How binoculars focus? The binoculars 101

how binoculars focus?

In order to observe clearly with the binoculars, you have to focus on the object or scene you want to watch. Two main focus systems can be found in binoculars and although it is relatively simple to perform this action, how does this work?

How binoculars focus? There are two focus systems encountered in the binoculars. One of the most common ones is the focus knob which adjusts the gap between the ocular lenses in order to focus on various distances for both sides of the binoculars, where the second system works similarly to the first, however, you won’t have a focus knob but every ocular eyepiece acts as a focus and diopter adjustment system. 

Further in this post, I am going to help you understand how every focus system works and how to improve your watching through the binoculars, how to focus and adjust the diopter properly.

How binoculars focus? The knob focus system

how binoculars focus - the knob focus system

Binoculars with the focus knob in the middle are the most common type of binoculars found on the market. This is also the easiest to use because all you have to do in order to focus is to rotate the knob to the left or right

When you perform this action, the gap between the ocular lenses is increasing in size or decreasing. The ocular lenses from both sides of the binoculars will move at the same time.

Moreover to this, usually on the right ocular, there is a diopter adjustment ring which function is to adjust the diopter of your eyes in case you have any eyesight problems or you are wearing glasses and you want to watch through the binoculars without glasses.

The diopter adjustment ring will extend or contract the gap of the ocular lenses on only one side of the binoculars, the side where the ring is situated, and further to this when you are using the focus knob, the binoculars will focus by performing the same action but with a small degree in gap between the binocular sides, as fixed per diopter level.

In order to best focus with the binoculars with a focus knob, there is an action to perform before you use them, this is to adjust your diopter level. This will follow in the post.

How binoculars focus? The independent eyepiece focus system

The independent eyepiece focus system is not that often found in the regular binoculars and can be easily seen in larger size binoculars such as the astrobinoculars. One example would be the Celestron 25x100mm as the one I photographed above (not the first one, that is my masterpiece sketching…).

You will easily notice that on this system of focus, there is no central knob to assist you to focus on the scene or object you want but you have two rings, one on each ocular. This will be a bit more difficult as it will require more time to focus with each eye at a time.

Furthermore to this, each ring will act as a focus and diopter adjustment at the same time.

The easiest way to focus with each eye is to cover the opposite eye with your palm or a cup and use the ring from the ocular to focus, then repeat the process to the other eye by swapping over.

Why binoculars may not focus properly? The diopter adjustment system

binoculars diopter adjustment

When you use your focus knob (speaking about the binoculars with the focus knob) and you focus on an area, scene or object, where one eye will be in perfect focus and the other one won’t, the most common issues, and this is not an issue at all, is that you need to adjust the diopter of your binoculars to your eyes.

The process is relatively simple and is the best to follow as listed below:

  1. On the outdoors area, select an area or object (e.g. a tree) you want to watch through binoculars, which is not going to be either too far from you or too close.
  2. Watch through the binoculars and cover the right side objective lens with your palm or a cup (or simply close your right eye) and use the focus ring until the object or scene you selected is in perfect focus
  3. Now cover the left objective lens with the palm, a cup or simply close your left eye, so you can watch through the right eye only. If the object or scene you want to observe is not on perfect focus, use the diopter adjustment ring (do not touch the focus ring anymore for now) until the object/scene is in perfect focus.
  4. Your binoculars are adjusted per your eyes and in the future, all you have to do is use only the focus knob when you want to focus on what you are going to observe. Don’t touch anymore the diopter adjustment. If one of the eyes will get out of focus again, repeat the process.

How do autofocus binoculars works?

I am going to break down a myth here: the autofocus binoculars will NOT focus automatically and will not have a computerized and motorized system to focus automatically.

The term is often used for the binoculars where there is no focus ring or knob at all, and everything above 40 yards (plus/minus depending on the model) should be in perfect focus.

One ring you are going to find on these binoculars which is the diopter adjustment ring, to help you adjust your binoculars per your eyes.

Related Q&A and extra tips to share

Q1:

What does 10×42 mean?

Usually, on the focus knob, there are two numbers divided by an ‘x’ such as 10×42 or 12×50. The first number is the magnification where the second number is the objective lens diameter.

Q2:

What is the exit pupils

The exit pupil is the diameter of the light beam going out of the eyepiece into your eyes pupils. This is calculated by dividing the objective lens diameter to the magnification (e.g. 10×42=42/10=4.2mm). The human’s eye pupils are open between 2-3mm during daylight and up to 7mm during the night.

Q3:

Can binoculars affect your eyesight?

Not using correctly the focus adjustment or set up your diopter right are just a few of the things that can affect your eyesight in time. I wrote an article which can be found HERE where this is in-depth covered.

Conclusion

focus knob on binoculars

I never liked the binoculars with autofocus as they are not good for a variety of things such as birdwatching or indoor concerts, anything where you need to observe under 40yards/36meters. The beauty of the binoculars is not only to observe what is far but what is close as well.

The binoculars with individual eyepiece focus system is a bit more hard to use and as mentioned above, is often found on the “big daddies” of the binoculars. But this is not always the case. If you are going to buy a pair of binoculars, have a look at the focusing system and pick a model with the knob focus. It’s easy, simple and takes seconds to focus.

Thank you for reading our post and I hope to see you around. Take care for now! If you want you can have an extra read about the binoculars focus and adjustment on Wikipedia (external link)

About Gabriel Mihalcea

Passionate about binoculars, photography and blogging with years experience behind, I love to split my time and observe the beauty of this world with different eyes.

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