Depth of Focus and Why Is It Important

Why is Depth of Focus Important? And What the Heck is Parfocality?


The less time spent focusing means faster surgery and less stress caused by muscle and eye fatigue

Depth-of-Focus vs Depth-of-Field:

The vertical portion of the surgical view that is in focus at any range of magnification is Depth of Focus (DOF).  The marginal area of focus on either side of your surgical view relates to Depth of Field. They go hand in hand but we will concentrate on the former.

Here is how you can maximize your DOF, increase your resolution as well as reduce constant focusing:

Initially, if you focus on object #1 at low magnification (2x), your object will be out of focus at 12x high magnification. If you start at high magnification and focus on object #2, you will be in focus throughout the range of magnification (#3). As a result, you have Parfocus.

Depth of Field
Parfocality Diagram

Depth Perception:

Depth of Focus and Depth of Field are mechanical. The visual ability to perceive the world in three dimensions and enabling judgement of distance is Depth Perception. The auto iris and/or manual iris plus the stereo base of the optics enhance depth perception. In addition, the wider the stereo base the better depth perception.

Haag-Streit Surgical Microscopes have the widest stereo base of any operating microscope.

Stereo Base

Personal Parfocus adjustment:

It’s one thing to keep the microscope in parfocality, however it’s critical that you obtain your own personal parfocus settings. Here is how:

A. Position the microscope above a flat, stationary surface.

B. Using a pen or pencil, makes a dot on a piece of white paper to serve as a focus target and place it within the illumination field of the microscope.

C. Set both of the eyepiece diopter settings to “0”.

D. Set the microscope on its highest magnification setting and focus until a sharp image is obtained.

E. Being careful not to physically shift the microscope position, change the magnification setting to its lowest position. Then, focus left and right eyepieces, one at a time, by turning the diopter ring until the image is clear and sharp. Tighten the diopter lock button to lock in this position and record the settings for future use.

F. Each operator will then have their particular settings which are to be dialed in whenever that particular operator uses the microscope.

This procedure does not have to be repeated by the same operator each time the microscope is used. However, due to changes in eye correction associated with time, it is recommended that this procedure be repeated by the operator once or twice per year.

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Focal Length and Working Distance

Do you have enough room under your microscope?


Focal length and working distance are critical. You need enough room under your microscope to accommodate your instruments plus you want to consider how much anatomy you want in your Field of View. Surgical microscopes come in two types: fixed focus and variable focus.

Fixed focal length

For ophthalmic procedures, fixed focal length convergent lenses are used. Usually f=175 or f=200.  Some operating microscopes for ENT procedures also may use a fixed front lens.

Variable focal length

For neurosurgical, spine and ENT procedures, the focal length is variable. For example, the objective lens combination can be configured so the focal length is from f=200 to f=450. So any object within f=200 to f=450 will be in focus.  Any object outside f=200 to f=450 will never be in focus. The minimum working distance from the surgical site objective is 200 mm . So if an instrument is 300 mm long you would not want the microscope at 200 mm.

Also, remember that with increased focal length (working distance) more of the objective will be in view. This is called Field of View (magnification aspect discussed later).


The “Front Lens” of the microscope is the convergent (double convex) lens. The focal length is the distance between the front lens and the object at F. It is usually imprinted with f=xxx on the lens’ bezel.

Working Distance:

The working distance can then be approximated. For example if the front lens has a focal length of f=200, as a result the object will not focus at less than 200 mm working distance. In addition, using a 300 mm instrument would not be advised.

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