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How to Buy the Right Microscope

Intro» Construction» Optics» Light» Focus» Components» Used» Conclusion

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.

Coarse Focus

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.

Fine Focus

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 good.

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

Metal Focus GearBeyond the 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.

Slip Clutch

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

Intro» Construction» Optics» Light» Focus» Components» Used» Conclusion

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