One of the most important things that guarantee enjoyable binocular use and good results is understanding how to properly focus them, adjusting them for your eyes. A good focus will yield crystal-clear, sharp images that will open up a new world for you.
Whether we discuss new hunting binoculars, astronomy scopes designed to look at the Moon and constellations or military equipment, everything that is not basically a toy will have focusing rings. These can be placed either in a central position (in which case the binos feature central focusing) or behind each eyepiece (devices with this emplacement are referred to as individual focus devices).
The best results are obtained only with proper set-ups, as focusing eliminates dark spots, blurry images and prevents the apparition of the so-called double vision.
Central focus designs are more common than the individual focus ones and for this reason, we will begin with this type. The rotating knob or ring used to focus them is located in the center, somewhere between the scopes, in an easily reachable position for either right or left index finger.
If you haven’t already bought the product, before deciding, make sure that the knob is large enough and with grooves, so it can be turned easily in wet weather or with gloves on. Besides from this, on a central focusing binocular you will find the adjustable diopter ring, usually found on one of the eyepieces.
The central knob will bring the image into focus simultaneously affecting both eyepieces – it is from there that you will make adjustments while changing the targets. To compensate for varying diopter between left and right eye, the diopter ring has to be turned as well until a satisfactory result has been obtained.
Before starting the actual focusing operation, make sure that interpupillary distance (also abbreviated as IPD) is properly set. This distance is measured between one pupil and the other, and getting a correct IPD setup will generate a clear image, without doubling the object or vignetting the view.
Apart from this, you will have to set-up eye relief (especially if you wear glasses) – this is the distance measured from the eyes to the ocular lens, at which the entire field of view is available. Glass wearers will need a distance between 14 and 18 millimeters, while non-eyeglass wearers can simply adjust this according to their preference.
Now, in order to focus the item, you will first need a target that is situated at long range and does not move. Look through the eyepieces and close the eye corresponding to the diopter adjustment ring. If you close that eye for whatever reason, block out the objective with a piece of plastic or wood.
Now look through the unblocked tube and turn the central focusing knob until the image obtained is as sharp as possible. After this, open your eye (or remove the cover placed on the objective) and, if the result is a perfectly clear image, no other modifications are required.
However, if it is still blurry or not that sharp, diopter adjustment will be required. Close your other eye and adjust it until the view is what you desired, and then you will be ready to go. Keep in mind that the center focus adjusts both eyepieces, while the diopter adjustment ring will only adjust the one on which it is mounted – that is why you will start by looking through the eyepiece without the diopter ring.
We will discuss individual focus devices (abbreviated as IFs), the model that allows the user to focus each eye individually. They are also called no-focus (these are cheap items with a fixed focus distance that cannot be modified – they are not recommended) or self-focusing binos (these are okay).
Binoculars with individual systems are set focused at a certain distance, but this is their main advantage – the fixed depth of field permits users to pick any target beyond that distance without having to refocus. An IF product is adjusted in basically the same way a central focus one is corrected.
Now, since each eyepiece has its own diopter adjustment ring, you can begin by closing any eye you want. Turn the eyepiece until the image obtained is as clear as possible, and repeat the procedure to adjust the other scope. Since almost all adjustment rings have a scale, you can note the indication so that you do not have to follow the steps above every time somebody else readjusted the device.
There are some advantages that the IF system offers, compared to the central focusing one. They are usually more rugged and resistant, tending to be waterproof and dustproof. The need to constantly refocus is reduced as your eyes have a natural ability to focus an image within a fixed depth of field.
Reliability is also increased as they have fewer moving parts (basically, only two if we neglect the articulation between the scopes). However, close range objects cannot be focused; they tend to be expensive and are usually meant for very long range target acquisition. In order to get a better picture, think of a golf binoculars in comparison to a military or naval device.
The first one will most likely have a central focusing system while the second will be an individually adjustable design; with the golfing one, there is no problem in, let’s say, checking what your children are looking at while playing outside but if you want to go ship spotting, it is unlikely that a ferryboat 5 kilometers away will come clear in view.