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Testing Binocular Stability (1 Viewer)

typo

Well-known member
I think it might be timely to reclaim some reality by going back to one of my little back yard studies on the subject of binoculars and stability, and offer a little new data. I know it's a topic that most experienced users feel they understand how it relates to themselves, but I know I'm not alone in trying to quantitate the effect and pin down some of the main component factors. It might be time to give the data a fresh airing.

Magnification is the relative increase in the apparent angle of view compared to the real angle of view. It will also potentially be matched by the same increase in apparent acuity, which can be readily checked by tripod mounting a binocular. At the same time it will similarly magnify the amplitude of any binocular movement, and with it, the angular speed which is a cause of target blur.

It's important to understand that the reference point for any individual's visual acuity is when both the user and target are completely stationary. Various scientific studies have concluded that it takes approximately a quarter of a second for the retina to collect and process the maximum resolution image to be acquired for interpretation. Four frames a second is too slow to be useful, even when walking, let alone running or driving. Any movement of a target in this time span would impair acuity. It was a scientist from Eastman Kodak in the 1960s that first realised that optimal perceived sharpness occurred at lower spatial frequencies. While the 'normal' acuity range was 1 to 2 arcminutes, optimal perceived sharpness was at 5 to 10 arcminutes. We now know that at this lower resolution information is acquired and interpreted much more quickly, in as little as one thirtieth of a second. This also allows the eye to track movement much more readily, but it is at the expense of retinal resolution. How sensitive an individual is to this loss of acuity appears to be enormously variable. It sees that some are quite oblivious of this 5 fold loss in detail while others will complain about just 0.05 fold reduction.

I think I read that there are something over one hundred individual muscles in the hands, arms and shoulders that may be involved in holding binoculars to your eyes. Many of those will be continually varying their tension in order to maintain the position of the hands, and in doing so, contribute to the resulting shake we see in the view. I recall reading Schober's 1963 paper (no longer accessible) that they had identified the primary oscillation frequencies associated with different groups of muscles that contributed to binocular shake (summarised by Vukobratovich 1989) but I've not found any other scientific study of the subject. I hope it is obvious that the apparent amplitude and frequency of the oscillations have a consequence for visual acuity, whatever time frame we are considering.

I should also point out the negative effect of motion will also depend on the starting static acuity at the time, and at least potentially, the effective resolution of the binoculars. The effect of shake may be lower for those with poor eyesight and/or bad binoculars.

Naturally there are large differences between individuals, due to physique, age health and other factors, that may affect their ability to hold a binocular steady, but there are other important variables as well, as many have pointed out. There have been several reports on specific comparisons by forum members. Some used homemade targets like letters with different point sizes, but a smaller number used calibrated line charts for greater accuracy. I don't recall anyone going beyond 12x, but I'm happy to be corrected.

I think it was probably the first time I visited a specialist optics shop that I realised that even binoculars with the same magnification, had quite a lot of variability in the steadiness of the view. It was not until about 5 years ago I decided to spend an afternoon with 10 different binoculars, between 6x and 12x, a resolution chart, some fishing weights and some food wrap and try to work out what the binocular characteristics were that gave me the steadiest view. I had checked them all out for resolution and effective resolution previously, and three were slightly limiting for my eyesight, but they did not influence the results. I can't find the original data now, but the results clearly showed that there were some obvious physical parameters that contributed to a steadier view in my hands. My very personal criteria for a steady view were, in order of priority, as follows.

An approximate weight of 850g.
A balance point 80mm or less from the eye.
A roughly parallel barrel diameter of approximately 50mm around the balance point
Minor contributing factors were a slight advantage for models 150mm or less in length and a largish bridge that allowed my fingers to interlock at my IPD.

Of the binoculars in that comparison my Zen Ray Prime 10x42 was a clear winner with a mere 26% reduction in efficiency, but by adding weight and altering the balance point and girth of the lower powered binoculars I could pretty much match the steadiness of the ZR with a few of thel other binoculars. The 10x56 at ~1.2kg and 12x50 at ~1.3kg I used that day obviously weighed significantly more than my optimum 850g, but the steadiness could still be improved by adding weight at the eyepiece end to improve the balance and making the grip more relaxed.

What I have found in my notebooks are dozens of records since then, where I have carefully compared a new or review binoculars, or even done basic chart comparisons in shops to the ZR. One or two have matched it's efficiency in that time but none have surpassed it. It still remains my reference binocular for steadiness and I see no indication that my preferences have changed over that time. Naturally this is particular to me, and others are bound to differ, but I think my parameters are reasonably consistent with the majority of anecdotal reports on the forum, and seem supported by discussions I've had with several experience retailers.

I'm now five years older, and I have to admit there have been both visual and physical changes in that time. I thought it might be useful to get out three of those original binoculars and test to how things are now. I took two sets of readings this morning, roughly one hour apart. The light level had increased for the second test and with it my baseline acuity improved a little. I used a commercial printed USAF1951 chart at 11m, though the distance was in some cases altered for improved accuracy. All measurements were free-standing with just a light weight fleece on my arms. For the 7x and 10x, the best reading in 1 minute was recorded. Due to the fairly rapid increase in the levels of shake over time when using the 12x50, I used three periods of approximately 20 seconds with a short rest in between instead. The results were converted to arcseconds, and then using the baseline acuity, converted to magnification equivalent and percentage efficiency, and the two results averaged.

Opticron Classic 7x36 …. 4.4x or 62%
ZenRay Prime 10x42 …. 7.7x or 77%
Unbranded, promotional sample 12x50** …. 5.7x or 48%

** This was a 1.3kg, poorly balanced binocular, and even this result was challenging to spot, but I should point out it was often possible to resolve greater detail in either the vertical or horizontal axis separately rather than both simultaneously. The best single axis value was a single vertical glimpse of 7.8x or 65% in a total of 2 minutes viewing, of course this has neglidgeable practical value for the majority of users.

These values agree fairly well with those from 5 years ago, though the 7x value was a little lower than the original study. This was most likely accounted for by the caffeine effect on the first reading.

In the intervening years I have had much better results with a 12x50 binocular than the model above. In particular the lighter and better balanced Meopta Meostar HD 12x50. I have yet to find a 15x56 that I thought stable enough in my hands to be worth evaluating in detail.

It would obviously be quite wrong to interpret what I've found as a problem of magnification. It is clear, for me at least, that the steadiness of the view is primarily a question of load, leverage and tension on my hands arms and shoulders, at least in the 6x to 10x magnification range. Exceeding my physical optimum and increasing magnification rapidly degrades binocular efficiency as the eye trades acuity for frame rate in an attempt to track the oscillating target. It appears that practical limit is exceeded by all the 15x and higher magnification binoculars I've managed to try so far.

Obviously I am unable to say how relevant my results are to others. I know others have done their own assessment at different times. This might be a good occasion to give the data a fresh airing.

David
 

tenex

reality-based
Thanks to both David and Binastro (in the other thread) for helping to move beyond purely theoretical questions and arguments whose interest had been exhausted. I do use a 15x56 myself and generally find its image more useful across wide open spaces than my usual 10x, but have never analyzed whether I'm seeing comparable fine detail or that's what matters most. It might involve another factor instead, like coarser detail or detecting color. (I've been curious to experiment with handholding 25x (or more) as has lately been discussed but have no such instrument myself.)
 

Troubador

Moderator
Staff member
Supporter
Thanks to both David and Binastro (in the other thread) for helping to move beyond purely theoretical questions and arguments whose interest had been exhausted. I do use a 15x56 myself and generally find its image more useful across wide open spaces than my usual 10x, but have never analyzed whether I'm seeing comparable fine detail or that's what matters most. It might involve another factor instead, like coarser detail or detecting color. (I've been curious to experiment with handholding 25x (or more) as has lately been discussed but have no such instrument myself.)

I agree Tenex and on the few occasions I have grabbed our 15x56 for a hand held view in the west of Scotland, my familiarity of the bird species there probably means I only needed the coarsest detail or even just general impression and shape to identify the bird, so it is more than likely that fine detail was not even looked for.

Lee
 

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