Objective Lenses: Achromatic vs Semi-Plan vs Plan

Besides some special microscope objective lenses, there are three kinds of objective lenses which are achromatic, semi-plan, and plan.

First of all, all these three objective lenses correct for color, although there's no "achromatic" in the name of "semi-plan" or "plan".

The difference between these three lenses is the focusing area which can be seen from the eyepieces when using high power objective lenses such as 40x, 60x and 100x. The difference from low power lenses are not obvious, which means that can be ignored.

At high power, an achromatic lens has about 65% of focused area acrossing the center. A semi-plan one has about 85% whereas the plan one has 95%.

Due to the correction from the different lenses, the price for those lenses are also different. Here are some links for the objective lenses:

Achromatic:
http://www.microscopenet.com/achromatic-objective-lenses-compound-microscopes-p-377.html 

Semi-Plan:
http://www.microscopenet.com/semi-plan-achromatic-objective-compound-microscopes-p-317.html

Plan:
http://www.microscopenet.com/plan-achromatic-objective-compound-microscopes-p-316.html

Common Myths about Microscopes

1. The microscope with the largest power is the best.
As we discussed above, you need to think about many elements, not just the magnification. Even if you have a very high power microscope, sometimes you still have to use the low power to view your specimen since lower power allows you to have a wider field of view and a larger working distance.

2. "Big eye" eyepieces are better than "small eye" ones.

"Big eye" means the lens glass on the top of an eyepiece is big in diameter. Actually the field of view is decided by the diameter of the bottom (not the very end, but close to the bottom) of the eyepiece. You may find the size of the viewfield from the specification of the eyepiece. For example: WF10X/20, means the viewfield is 20mm in diameter, WF10X/22 means the viewfield is 22mm in diameter. WF10X/22 will have larger viewfield than WF10X/20, no matter if the top lens is "big eye" or not. Of course, for the same viewfield, "big eye" is better than "small eye". If the viewfield is not specified, you may measure it with a vernier caliper of a regular ruler (may not be accurate but can see the difference). Find the smallest ring of the eyepiece frame through the bottom of the the eyepiece and measure it.

How to select Compound Microscope

Compound microscopes are high-power microscopes designed for observing cells, virus, and tiny organisms. Usually the specimen is on a slide. The common magnifications are 40x, 100x, 400x, 1000x, 1600x. They can be used in labs, hospitals, universities, schools and homes. You need to consider about the following aspects when you select a compound microscope.


1. Magnification:
 The total magnification can be calculated by the multiplication of the eyepiece power and the objective power. For example, if you have a pair of eyepieces of WF10X, and the objective is 4x/10x/40x/100x, the total powers are 40x, 100x, 400x and 1000x. For the low-end compound microscopes, 400x is maximum since the image quality is not satisfactory when the magnification goes above it. But it's still good enough for students to study the cells and tiny organisms.


2. Nosepiece:
Nosepiece is a rotating objective turret. Common nosepieces are: Triple(3), Quadruple(4) and Quintuple(5) . More objectives on the same nosepiece give you more convenience to change magnification. They are also convenient to change different type of objectives by just turning the nosepiece. The more objectives you have on one nosepiece, the more cost it would be.

There is another type of nosepiece called reversed nosepiece. A reversed nosepiece is inclined towards the arm of the microscope body. Some others call it backward nosepiece. It allows larger operating space and prevents the objectives from being breathed on.


3. Objectives:
The common objectives are 4x, 10x, 40x, 100x. There are some 20x and 60x. There are different quality lenses for objectives. Standard achromatic lenses help prevent color distortion, semi-plan lenses have improved clarity and flatness while plan lenses do the best to make the image sharp. Most 100x objective is an oil-immersion lens. This allows you to see sharper images. To use it, you need to put 1-2 drops of immersion oil on top of the slide cover. There are also some special objectives, e.g. objectives for darkfield.


4. Viewing head (monocular, binocular and trinocular):
Monocular microscopes only have one eyepiece. That means you may only use one eye to observe the specimen. If you have an electronic eyepiece (USB Digital Camera), you will have no way to view it with your bare eye at the same time. The pros for monocular are inexpensive and light weight. Binocular is the most common selection. It's comfortable for your eyes if you get used to it (it may take some time for beginners to get used to it). Trinocular has a third eyepiece tube for electronic eyepiece. For some models, you may have to pull or push a lever to switch the views. Of course, trinocular is more expensive than the other two forms. The heads commonly have two kinds of inclined angles --- 45° and 30°. There are two kinds of inter-pupillary distance adjustment forms --- sliding and hinge. People have different preferences for the inclined degrees or inter-pupillary distance adjustment forms, cannot say which one is definitely better than the other.


5. Condenser & Diaphragm:
The light from the illuminator is condensed and focused through the lenses in the center of stage. The part containing the lens is called condenser. There are single lens condenser and two-lens Abbe Condenser. Condensers often come with diaphragms. A diaphragms is used to restrict the light traversing through the condenser. There are disc diaphragms and iris diaphragms. The disc diaphragm is used on most microscopes without 100x objective. It's a rotating disc with different sized holes on the circle, fixed under the stage to control the amount of light passing through. Smaller holes are for low magnification (4x and 10x objectives) and larger holes are for high magnification (40x objective). Iris diaphragms are for the microscopes with 100x objectives. An iris diaphragm has a sliding control lever rather than a rotating disc. It has better control than a disc diaphragm.


6. Illumination:
Except special microscopes, regular compound microscopes only have one illuminator on the bottom, unlike stereo microscopes. Tungsten lights are the least expensive but are large and have short lifetime. Halogen lights are compact, bright and have long lifetime. Fluorescent lights are brighter than halogen and the color temperature is cold.


7. Focusing:
The focusing knobs move the stage up and down to make the slide being focused. Some have separate fine and coarse focusing and the others have coaxial knobs. Although most high end microscopes have coaxial knobs since people think it's easy to reach but still, different people have different preferences.


8. Stage:
Stage is the platform that holds the slide. The simplest plain stage is just a metal plate with two clips to grip the slide. It can be attached with a two-way moveable stage with rulers. Mechanical stages are better than plain stages since they can make two-way fine movement. For mechanical stages, there are single layer stages and double layer stages. Double layer stages are moving more smoothly. Some double layer mechanical stages can hold two standard slides, easier to compare two specimens.


9. Accessories you may need:
a. Microscope Digital USB Camera (electronic eyepiece): for watching though computer monitor or TV, taking pictures of the objects.
b. Camera and video monitor adapters.
c. Extra condensers and attachment: bright field, darkfield, aspherical lens, polarization set.
d. Optional illumination: various Kolher illumination sets.
e. Thermal control stage: keep the stage at a certain temperature to make the specimen alive.
f. Attachable mechanical stage for plain stage.
g. Extra eyepieces: P16X is another commonly used eyepiece.
h. Slides and immersion oil: blank slides and covers, prepared slides, immersion oil.
i. Spare bulbs.

How to select Stereo Microscope

Stereo microscopes are low-power microscopes designed for observing insects, plants, rocks, jewels, stamps and coins, etc., also used for inspections. It is also possible being used to observe slides. The total magnifications are 10x, 20x, 30x, 40x. In some cases you may need 60x-90x. You need to consider about the following aspects when you select a stereo microscope.

1. Magnification:
The total magnification can be calculated by the multiplication of the eyepiece power and the objective power. For example, if you have a pair of eyepieces of WF10X, and the objective is 2x and 4x selectable, the total power is 20x or 40x. Usually 20x-40x is enough for regular use but it's not bad to have extra eyepieces with higher power (e.g. WF20X) if budget is not an issue.

2. Zoom or fixed power:
Zoom microscopes have zoom objectives, just like your zoom cameras. The magnification is gradually changed by turning the zooming knob. The common zoom ratios are 1:4 and 1:6.5, or higher. The higher ratio, the better, but the more expensive. The popular objectives are zoom 1x-4x and zoom 0.7x-4.5x.

Fixed power stereo microscopes are inexpensive. They have fixed power objectives. Usually they have one or two rotatable objectives, e.g., 2x, or 4x, or 1x/3x, 2x/4x. The cost for having zoom or fixed power is significant.

3. Number of eyepieces (monocular, binocular and trinocular):
Monocular microscopes only have one eyepiece. That means you may only use one eye to observe the specimen. If you have an electronic eyepiece (USB Digital camera), you will have no way to view it with your bare eye at the same time. The pros for monocular are inexpensive and light weight. Binocular is the most common selection. It's comfortable for your eyes if you get used to it (it may take some time for beginners to get used to it). Trinocular has a third eyepiece tube for electronic eyepiece. For some models, you may have to pull or push a lever to switch the views. Of course, trinocular is the most expensive out of the three forms.

4. Working Distance:
Working distance is the distance between the bottom of the objective and the top of the specimen when it is focused. The larger working distance, the better. Large working distance allow you to have more options for illumination (like ring light, gooseneck cold light) and spare room for operation. The working distance could be significantly changed by adding auxiliary objectives. Usually a 0.5X auxiliary objective could get double working distance.

5. Illumination:
Illumination is important for microscopes. Two lights (upper and lower) are better than one. Being able to adjust brightness is better than not being able to. For boom stand microscopes, ring lights or gooseneck cold lights are the best options. Tungsten lights are cheap but have large size and short lifetime. Halogen lights are compact, bright and have long lifetime. Fluorescent lights are brighter than halogen and the color temperature is cold (5000K-7800K). LED ring lights have the longest lifetime and produce very little heat, better for long time operation.

6. Quality of lenses and mechanism:
The quality of lenses and mechanism has direct impact on the sharpness and accuracy of the image. The appearance of the product will show the quality of the products. You may have a basic idea from the photos.

7. Accessories you may need
a. Microscope Digital USB Camera (electronic eyepiece): for watching through computer monitor or TV and taking pictures of the objects.
b. Darkfield: for observing three-dimensional objects-- diamonds and gemstones. It creates beam lights to watch the details not available through other forms of lights.
c. Jewel tweezer: for jewel or other object inspection. 3-way movement and rotation make it easier to watch from different angles.
d. Boom stand: for viewing objects with large area.
e. Extra light source: ring light or gooseneck cold light could be a backup light source.
f. Extra eyepieces: get wider range of magnification.
g. Auxiliary objective: usually a 0.5x auxiliary objective gives you large view field and double working distance.
h. Spare bulbs, ring light tubes with different color temperatures.