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Re: [OM] Extension Tubes

Subject: Re: [OM] Extension Tubes
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Thu, 12 May 2016 21:12:00 -0400
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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