extension tubes

Has anyone written a calculator program to figure focusing ranges for a
zoom using extension tubes?

Inputs: zoom short and long focal length, nearest focus distance and
extension tube length

Output: nearest and farthest focus distance

Thanks,
Jim

JimKramer wrote:
Has anyone written a calculator program to figure focusing ranges for a
zoom using extension tubes?

Inputs: zoom short and long focal length, nearest focus distance and
extension tube length

Output: nearest and farthest focus distance

This is straight forward if you assume the zoom is a simple lens, ie
entrance and exit pupils equal and front and rear nodal points in the
same place in the middle of the lens.
FL Focal length setting
EX Extension
D Subject to lens distance
FD Marked Focus Distance

D = 1/((1/FL)-1/((FL*FD)/(FD-FL) + EX))

The problem is that zooms are not simple lenses. They are retrofocus,
telephoto, or in some cases both. The nodal points and entrance/exit
pupils are not the same and vary with the zoom setting. It is possible
to measure these zoom lens characteristics at each focal length and
correct the formula. This is after all what the zoom macro lens
designer did to determine the macro ratio. A much simpler (and more
accurate) solution is to buy some extension rings and measure the
ranges on your zoom lens for yourself. You can just hold the lens in
front of your camera with a gap equal to each standard extension ring
and measure that way if you don't feel like investing in rings.