/v = 1/f gives u = infinity, as expected.

**Nick_3536** · 24-Mar-2005, 21:03

Thanks. The formula proves I'll need a recessed board.

**Emmanuel BIGLER** · 25-Mar-2005, 04:32

If you want to be very precise in principle you should in the formula 1/F = 1/U + 1/V take into account the distance between principal planes. u and v are accounted from those separated points.
For most wide angle view camera lenses, principal plane separation is small and will not help you very much but does add in the right direction a few useful millimetres to the flange focal distance. The only way to increase the flange focal distance for short focal lenses is to use an assymetrical design of retrofocus type (common in 35mm and medium format for reflex camras) where the FFD is significantly bigger than the focal length.
Ultra wide angle view camera lenses in the 24 mm(new 'digital' series) to 60 mm range do exhibit a small amount of retrofocus effect in order to ease focusing and movements.