Reducing the Minimum Focusing Distance
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By purchasing a 1.25" eyepiece adapter, you can use extension tubes to reduce the minimum focusing of distance of
a spotting scope. As you increase the the distance between the eyepiece and the objective lens, you reduce the
infinity focus but at the same time reduce the minimum focus distance of the spotting scope. This provides additional
flexibilty when you run across small nature subjects the you can approach which are closer than the minimum
focusing distance, or when you want to make a composition not otherwise posible.
Depending on the speed with which you think you might need to change to extension, there are several convenient
ways possible to add extension. The simplest way is to buy a couple of one inch extension tubes that screw into the
bottom of a 1.25 inch eyepiece using the filter threads. These extension tubes should be threaded using the same
standard eyepiece threads so they can be stacked if you want more than one inch of extension. I use the extension
tube from the Televue Barlow lens with the glass removed, which works well with my 40mm Plossl and Coolpix
5400. I have also used the modified prime focus adapter as an eyepiece holder without the Barlow, and extension
can be adjusted very quickly by loosening the set screws and lifting up the eyepiece.
This method can reduce the minimum focus distance to around 8 to 10 feet from about 20 feet.
The moth above, about 1.5 inches long, was in a forested area, taking refuge from a shower. The above
photograph is uncropped. Using extension tubes and a flash, I was able to take this photograph from about
12 feet. The same frame could be taken at twice the distance with twice the magnification, but having the
subject nearer with less magnification yields a better photograph as well as making more efficient use of
flash.
