Re: [OM] Extension Tubes

[Chuck Norcutt]

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