Microscope Depth of field and depth of focus explained

 A common limitation you’ll run into with microscopes is depth of field.

Depth of field (sometimes called depth of focus) dictates how much of your sample will be in sharp focus, in a vertical sense rather than horizontally.

It can be altered, just a bit, and sometimes even worked around, but there’s no escaping it. 

What is depth of field?

If you’ve ever seen a photo or a movie where the background is blurry while the target is still in focus (like the close-up of a yellow dandelion against a hazy field), you’ve encountered the practical effects of depth of field. The same is true for microscopy.

Every lens, whether in a camera or a microscope, has a particular focal point – a set distance from the lens where everything is in perfect focus. 

Moving closer or farther away from that focal distance will cause an object to become progressively blurrier as it moves out of focus.

Depth of field is a measure of how far your object can be away from the focal point while still remaining acceptably in-focus. It denotes the range of distance where objects are still sharp and detailed.

A shallow depth of field will provide only a narrow band in which objects can be in-focus, while a deeper depth of field will allow you to see the details on objects much farther away from the focal point.

Dealing with depth of field

In more practical microscopy terms, depth of field determines how much of your sample will be vertically in focus.

Some of the Dino-Lite digital handheld microscopes have the function Extended Depth Of Field EDOF this enables you to capture images that are fully in focus.

A larger depth of field will allow you to see more of your sample, or even allow you to view it at an angle due to the larger focal area. For this reason, it’s particularly useful for electronics inspection, repairs or soldering.

But while a greater depth of field is often desirable, it can’t really be achieved without setbacks.

Depth of field can be easily increased or decreased using stereo microscopes by fitting a microscope barlow lens.

You can increase the depth of field by switching to a lower magnification or numerical aperture (NA), but there are the obvious drawbacks of lower resolution and reduced magnification.

Focus stacking

Also known as Z-stacking, this is a technique where you take a series of images of your sample at different focal depths, then use stacking software to compile the collection of layers into a single complete picture.

The more layers you have, the more detail you can include. It’s a fairly convenient way to overcome the limits of depth of field.

There’s no easy way around depth of field, but with a little time (or money) you can alleviate its influence.