It's effect on back focus is that there is a specified distance at which the reducer has the 37% reduction. Farther back, the focal length of the scope shortens. For instance, with a 2" star diagonal, the f/ratio could be f/5.5 or even f/5. It doesn't change the overall amount of focus available in the scope, though.
Eye relief is not affected, as far as I know.
I used one for 11 years and never noticed that focusing became more sensitive. I did increase the focus knob diameter to 64mm, however, to slow focusing down and make fine focusing easier. That larger focus knob also made focusing at f/10 easier.
A focal reducer does NOT mean the scope has more illumination at the edge of a particular true field, though. If the scope starts displaying significant vignetting with a 1.2 degree field at f/10, it will at f/6.3 as well. So a focal reducer doesn't allow any larger a maximum possible field in the scope than the scope can have without it. it does, however, allow that maximum field with smaller, lighter, and less expensive eyepieces. You could easily spend as much for one 2" long focal length maximum field eyepiece as you would for a whole set of decent 1.25" eyepieces.
Don
Quote:
Originally Posted by Solanum
Thanks for this, I was actually going to ask about a focal reducer in a new thread, what are the downsides? Presumably there is less back-focus? Does this also affect the eye-relief? Does focusing become more difficult (the travel on the focus knob is a lot less than I am used to, which makes it a bit more fiddly)?
Everard
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