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<!-- comment -->
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Microscope objectives: Back focal plane</title>
<link rel="stylesheet" href="stylesheets/style.css">
<link rel="stylesheet" href="stylesheets/prism.css">
<script src="javascript/python-highlighting/prism.js"></script>
<script async src="javascript/Minimal-MathJax/MathJax.js?config=TeX-AMS_CHTML"></script>
<script src="javascript/update_figures.js"></script>
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</head>
<body>
<section>
<a href="https://amsikking.github.io/">Home page</a>
<h1>amsikking: Microscope objectives</h1>
<a href="./index.html">Index</a>
<h2>Back focal plane</h2>
<p>
The back focal plane (BFP) is an important design and build parameter
that may affect, for example, the kind of tube lens that is used with
the objective to form a finite image. It is also common to image the
BFP onto other optics like scanning mirrors or adaptive optics etc, and
so knowing the size of the BFP may be important. Fortunately, we know
from the infinity correction, that the BFP diameter (\(D_{BFP}\))
scales with the focal length and numerical aperture of the objective
according to:
\[ D_{BFP} = 2 f NA \tag{1}\]
However it may also be useful to know the <em>location</em> of the BFP
for mechanical and alignment considerations, for example how far the BFP
is from the objective flange. Not all manufacturers provide this
information, so it may be down to the builder to find out for themselves.
In practice the BFP is often located <em>inside</em> the objective, i.e.
a focusing laser beam into the rear of the objective will produce an
output of minimal divergence when the focus is some distance past the
flange. Sometimes the only way to find out is to do this test and measure!
</p>
<figure>
<img src="figures/back_focal_plane.png" alt="back_focal_plane.png">
<figcaption>
(<a href="figures/objective_sketches.odp">.odp sketch</a>)
</figcaption>
</figure>
</section>
</body>
</html>