So that you can see correctly and use the binoculars optimally, it is necessary to adjust the instrument to your eyes and focus individually. If the image of the binoculars is out of focus, the brain tries to compensate for the blurred vision and the misaligned images. This can lead to dizziness, headache, eye fatigue, eye strain, or eye pain.
How do you focus Binoculars?
- Fold-out the eyecups for the optimal distance between the binoculars and the eyes
- Adjust the binoculars barrels according to your eye distance. If this is correct, your view is a perfect circle
- Look at an object
- Close your right eye and use the middle focus wheel to adjust the view for your left eye
- Next, close your left eye and adjust the diopter adjustment to the right eye until the image is sharp
- Open both eyes and enjoy a sharp, razor-sharp image.
- Use now the middle focus wheel only to set the focus again when viewing objects in different distance
How binoculars focus an image
Binoculars provide binocular vision, thus the name of the instrument. Binocular vision allows us to perceive depth and see 3-dimensional, and estimate distances between objects. However, this only works well, if the binoculars focus on the viewed object and the instrument is collimated, meaning the optical axis of the two binoculars barrels are precisely parallel.
Light rays passing through an optical lens are refracted (bent) and collect (intersect) in a point, the focal point. You know this effect from using a simple magnifying lens. For the best result, you have to move the lens back and forth a bit until the image is big and sharp.
The objective lens of the binoculars collects the light in the same manner. At the focal point (F) of the objective lens appears a small projection of the viewed image, the focal point is also known as “image point”.
An ocular (eyepiece), which in its simplest form is a magnifying lens too, is used to enlarge the image. To present a clear and sharp image to our eye, this image point must be at exactly where the focal length of both lenses converges.
Every time when the binoculars are moved and we look at something else at a different distance, the projection of the image point shifts a tiny bit. By adjusting the position of the ocular, we compensate this shift, until the ocular focuses exactly at the image point projected by the objective.
How to Focus Binoculars
If an object is not in focus and appears blurred, eye fatigue will be the result. Learn how to focus binoculars in a few simple steps. When all is properly adjusted you will have a crystal clear, almost three-dimensional image, and your eyes don’t have to work hard when viewing through your binoculars.
Adjusting the eyecups
Most good binoculars have adjustable eyecups. Those are rubber or plastic caps attached to the binoculars ocular which assure the correct distance of the eye, more precisely of the pupil, to the eyepiece first lens. This distance is called Eye relief.
When correctly adjusted you will have a clear bright picture, without infalling stray light and the full field of view that your binoculars are rated at. If the pupil is is outside the eye relief, the field of view will be reduced.
When using glasses while viewing with binoculars, fold in or turn in the eyecups completely.
Adjust the eyepiece distance of the binoculars to the interpupillary distance
The distance between the pupils of your eyes is called the pupillary distance (PD). Since the distance varies depending on the person, the binoculars must be set individually to get the best field of view.
Adjust the pupillary distance by moving both barrels of the binoculars which brings the eyepieces closer together or farther apart. When viewing a distant object and it is correctly adjusted the two images will converge and form a circle.
How to focus binoculars
Why Properly Focusing Binoculars is Important
Even the best binoculars are of little use if you don’t focus them properly. The exact synchronous focusing of the two beam paths is the prerequisite for relaxed binocular vision. Unsynchronized focusing or misalignment of the optical parts (loss of collimation) can be tiring and will cause headaches as the brain of the observer tries to compensate for the blurry or misaligned images.
Also important for comfortable and fatigue-free observation is the quality of binoculars, how to assess this is described in this post.