The objective lens of a compound microscope is the one at the bottom near the sample.
At its simplest, it is a very high-powered magnifying glass, with very short focal length.
This is brought very close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube.
The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample.
Microscope objectives are characterized by two parameters:
- magnification and
- numerical aperture.
The magnification typically ranges from 4x to 100x.
It is combined with the magnification of the eyepiece to determine the overall magnification of the microscope;
- a 4x objective with a 10x eyepiece produces an image that is 40 times the size of the object. Numerical aperture for microscope lenses typically ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 mm to 2 mm, respectively.
A typical microscope has three or four objective lenses with different magnifications, screwed into a circular “nose piece” which may be rotated to select the required lens.
These lenses are often color coded for easier use.
The least powerful lens is called the scanning objective lens and is typically a 4 x objective.
The second lens is referred to as the small objective lens and is typically a 10 x lens.
The most powerful lens out there is referred to as the large objective lens and is typically 40-100 x.
Some microscopes use an oil-immersion or water-immersion lens, which can have magnification greater than 100, and numerical aperture greater than 1.
These objectives are specially designed for use with refractive index matching oil or water, which must fill the gap between the front element and the object.
These lenses give greater resolution at high magnification.
Numerical apertures as high as 1.6 can be achieved with oil immersion.