<p>Say I have a scope with a given diameter <span class="math-container">$D$</span>, a given focal length <span class="math-container">$f$</span>, and a given exit barrel diameter <span class="math-container">$B$</span>. It has a maximum true FoV of <span class="math-container">$B/f$</span> (supposing <span class="math-container">$B&lt;&lt;f$</span>).</p>

<p>Would adding a 0.5x focal reducer exactly amount to having a <span class="math-container">$D/(f/2)$</span> scope with barrel size <span class="math-container">$B$</span>, for all intends and purposes (i.e. be optically indistinguishable from an actual scope of diameter <span class="math-container">$D$</span> and <span class="math-container">$f/2$</span> focal length) ? I feel like it should, but I'm unsure whether maximum true FoV would indeed become <span class="math-container">$2B/f$</span> as on a "natural" <span class="math-container">$D/(f/2)$</span> scope.</p>

<p>(You may ignore light absorption from the focal reducer' lens(es))</p>

<p>Conversely, say I have the same initial scope with a given diameter <span class="math-container">$D$</span>, a given focal length <span class="math-container">$f$</span>, and a given exit barrel diameter <span class="math-container">$B$</span> (it still has a maximum true FoV of <span class="math-container">$B/f$</span> (supposing <span class="math-container">$B&lt;&lt;f$</span>)).</p>

<p>Would adding a 2x Barlow exactly amount to having a <span class="math-container">$D/(2f)$</span> scope with barrel size <span class="math-container">$B$</span>, for all intends and purposes (i.e. be optically indistinguishable from an actual scope of diameter <span class="math-container">$D$</span> and <span class="math-container">$2f$</span> focal length) ?</p>

CC BY-SA 4.0
2020-06-04T04:06:35.557Z
Other things like added chromatic and other aberrations should be ignored as well?
uhoh
CC BY-SA 4.0
2020-06-05T07:13:38.280Z
I'm interested in those aspects as well, but maybe as a separate addendum. I'd like to first know the answer for a perfect optical system (still subject to physical limits such as resolution due to aperture, etc.). Then, if some aberrations happen to be doubled or not or whatever, that can be indicated as well I guess.
Parker Lewis
<telescope><optics>
Telescope focal length, reducer and/or Barlow
Parker Lewis
# Telescope focal length, reducer and/or Barlow

(Asked by: username_0 on 2020-06-03)

Say I have a scope with a given diameter $D$, a given focal length $f$, and a given exit barrel diameter $B$. It has a maximum true FoV of $B/f$ (supposing $B<).

Would adding a 0.5x focal reducer exactly amount to having a $D/(f/2)$ scope with barrel size $B$, for all intends and purposes (i.e. be optically indistinguishable from an actual scope of diameter $D$ and $f/2$ focal length) ? I feel like it should, but I'm unsure whether maximum true FoV would indeed become $2B/f$ as on a "natural" $D/(f/2)$ scope.

(You may ignore light absorption from the focal reducer' lens(es))

Conversely, say I have the same initial scope with a given diameter $D$, a given focal length $f$, and a given exit barrel diameter $B$ (it still has a maximum true FoV of $B/f$ (supposing $B<)).

Would adding a 2x Barlow exactly amount to having a $D/(2f)$ scope with barrel size $B$, for all intends and purposes (i.e. be optically indistinguishable from an actual scope of diameter $D$ and $2f$ focal length) ?

**Tags:** telescope, optics

> **Comments:**
> **username_1**: Other things like added chromatic and other aberrations should be ignored as well?  
> **username_0**: I'm interested in those aspects as well, but maybe as a separate addendum. I'd like to first know the answer for a perfect optical system (still subject to physical limits such as resolution due to aperture, etc.). Then, if some aberrations happen to be doubled or not or whatever, that can be indicated as well I guess.  

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## 0 answers

