The central focus is also called the middle axis handwheel adjustment. From the picture above, you can see that she adjusts the focus through the middle handwheel to adjust the overall focus. Bi-tuning is what we often call a telescope with a diopter, and the focus should be adjusted on both sides separately. Generally, adjust the left eye first and then the right eye.
In essence, there is no difference between the middle and binoculars. Their purpose is to make the telescope see more clearly according to the user’s situation. It’s just that the internal adjustment mechanism is different. One is for overall adjustment, and the other is for adjusting two eyepieces separately. The effect when used is almost the same.
Central focus is also the most widely used in binoculars, because it is easy to operate and save time. The military binoculars all use a dual-tuning structure, because the dual-tuning is the external adjustment of the eyepiece, unlike the middle-tuning to adjust the lens or adjust the prism. The structure is much more stable, and the airtightness is better. Although the adjustment is not as convenient as the middle adjustment, the dual adjustment is chosen considering the stability of use.
Some friends think that those with myopia should choose bi-tonal ones, and those with good eyesight should choose mid-tonal ones. Because the dual-tone can adjust the diopter of both eyes according to myopia. This is wrong. If you observe carefully, you will find that the middle adjustment also has a fine adjustment knob on the right eyepiece, as long as you adjust the focus and then fine-tune it is the same.