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Re: [OM] Extension Tubes

Subject: Re: [OM] Extension Tubes
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Fri, 13 May 2016 14:51:39 -0400
f is simply the focal length of the lens. When an object is at infinity the image is formed at distance = the focal length.

S1 is the object distance from the lens and S2 is the distance to the image formed by the object. If S1 = infinity then S2 = f. As the object comes closer the image distance starts increasing until when S1 = f then S2 will equal infinity.

But all of these distances are relative to the optical center of the lens. Not easy to determine in a modern lens.

Chuck Norcutt

On 5/13/2016 10:22 AM, Chris Trask wrote:
     This is a bit confusing for now as my knowledge of optics is insufficient 
to comprehend the meanings of f, S1, and S2.  References 23 and 24 on that 
Wikipedia page will help, but I need to read those carefully.

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.


When the going gets weird, the weird turn pro
     - Hunter S. Thompson

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