Chapter 2. Microscope Optics
As you might imagine, the optics, or lenses, are the most important component
in a good microscope. Remember, however, they are just a part of the whole
package. Great lenses without a quality focus system will be next to worthless
(more on focusing later). So, while you read, remember that you'll want to
consider and evaluate a scope as a whole unit, lenses and all.
Achromatic Glass Objectives
"DIN" is an international standard for microscope objective lenses. "DIN"
stands for "Deutsche Industrie Norm". Occasionally, you might see "JIS",
which is a Japanese standard.
You would be wise to purchase a scope that adheres to the DIN standards (of
threading and length). Doing so, you'll be assured that in the future should
you lose or damage one of your objective lenses (the lenses usually found
in a cluster beneath the head of the scope), you can replace it with a lens
from nearly any microscope company in the world.
When I was a young boy, I had a nice toy microscope that was not DIN. I took
it to school one day and the lenses were stolen. The manufacturer was nowhere
to be found, and lenses from other manufacturers would not fit. What remained
of the scope was rendered worthless - all for lack of a standard!
Another important term relating to objectives is "achromatic". This term
makes reference to several things.
First, the lenses are constructed to be "color corrected". Each objective
lens on a microscope can be built with ten or more glass lenses. If the design
and construction is not done properly, some colors are sent out of the focal
plane, and thus are unseen. (Focal plane refers to the area in focus, which
is by definition a specific distance from the objective.)
If the lens is not color corrected, there are things that you simply will
not see with your microscope. Achromatic lenses are color corrected.
Secondly, the achromatic standard specifies that the center 60% of the field
of view appears as flat and focused without aberration. (An aberration is
an optical distortion caused by a defect in a lens.)
When constructing fine lenses, very exacting processes are followed. With
an Achromatic lens, any chromatic (color) and spherical (focus/flatness of
field) aberrations will be in the outer 40% of the field of view. Usually,
the outer rim of the field of view will appear to curve up out of focus.
This is normal, and since our tendency is to center the subject, most will
not even notice these tendencies.
You may think, "I want my lenses to be 100% free of aberration!" Lenses of
this type (called "Plan Achromatic"), are expensive, and are usually found
on the fine medical and research scopes, usually costing $1000 dollars or
Achromatic lenses will serve you well for most student, school, and hobbyist
The next grade is "Semi-Plan". Aberrations are generally located in the outer
20% of the field of view. Finally, "Plan" optics are exceptionally flat field
to the edge, and for all practical purposes, are 100% free of aberration.
By contrast, cheap toy store microscopes usually have plastic lenses with
All of our scopes from the and Student Series up are DIN threaded and have fine
achromatic objectives. Scopes in our Revelation and M2 professional line
also offer semi-plan and plan objectives.
Our customers (which have included doctors, nurses, lab technicians,
pathologists, teachers, homeschoolers, and hobbyists) have been delighted
with the superb optics on our scopes.
Did you know that GreatScopes has a Low Price Guarantee?
We've taken a look at the objectives, now we'll talk about the lens closest
to your eye, the eyepiece (also called the ocular). You should look for a
microscope with a "wide field" eyepiece. On a wide field eyepiece, the lens
opening is significantly larger than one that isn't wide field.
This will help you in two ways.
First of all, it is easier to position your eye to see into a wide field
eyepiece. The reason is simple. Imagine trying to peer into a box through
a pinhole. It would be pretty tough. Now think about trying to look in through
a half-inch hole. The larger the hole is, the easier it is to see within.
It is the same way with the microscope.
The lens in a wide field eyepiece is usually 18mm, generally as large as
a U.S. dime. This makes it easier to position your eye for viewing.
Best of all, it also makes it much easier for children to see. I have seen
two and three year old children look into a wide field lens with no problem,
and see what had been brought into focus. (Young children don't have the
fine motor skills to focus and such - they'll need help with that- but they
will have no problem seeing while using a wide field eyepiece.)
A wide field eyepiece will also widen your field of view, that is, the width
of what you see. As a result, you will not have to move the slide as much
while you are viewing.
Let's discuss the topic of changeable eyepieces.
If your microscope features changeable eyepieces, change them quickly when
doing so. The reason is that each time you change eyepiece(s), you can introduce
dust into the microscope in the places that are hardest to clean. It takes
less than a second if you prepare for it properly.
Your best bet is to make a decision concerning what eyepiece and objectives
you need or intend to use most, install them on the scope, and leave them
alone as much as possible.
To keep down dust intrusion in your scope, keep it covered with the dust
cover when not in use, and minimize the removal of objectives and eyepieces.
Please understand that microscope resolution (or the ability to see close
but separate points as distinct) comes from the objective lenses, not the
eyepieces. All an eyepiece can do is magnify the resolution that is already
provided by the objective.
Perhaps an illustration from photography will best explain this concept.
If you were to take a photograph of your hand, and then magnify that photograph
1000 times, you will not see microscopic skin cells. Once the picture is
taken, the resolution, or the amount of detail, is forever locked in. Intense
magnification will only reveal the graininess of the film used, not increased
detail in the subject photographed.
In a similar fashion, it is the OBJECTIVE lenses that provide the resolution,
that is, the amount of detail that will be captured and relayed to the eyepieces.
The eyepieces can only magnify that detail, but cannot add to it. By nature,
a higher power objective of the same grade is going to collect more detail
than a lower powered one.
A 40x objective and a 10x eyepiece will result in a higher resolution (sharper
detail) image than a 20x objective and a 20x eyepiece. Total magnification
is the same (achieved by multiplying the two numbers), but the detail, the
resolution, will be better with the 40x objective.
GreatScopes' microscopes feature wide field eyepieces. All of our
student/hobbyist scopes employ the widely accepted 40/100/400x configuration,
which provides an excellent range of magnification. Several student models and all of
our professional line also have 1000x.
Monocular or Binocular?
We are frequently asked if it is better to have one eyepiece or two.
There is not a "one size fits all" answer to this question. The answer depends
partially upon your situation - that is, how you will use the scope, and
on your budget.
If you will be using your microscope day in and day out for hours at a time,
you need binocular (two eyepieces). There is no question about it. Binocular
viewing is much more comfortable because you don't have to train your brain
to ignore the information from one eye. You'll notice that just about every
professional microscope on the market is binocular. Those who use these scopes
comfort associated with two eyepieces.
That having been said, if your primary intent for this microscope is for
use by a child, you actually may find that a monocular (one eyepiece) microscope
is more appropriate for them. Sometimes children can have a difficult time
with the interpupillary adjustment. Just like a pair of binoculars, a binocular
microscope is adjustable to allow for different size people. The eyepiece
distance is adjusted until a single image is seen. Sometimes little ones
have a tough time with this. If you must have binocular though, just have
them move their eyes over one space and just use one eyepiece until they
are a little older.
The next Chapter, Microscope Lighting will teach you the four major types of lighting, and help you decide which is the best for you. Go to MICROSCOPE LIGHTING.