Microscope
Magnification Specifications

Field of View or Field Diameter is very important in microscopy as it is a more meaningful number than "magnification".  Field diameter is simply the number of millimeters or micrometers you will see in your whole field of view when looking into the eyepiece lens.  It is just as if you put a ruler under the microscope and counted the number of lines. 

The chart below will tell you (approximately) what to expect when looking through a microscope with varying combinations of eyepiece and objective lenses.  As an example (in green below), a dual power stereo microscope with 10X eyepiece lenses and 1X and 3X combinations of objective lenses, would have total powers of 10X and 30X and your field of view would be 20 mm and 6.7 mm respectively.  

This means that an object 20 mm (2 cm, or about 3/4 inch) wide would fill up the whole viewing area at 10X and an object about 6.7 mm wide would fill up the whole area at 30X.  As you can see, having the highest power may not be best for your particular application.  When you move to greater magnifications, you sacrifice field of view. 

Try this

Get a metric ruler and place it on the stage of your microscope.  Illuminate from above (if you are using a compound microscope, get a transparent ruler or illuminate it with a flashlight).  See how many millimeters you can see from left to right.  What would be the field of view of this image?  (answer at bottom of page)

Other considerations:  The working distance is the distance from the bottom of the microscope (lens) to the part of the specimen that is in focus.  As you increase the magnification, you decrease the working distance.  If you need to work under the microscope, you will need a large working distance.  Some special microscopes have extended working distances for these purposes.  Zoom microscopes have a fixed working distance throughout the zoom range.  When using a 100X objective lens (1000X total power) your working distance might only be 0.04mm (40um).  The lens will be extremely close to the specimen!  The working distance and the amount of vertical motion of the microscope will also affect the maximum specimen height.  Maximum Specimen Height is how tall an object you can put on the stage and still be able to focus on the top part of the specimen.

Note:  The numbers below will not be the same for all brands of microscopes.

Objective
Lenses
Eyepiece Lenses
  5X 10X 15X 20X
  Eyepieces Eyepieces Eyepieces Eyepieces
  Total
Mag.
Field
Size
Total
Mag.
Field
Size
Total
Mag.
Field
Size
Total
Mag.
Field
Size
1X 5X 22mm 10X 20mm 15X 13mm 20X 9.5mm
2X 10X 11mm 20X 10mm 30X 6.5mm 40X 4.8mm
1X 5X 22mm 10X 20mm 15X 13mm 20X 9.5mm
3X 15X 7.3mm 30X 6.7mm 45X 4.3mm 60X 3.2mm
2X 10X 11mm 20X 10mm 30X 6.5mm 40X 4.8mm
4X 20X 5.5mm 40X 5mm 60X 3.2mm 80X 2.4mm
10X     100X 1.8mm
1800um
       
40X     400X 0.45mm
450um
       
100X     1000X 0.18mm
180um
       

High power microscopes usually have 10X eyepiece lenses so we only list one column above the 4X objective.  If you wanted to look at an amoeba that is 150um long (0.15mm) then you would use either the 40X or 100X objectives..  There are 25 mm in an inch and there are 1,000 um (micrometers) in a single millimeter.

See what a penny looks like at various powers

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Answer to question above:  7.2mm

 
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