Chapter 4. Microscope Focus
Okay, we've discussed good optics and lighting,
now we've got to focus.
The focus system on a microscope brings the subject that you want to observe
into the focal plane of the objective lenses. A microscope's focus system
will have one or two focus knobs, and perhaps a "slip clutch" - something
we'll talk about shortly.
Every microscope has at very least a coarse focus. If a scope has just one
focus knob, it is a coarse focus. This knob will move the subject rather
quickly through the focal plane - that is, it doesn't take a whole lot of
turning to get something in focus.
At times it is more difficult to hone in on a sharply focused image using
only a coarse adjustment. Nevertheless, many people find that a single, coarse
focus knob is all they need.
A fine focus adjustment was at one time a feature just for high-end instruments.
Increasingly though, it is being seen on student microscopes - and that is
To understand the need for fine focus, you've got to think about what is
going on when something is under magnification.
We remember that the subject is being magnified horizontally (i.e., a pinhead
is magnified and stretched out as big as a dinner plate), but we forget that
vertical magnification is taking place as well.
Think about it, at 400x, something as thin as a sheet of paper, is magnified
to the thickness of a 800-page book, every page with information on it!
(Remember, both sides of the pages in a book are numbered, so it takes 800
pages to get 400 sheets of paper.)
Now as you focus, you need a light touch to look at the various levels (the
800 pages) of the object. Fine focus gives you that touch.
Using coarse focus to focus, at high power, on the various features of say,
an ant's eye, a fly's wing, or some cell tissue, is a difficult task. In
fact, without fine focus, many students never notice that those features
are there. With fine focus, however, nothing could be simpler.
Here's something to remember if you are considering saving a few dollars
by doing without a fine focus. If your scope was built without a fine focus,
it will never have it. It cannot be added later.
Another thing to consider, if you do not have fine focus, you really should
not attempt to add magnification over 400x to your scope - because focusing
can become quite difficult.
A fine focus adjustment also makes a microscope easier for children to use.
When is fine focus NOT fine focus?
We recently examined a relatively inexpensive microscope that is heavily
marketed on the Internet these days. While claiming to have coaxial "fine
focus" we noticed right away that it was exceptionally coarse! One revolution
of the fine focus knob moved the stage just over two millimeters. After measuring
that, I made the same measurement on the 3088 microscope: One turn moved the stage
one tenth of one millimeter. The 3088 provides the finesse that is needed
in a fine focus, while the cheaper scope did not.
Fine focus is standard in our popular 3088, and is found on all of
GreatScopes professional scopes as well.
Focus Gear Construction
knobs, much of the focus system on a scope is hidden from view, inside the
scope, but is important nevertheless. If you intend for your microscope to
serve you for many years to come, you'll want to be sure that the internal
focus gears themselves are metal. Many otherwise sturdy scopes use plastic
or nylon gears, raising durability issues. If your microscope is an investment
that you want to last, remember that plastic and nylon just will not hold
up in the long run.
You will usually have to ask the dealer to be sure that you are getting focus
based on a "metal gear system with no plastic parts in either the coarse
or fine focus". You will be surprised how many otherwise "metal" microscopes
have plastic or nylon gears!
(Sadly, on the cheaper microscope mentioned in the section above, the stage
was held to the focus train by two metal screws - screwed into a PLASTIC
bar. That may last months, it may last years. But it certainly won't hold
like the chromed steel machine screws in metallic alloy components on the 3088, make no mistake about it.)
All of our microscopes have long lasting metal focus gearing and components.
On student microscopes, at the top and bottom of their focus range, young
users will at times have the tendency to want to continue cranking down (or
up) on the focus once it has reached the end of its range. A microscope that
is equipped with a "slip clutch" will allow the focus knob to slip (i.e.
turn in place) without damaging the scope's focus gear system.
All of our student microscopes have a slip clutch.
The next chapter, Microscope Components, will teach you some of the terms you'll see in various microscope descriptions, and how these components might be helpful on your microscope.
Go to MICROSCOPE COMPONENTS