Diopter adjustment
A focus ring on the eyepiece that sharpens the screen or tube image for your own eyesight. Set it once, then focus the front lens on the animal.
What it means
The diopter is the adjustment on the eyepiece that focuses the image for your eye. Its range is given in diopters, a measure of lens strength, often written as something like “−5 to +3”. Thermal scopes, monoculars and night vision housings nearly all have one.
There are usually two focus controls. The diopter, at the eye end, sharpens the screen or tube picture for your eyesight. The objective focus, at the front, sharpens the scene at a given distance.
What it changes in the field
If the diopter is wrong, everything looks soft no matter how you set the front focus, and you may blame the sensor or tube. Set the diopter first, by looking at the menu text or reticle until it is crisp, then focus the front lens on the animal.
If you wear glasses, a wide diopter range may let you use the device without them, which keeps your eye close enough to the eyecup to see the whole picture. If you share a device, the next person may need to reset it.
What to look for
- A range that covers your prescription if you want to use the device without glasses.
- A ring stiff enough that it won’t drift when the device rides in a pocket or on a sling.
- On helmet-mounted night vision, a diopter you can reach and turn with gloves on.
How we use it on this site
We record the diopter range for night vision housings when the maker publishes it. It is not scored.