The Binocular Blog - Part 7: Focusing Fundamentals

I'm surprised that so many people don't know how to get the best out of your binoculars even now. Everything about anything will find space on a YouTube video, and it applies to focusing a binocular too. Despite that, I find a lot of people complain, "This binocular doesn't focus well!"

Focusing a binocular correctly is not rocket science and is done easily if you follow the steps in correct order. Do also remember, binoculars are meant to look at objects at different distances and therefore the focusing knob or wheel will never be in a constant position, especially if you are observing moving objects. If you are tired of constantly focusing, there is an option, but one which I won't recommend. The focus free binoculars. I will give a brief description just to get it out of the way.

Focus free binoculars

Normal binoculars don't have an auto-focus mechanism inbuilt but there are binoculars that are 'focus-free'. I once again say that it is not one I'll recommend to anyone unless everything you want to look at something that lies more than a 100 feet away. Even if they are advertised as auto-focus or self-focusing is only a misleading term. 

These binoculars are set to focus permanently at anything from around 50 or 60 feet up to infinity and have no focusing knob or wheel. They have a great depth of field but trying to see anything close will be a futile exercise. They are suitable for young people whose eyes can accommodate (adjust focus) effortlessly. The other problem is that if you wear glasses, and the spectacle power of your eyes are different, there is no way to adjust focus for individual eyes. For overcoming this problem you may have to get an Individual Focus binocular which, while it still has no focus knob, has diopter rings on both sides so that your eyes can be 'matched'. 

If your hobby includes watching butterflies, dragonflies, other insects and small birds in bushes, this is not the type of binocular you should consider. Now, since that has been clarified, I hope you understand, binoculars can't 'auto-focus'!

In case you took my word and decided to go for a normal binocular that requires adjusting focus constantly. The rest of this post is for you. With any binocular, if you have to see the object you view clearly, the instrument must be adjusted to suit your eyes. There are three parts that come into play to get a proper focus.

1. The Hinge to set your interpupillary distance (IPD)

Most midsize and full size binoculars have a central hinge on which the two barrels are mounted. Some compact binoculars have a double hinge since they are designed be palm sized. Please see the previous post to know more about interpupillary distance. 

Single central hinge in a roof prism binocular

Double-hinged compact binocular

Hinge in a porro prism binocular

2. The Focus knob or ring to adjust focus on the go. They are usually found in the mid-axis along the hinge. The position of the knob or wheel, as the case maybe will vary with models. Most binoculars nowadays use a central focusing knob or wheel mounted on the bridge between the two optical parts just behind the hinge. There is a slight difference between the Porro prism and the roof prism binoculars. 

Focus ring on the hinge (Porro prism)

Focus ring behind the hinge (Porro prism)

The focusing wheel on Porro prism binoculars moves the eyepieces forward and backward to achieve optimum focus. This increases the length of the binocular and also makes it difficult to make it dustproof or waterproof.

Eyepieces move in and out while focusing on a Porro prism binocular

Whereas, in the roof prism binoculars the focusing is done by shifting a group of lenses within the barrel so there is no change in the length of the instrument. 

 

Focus knob behind the hinge (Roof prism)

 

When you buy your binoculars, first hold it to your eyes and see if your index finger rests comfortably over the focusing ring. This might seem to be a matter of least concern but when you want to move through your focus quickly an awkwardly placed finger might slow you down. Some binoculars go through only a full circle or a little less from close-up to infinity. Others might require more than that which can be frustrating when trying to focus on a rapidly moving target. See if you can get a binocular that won’t require too much rotation of the ring to achieve quick focus.

 

3. The Diopter ring to match focus in both eyes.

 Usually found on the right eyepiece. Some binoculars have it cleverly incorporated into the focus knob. Most of them have notches or ribs to mark off the diopters. Some models have numbers on them.

Diopter ring

Older models have numbers. The Zenith has from +2 to -2 diopters

The newer roof prism binoculars have notches or ribs instead of numbers

ART OF FOCUSING

To get precise focus with your binoculars , after you have familiarized yourself with the various parts involved in this process, you can do it in a few simple steps.

1. Adjusting the IPD

The first step is to setting your IPD (interpupillary distance). It had been discussed earlier (see link here) but here we are going to see how it is to be set. Unless your eyes are perfectly aligned with the eye pieces you will not able to focus precisely on your object of interest. Contrary to what you would have seen  in movies, where the scene viewed through a binocular is depicted as something enclosed in the shape of an 8 that has fallen on its side, the image viewed through a binocular aligned correctly will be circular!

If the angles are marked it is easy to remember your IPD

In roof prism binoculars the scale is usually absent as the focus knob is in that position.

Look at something in the middle-distance through the binocular, turn the focus knob till it comes into focus or is partially focused, then turn the barrels downward or upward till the two image circles become one single circle. That is your IPD. Note it carefully, either as a number or as the position by using a marker. Once that is done the next step is adjusting the focus.

2. Setting the eye cups

All binoculars come with eye cups that help you get the full field of view by keeping your eye at the correct distance from the eyepiece lens. That was what we discussed earlier as eye relief (see the link here). The eye cups can be foldable rubber types and pop-up or twist-up ones. If you wear glasses it should be pushed down (left) and if you don’t, the cups should be pulled up or out (right).

Eye cups down (left) & eye cups up (right)

        

3. Adjusting the focus 

This is done in two steps, one eye after the other. The first step is to focus the eye on the side without the diopter ring, usually the left eye in all but few cases. You have to block off the fellow eye (right) first and this can be done either by covering the objective lens with the rubber cover or your hand. Alternatively you can close the eye itself but remember, when one eye is closed, the other eye will also narrow a bit. With the objective lens closed, look at a stationary object like a building or car about fifty feet away. Then using the focus knob or wheel get the object in sharp focus. Remember, if you are not a spectacle wearer keep the eye cups pulled out and vice versa.

Once the left eye is done it, without altering the position of the focus knob or wheel, close the objective on that side. After opening the objective lens cover on the opposite side (right) look at the same object as before, then without touching the focus knob, turn the diopter adjustment ring of the right eye piece till the object is sharply focused. Now when you open the objective lens of the other side and look at the object with both eyes your binocular should focus correctly. If there is a problem you may have to repeat this step till you get a sharply focused image. Usually, it happens when we inadvertently turn the focus knob while turning the diopter ring, as they are rather close to each other.

Once you have gone through these steps, objects at varying distances can be focused by turning only the central focusing ring. When you buy a binocular ensure that your finger reaches the focusing ring easily when you grip the barrel. It makes focusing easy. 

 

Easily accessible focusing knob

I abandoned the Zenith in favour of a roof prism binocular for regular use because it was bulky, and found it difficult to turn the focusing wheel with my short fingers. 

Only finger tip reaching the focus wheel in a large binocular

Sometimes in some binoculars the diopter ring can get accidentally turned during use. You can mark your position on the ring with some indelible pen. Usually they are a little tight so won’t get misaligned easily. Some binoculars like my Vortex Viper has a push-lock mechanism that prevents such accidents. 

 

Locked (left) & unlocked (right) diopter ring in the Vortex Viper HD

When you have finally set the binocular’s focus, remember that this setting is only for YOUR eyes. If you share the binoculars, the instrument will have to be set to the user’s eye, in which case you will have to reset it to your eye again, later.

Most of us, over the age of 40 will need spectacles for presbyopia as our near focus recedes with age. We usually opt for bifocal or a progressive lens in our spectacles. If the segment for distant vision (at the upper portion) is not aligned properly to the eyepiece we will still have focusing issues. If you are presbyopic avoid the temptation to go for a monofocal, you might regret it if you want to look at the time or your camera settings or even your phone! It is better to make a pair with the bifocal segment slightly smaller and set lower down. Avoid using progressive lens in the field. They are best suited for the office!

I hope this makes focusing your binocular more easier now. When you settle for a model see how comfortable it feels in your hand and after that see if it focuses to your satisfaction. 

For those who are impatient to go through the entire blog series, I'm giving below direct links to all the posts. You can go directly to the page you are interested in, though I suggest you go through each post as all of them contain some information that will be useful when you consider purchasing a binocular for your self.

1.       Introduction
2.       Part – 1: Anatomy of a Binocular
3.       Part – 2: Purpose and Portability
4.       Part – 3: Magnification, Objective diameter & FOV
5.       Part – 4: Eye Relief
6.       Part – 5: Exit pupil & Interpupillary distance
7.       Part – 6: Image brightness
8.       Part – 7: Focusing fundamentals
9.       Part – 8: Weather sealing
10.   Part – 9: Purchase decision
11.   Part – 10: Care & Storage
12.  Afterthoughts and accessories