When you look through the lens of a microscope you see a circular area, the diameter of which is known as the field of view. To work out the field of view we need to know the field number and the magnification of the objective lens.
The field number is the diameter of the image area when seen under the eyepiece. This is often written on the side of the eyepiece, as shown on the right. In this case, the eyepiece has a field number of 20mm. So the image, when seen with the eyepiece only, has a diameter or 20 mm.
Consider the image on the right as seen through the compound microscope with the eyepiece shown above. The microscope field of view when the 10X objective lens is used is found with the following formula:
Field of View = Field Number (FN) รท Objective Magnification
Field of View = 20mm / 10 = 2.0 mm
The greater the magnification the smaller the field of view.
For example, if the field of view under low magnification using a 10X eyepiece and a 10X objective lens is 5mm the field of view when using a 40X objective lens would be
5mm X 10/40, which is 1.25 mm.
A ruler is placed under a microscope and viewed using the 10X objecive lens.
What is the field number on the eye piece of this microscope?
Click to see a picture of the same ruler seen under a different magnification.
a) What is the field of view?
b) What objective lens is used to magnify the ruler?
Depth of field is the range of depth that a specimen is in focus. A microscope that has a thin depth of field will have to be continuously focused up and down to view a thick specimen, as shown on the right. Different levels of this specimen are brought into focus by continuously adjusting the fine focus.
With higher magnification comes more shallower depth of fields.