I'm not that familiar with roof prism binos, having only had 4 in my hands over the past 20 years or so (porro fan!). The latest bino purchased is a bushnell legent ultra HD (8x42) that has a click out diopter adjust ring turn to adjust the diopter, then push back down to lock the diopter. Seems a bit awkward to use in addition to the problem that the adjust does not allow enough of a minute adjust and still lock down. As if the adjustment doesn't have a fine enough gearing to allow a finer movement then lock . Since I haven't gotten to a bino that has the diopter adjust on the center focus wheel, does that function in a similar fashion? I can always leave the adjust ring "up" unlocked to facilitate an adjustment (stiff enough for it to stay in position without a lock down). But am interested in why they have the various schemes to change and set the adjustment. Are they all equally robust in the long run (yrs down the road)? Thanks in advance, Pat
Hi, Pat, et. al.
Individual focus binoculars NEED a diopter adjustment for each eye; CF binoculars do not. When you turn the center wheel to focus the LEFT (or right in some cases) eye, you are adjusting the dioptric setting for THAT telescope. There is only ONE diopter adjustment need of center-focus binoculars. It’s there to address the DIFFERENCE between the power the observer’s eye and only 2-3% of them have the same in each eye. Once that difference has been established ACCURATELY, the observer need only adjust the center focus—which needs no diopter scale—for focusing at various distances.
DIOPTRIC ACCOMMODATION
(The importance of Learning to Stare)
Perhaps the most important aspect of observing is found in dioptric accommodation, which is also a component of observing that can be controlled by the observer if they understand the most efficient focusing procedures. But what is there to know about simply focusing a binocular? Quite a bit, actually.
We have all seen bird watchers, amateur astronomers, or others repeatedly adjust the focus on their binocular when we know their target is not changing its proximity. Why, then, should an adjustment be necessary? The problem lies with a
dioptric accommodation—a physiological correcting of a disparity of focus along the Z axis and the observer’s understanding of focusing procedures—novice or postdoc.
Your mother probably taught you,
“It’s not nice to stare.” However, for the most pleasing, fiddle-free observing experience you might want to forget that advice. There are two parts to focusing a binocular. The first concerns the binocular’s focus mechanism—the actuation of the focus knob, diopter ring, or flip lever and it is the one seen almost exclusively by the observer. Over time, the second part can be even more important. It works through the involuntary
stretching or compressing of the eyelens by the eye’s
ciliary muscles. Our impatient brains want to see things in focus quickly. This is a safety mechanism for mankind that has been with us since prehistory. But if given the chance, it will use those muscles to the detriment of the viewing experience.
Let us suppose you want to see a target at a certain distance and start turning the focus knob to achieve the best image. The target may become sharply focused for you at -3 diopters, although your, at rest, dioptric setting should be -1. Yes, the image is focused. But only because your brain was rushing ahead of your need and only achieved that focus at a strained setting. Then, as fatigue sets in, or your physiological focus starts to wander, you feel the need to start fiddling with the focus mechanism. Even so, without considering the importance of
STARING, you will, more than likely, repeat the mistake and find that the problem remains.
Learning to stare can preclude the eye’s involuntary input and limits focusing to that achieved
through the binocular’s focus mechanism, leaving the observer in control of the whole focusing operation, as opposed to having the two aspects of focusing fight each other while you try to attain a precise and
comfortable focus for an object at a given distance. Not understanding the critical importance of learning to stare, observers have devised a few routines to circumvent the real problem.
Two of the most common involve trying to trick the brain. * Both can be beneficial to the operation and, although tantamount to placing a Band-Aid ®on a bullet hole, both have been endorsed in a number of articles. The first has the observer place a hand over one objective lens while focusing the other telescope. Then, the procedure is reversed for the other side. For some, even that is not good enough. In the second method, the observer must place the cap on one objective and reverse the process when a good focus appears to have been achieved. Some observers promote one or both of these methods and some satisfaction may be realized. Yet, how many times might a moving target disappear from sight while using the first method or fishing around in your pocket for a lens cap to perform the second? The person who has learned to stare need not bother with either of these stop-gap measures.
RECOGNIZING FOR YOURSELF
Patiently focus your binocular as precisely as possible on a target a mile or more away. Observe your dioptric setting for that target. This is your natural, at rest, dioptric setting for a target at that distance.
Now, defocus the setting by between 2 and 3 diopters. Then, when placing the binocular to your eyes, you may not initially see a disturbing difference in the focus. † Over time, however, you may note the image is not as crisp as when you started the experiment. Finally, as you continue to adjust the focus to attain a sharp image, you may see that the dioptric setting has returned to the one you had at the outset.
By learning to stare and bringing the image to its best—at rest—focus, you are allowing the
mechanism to do its job and much of the unnecessary, and time wasting, attempts at focusing can be avoided.
