What Is the Depth of Focus of a Microscope?

By Brent Smith

An optical microscope relies on light in order to produce an image, so the amount of light has great influence on the image produced. Two terms used to describe the way this image is viewed are depth of focus and depth of field. There is a great deal of confusion between the two terms, but depth of focus is basically how clear an image is, and depth of field is basically how much of the object under the microscope can actually be viewed.

What is Depth of Focus?

Traditionally people called the depth of field "the depth of focus," but the depth of focus is now used when talking only about the image side of depth. Technically speaking, the depth of focus of an optical microscope is the range of image plane position at which the image may be viewed without appearing out of focus for an object or specimen that does not move or change location during the viewing. This means that the depth of focus is how clearly your specimen or object appears to a human eye.

Depth of Focus Compared with Depth of Field

The technical definition of depth of field is the range of motion along the optical axis an object or specimen can move without impacting clarity of viewing. This means the depth of field is how much the object or specimen can move and still be clear. The depth of focus is the opposite of the depth of field, taking place on the opposite side of the lens. Also, the depth of focus is measured in microscopic units of measurement, such as nanometers, while the depth of field is measured in macroscopic units, such as meters or feet.

Variables in Depth of Focus

While depth of field depends on the resolution of the individual microscope, the depth of focus depends on the magnification. This means the depth of focus can be improved by increasing your magnification. Both resolution and magnification depend on the quality of your microscope's lens, the size of the lens and how much light is actually reflected. This quality is also called numerical aperture.

Improving Focus Depth

While using a high-quality lens and increasing magnification, the focus depth is greatly improved. This also reduces the depth of field. For photomicrographers--people who use microscopes to take pictures of what they are studying--this is extremely important.

Equations for Calculation

One formula to determine the depth of focus quickly is to find the product of the focal length times the f-number (relative aperture) divided by 1000. This formula is best used when the focal length is a representation of the format size. For the most accurate results, the ideal formula is two multiplied by the f-number multiplied by the circle of confusion (rays refracting in the lens creating a spot of imperfect focus) multiplied by the quantity of one plus the magnification factor.