Go here <https://en.wikipedia.org/wiki/Lens_%28optics%29> and scroll
down about 40% of the page and see the illustration for the "thin lens
formula" under "Imaging properties".
In the illustration, if we assume that the lens is focused at infinity
an image would be formed at the focal length f. If you impose an
extension tube the distance to the image is now S2. So you must use the
thin lens formula to solve for the image distance S1.
1/S1 = 1/f - 1/S2
If we plug some numbers: f= 50mm, extension = 10mm, S2 = 60mm we get:
1/f = 0.02
1/S2 = 0.0167
S1 = 1/0.0033
S1 = approx 303mm
This will be an approximation since your real lens is not a thin lens of
negligible thickness and, of course, the object distance S1 is measured
from the optical center of the lens which is not precisely known to you.
Chuck Norcutt
On 5/12/2016 2:10 PM, Chris Trask wrote:
Looking though all of the books that I have on nature and close-up
photography I was not able to find any mention of how to calculate
the maximum focal distance for a given extension
--
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