Focus and Diopter Adjustment in Night Vision

It's one of the most common questions our technical support team gets: "my tube looks blurry, is it defective?" The answer has always been no. The issue comes down to a very common mix-up between two distinct adjustments found on every monocular or binocular night vision system: objective focus, and diopter adjustment on the eyepiece. Two rings, two functions — and it's essential not to confuse them.

Two rings, two functions

On a standard night vision tube, you'll find:

• The focus ring, usually located at the front, on the objective lens. It adjusts sharpness relative to the distance of the scene you're observing.
• The diopter ring, located at the rear, on the eyepiece. It compensates for your own eyesight, much like a pair of glasses would, by adjusting the sharpness of the image displayed on the phosphor screen — not of the scene itself.

That last point is where the confusion comes from: diopter adjustment doesn't focus on the landscape, it focuses on the phosphor screen inside the tube, tuning it to your eye.

The concept of infinity, and why it changes everything

In optics, "focusing at infinity" refers to setting the focus so that objects at a sufficiently large distance — in practice, roughly anything beyond about 30 meters — appear sharp. Beyond that threshold, light rays reaching the objective lens are considered parallel, meaning a single focus setting keeps everything sharp, from the near ground all the way to the horizon.

That's why, barring a very specific need (close-range observation under a few meters, reading a document), it's almost always preferable to set the objective focus to infinity rather than to some intermediate distance. An infinity setting delivers the greatest usable depth of field and avoids having to re-adjust focus every time the distance to your subject changes in the field — a fiddly move in the dark, with gloves on, or while moving.

Your Silicate Systems device: collimated and set at the workshop

Every device that leaves our workshop is collimated and set to infinity before shipping. This step is a core part of our quality control process: it ensures the tube's optical axis is properly aligned and that the objective focus is positioned optimally right out of the box.

In practical terms, this means that upon receiving your device, you do not need to touch the objective focus ring. The only adjustment that remains personal, and that we can't perform on your behalf, is the diopter — since it depends on your own eyesight.

The one step that's on you: setting the diopter

Diopter adjustment is unique to each user and should be set upon receiving the device, then rechecked whenever it changes hands:

1. Point the device at a target object, with the tube powered on.
2. Adjust the eyepiece ring until the image is perfectly sharp.
3. Once the diopter is set, test it on a real, long-distance scene: the image should be sharp with no need to touch the objective focus, since it's already set to infinity at the factory.

If the image is still blurry after this one diopter adjustment, contact our technical support rather than adjusting the objective focus ring, which shouldn't need touching in standard mid- to long-range use.

A poorly set diopter causes fatigue

Unlike a poorly set focus, which is immediately obvious, an incorrect diopter setting can go unnoticed for several minutes of use — before turning into eye strain, headaches, or a feeling of tension after an extended session. Your eye constantly compensates for a slight lack of sharpness, which strains the eye muscles far more than an image that's sharp from the start. We recommend redoing this adjustment every time multiple users share the same device.

The special case of binoculars

On a binocular system, each eyepiece has its own independent diopter adjustment, since it's rare to have exactly the same visual acuity in both eyes. Remember to set the diopter eye by eye, one eye closed at a time, before use — objective focus, on the other hand, stays set to infinity from the factory on our housings and shouldn't need any intervention.

What about eye relief?

Diopter adjustment shouldn't be confused with eye relief either, which refers to the physical distance between your eye and the eyepiece lens — often constrained by wearing a helmet or glasses. The two adjustments are complementary, not interchangeable.

The takeaway

Objective focus sharpens the scene; the diopter corrects for your eyesight relative to the tube's screen. Set to infinity, focus delivers the best depth of field for most uses — which is why our devices leave the workshop already collimated and pre-set to infinity. On arrival, there's just one thing left for you to do: adjust the diopter against the phosphor grain, for a sharp image from the very first use.