How to Focus Binoculars?

The interpupillary distance is the separation between the center of each eye. Each binocular must be adjusted in such a way that the center of each eyepiece coincides with the center of each eye. For this, most models can be adjusted by varying the angle formed by each pair of eyepieces and objective through the central axis of the instrument.

The distance must be varied until a clear circular image is obtained, and not double images of the objects. The observation should be comfortable. This operation is simple and can be done both day and night (observing the sky). It must be taken into account that the measurement of the interpupillary distance varies from person to person, so if the binocular is shared, this factor should almost necessarily be readjusted. In any case, it is swift and almost intuitive.

Independent focus adjustment

1. Before beginning the observation, there is an important operation to be performed. The focus of the focus eyepiece must be adjusted independently. This is usually the right and can rotate smoothly to achieve an approach independent of the central focus. This will allow obtaining clear and well-focused images with both eyes. Otherwise, one eye will capture focused images and the other will not.
2. It can be done in two ways, but both I would recommend doing them pointing to the sky directly, in preference towards some stellar group (open cluster, for example)
3. The first is to close the right eye and looking only from the left to get a good focus with the central focusing system. Then, and being careful not to vary the latter, the right eyepiece must be turned slowly, now with the right eye open and the left eye closed until a focused image is obtained. It is advisable to repeat the procedure until you make sure that no unintentional errors have been introduced, such as slightly varying the central focus when adjusting the focus of the eyepiece.
4. If the procedure worked, the stellar images would be punctual and there will be no differences in focus between both eyes. There is also the effect that when we get a good focus, the field appears especially sharp and well defined.
5. The second method is similar to the first but more direct. Simply pointing to the sky should be achieved with the right eye closed and the left open a good focus. Then, and with both eyes open and without varying the central focus, the focus eyepiece should be adjusted slowly until clear and focused images are obtained. It is recommended to revise the central focus and retouch any errors.
6. In both cases, the reference object (stars, planet, etc.) must focus on the field of the binocular, since in many cases on the edge of the field there is a deformation of the images, preventing the correct adjustment.
7. The focus eyepiece has small scale-like marks without units. When a good focus has been achieved, it is recommended to make a small pencil notation on the scale. In this way, you can save time in case of variations, but still, have to check to confirm that the focus is correct.

Using the tripod

1. Many models of binoculars bring with them a universal thread of the one used in the cameras, which allows them to be mounted on a common photographic tripod. Still, there is a problem: the thread (except in large models of binoculars) is placed horizontally (on the axis of the binocular), and when placing them on the tripod (with thread pointing up), the binoculars remain mouth below, with the objectives facing the floor.
2. To solve this, an L-shaped adapter is commercialized (or it can be built since it is very simple) that allows fixing the upper end of the greater length of the L in the binoculars, and the other on the tripod. The part of the adapter that joins the binoculars comes with a universal screw, while the part attached to the tripod has a universal thread so that the tripod screw can be screwed into it.
3. Placing the binoculars on a tripod is recommended when observing specific objects (variables, planets or the Moon for example) or when group observations are made and the object does not want to be lost from the visual field between observer and observer. For individual observations, it is often more convenient to hold them in hand and make sweeps of the sky.

Other tips

• It is essential to use a comfortable strap that holds the binoculars when you do not have them in your hands and protects them from possible falls.
• When observing, avoid holding them with unnecessary force, since this causes the image to tremble more than usual. You simply must hold them securely, but without force beyond what is required.
• At the end of the observation the binoculars should not be stored immediately. First, they must be left uncovered so that they stabilize thermally and avoid keeping them fogged. They should be stored with their covers on and in the case provided by the manufacturer, but without any plastic bag, since they can get to trap moisture and this could damage the optics of the instrument (generating fungus, stains and others).