How to Determine the Proper Diopter Numbers on your Leeds’ Firearms and Toolmarks Comparison Microscope

What is a diopter?

Diopter is a unit of measurement of the refractive power of a lens that is used by Optometrists when writing prescriptions for corrective lenses.  For example, someone with myopia (near sighted) might use glasses or contacts with  - 1.5 diopter of  correction.  If you choose to use a microscope without your corrective lenses, focusing eyepieces allow you to dial in a corrective factor.  Each division on the focusing eyepiece and focusing stud is equal to 1 diopter of correction.  Please note that focusing the eyepieces cannot correct for astigmatism, so for people with astigmatism it is recommended that they wear their prescription lenses.

Why is setting the proper diopter number important?

When properly adjusted, these numbers assure that your microscope is parfocal from high to low magnification, and to the camera image for live viewing and image capture. This also assures that a reticle, (crosshair, scale, etc.) which may be mounted in the eyepiece, is in the same focus plane as your primary image.  Most importantly, it assures that the users’ eyes are at rest - or focused to optical infinity - allowing for more comfortable sample examination.  When equipped with a training bridge (dual viewing), Leeds’ Firearms and Toolmarks Comparison Microscope allows the second observer to adjust their own eyepieces so that their image is in focus with both the primary observer and the camera.

Focusing eyepieces and diopter setting

Looking closer at the focusing eyepiece, they are typically mounted in the right eyetube of the main observer’s binoculars. On the Leeds’ Firearms and Toolmarks Comparison Microscope two focusing eyepieces are normally supplied; one with a crosshair for when using the scale on the left stage, and a second focusing eyepiece without the crosshair for routine observations.  Leeds Forensic Systems recommends the two focusing eyepieces because many forensic examiners find the crosshair image distracting when examining samples.  If the crosshair in the eyepiece  is not an issue for the examiner, it can be used for all observation methods.

How to determine the proper diopter number setting

First find a flat sample with large features which will easily allow focus at both extremes of high and low magnification.

Dial the eyepiece on the right side and the stud on the left eyepiece to the most positive (+) position. This will extend the eyepiece mechanically to its greatest height. Please note that the right eyepiece will try to rotate when focusing it; grasp the ring at the base of the eyepiece when focusing to prevent it from turning. This also can be done by tightening the base of the eyepiece in its mount, but it would make the exchange of the left eyepieces more difficult.

Finding your proper diopter numbers

1. Turn the right focusing eyepiece to the most positive position (+).
2. Turn the focusing stud below the left eyepiece to its most positive position (+).
3. Using the focusing mechanism focus on the flat sample at the highest magnification on the zoom body (55X, 80X, or 102X).
4. Zoom down to lowest magnification (5.5X or 6X). At the lowest magnification the image will be out of focus; use the adjustment on the focusing eyepiece to bring the right eye image into focus.
5. Use the focusing stud on the left eyepiece bring the left eye image into focus.
6. Look at the diopter adjustments on both the right and left eyepieces to determine the position, record the numbers for both the right and left eyepieces and perform the procedure two more times. After the third time, there will be a set of three settings for the right and left eyepieces.  Use the average of these numbers for setting the right and left eyepieces diopter.

After this procedure of determining your proper diopter numbers, when focusing on the sample at high magnification and zooming down to low magnification, the image should remain in focus. Please record your diopter numbers for future reference so you can quickly adjust the microscope to your personal diopter settings.