Focussing: Just Do It! (1 Viewer)

[Tringa45]

This post, is not about technique. On the contrary, it is intended to dispel the concept that one should adopt a special technique to relax the eye muscles, or that one should focus from near to far, which could be problematical in some field situations!
Following on from my thread on "Dioptre Setting - Fallacy and Fact", in which I explained and experimentally verified that the chosen object distance for dioptre setting is irrelevant, there are still some doubters, who insist on performing this at extended distances. This is a de facto admission that they have failed to understand the workings of a Keplerian telescope, which I will recap at the bottom of the post to avoid repetition for the cognoscenti and to avoid boring others.
There was also a misleading suggestion that one's own ability to accommodate plays a significant role in focussing a binocular and that the clear results obtained in the above experiment could be attributed to my age and lack of accommodation - pernicious and discriminatory!

Consequently I decided to enlist the help of my neighbour's daughter. Miss C is a smart kid and wants to be a researcher later in life, so she was a willing participant.
I first did a rough check of her visual acuity. She was able to read license plates with either eye at 50 m and could focus on my 1951 USAF glass slide at 10 cm. This corresponds to 10 dioptres of accommodation but she probably falls within the norm for a 10 year-old of between 15 and 33 dioptres, in any event more than that of any active member on Birdforum!

The experiment was repeated with my tripod-mounted Swarovski 10x42 EL SV. Initial tests were performed using both barrels and I was able to mark focus settings with tape on the focus knob with respect to the just visible dioptre scale underneath. There were some variations of around +/-1 dioptre due to reliance on accommodation and unfamiliarity with a binocular. We then went on to repeat the dioptre tests, focussing with the right objective covered and setting the dioptre with the left objective covered.
In between, I set the dioptre knob to a false value and asked Miss C to try to attain the sharpest focus possible. This demanded some rather intense concentration so she took short rests between measurements.

I must admit to being surprised at the results. Miss C consistently set the dioptre to minus 0,5, both on a sign at 80 m distance and on the 1951 USAF glass slide at 4 m!
In binocular terms this was a focus travel of 23,75 dioptres and shows conclusively that object distance is irrelevant to dioptre setting, that any special focussing techniques are superfluous and that accommodation ability is not relevant to achieving precise focus.

Lastly, to recap, a binocular is a passive afocal instrument. The objectives form reversed left to right and inverted images, which are rectified by the prisms.
For an object at infinity the images would fall in the focal plane of the objectives and for closer objects the images would be further from the objectives according to the formula, 1/f =1/d1 + 1/d2, where f is the focal length and d1 and d2 are object and image distances respectively. These are on opposite sides of the objective , so d1 will have a negative value.
These images are viewed through the eyepieces, which can be regarded as (rather complex) loupes. The magnification is the ratio of objective focal length to eyepiece focal length.
Consider a user with normal vision viewing an object at infinity. He or she would adjust the focusser of the binocular to place the image in the focal plane of the eyepiece. Rays from any point on the object would then emerge parallel from the eyepiece, i.e. at infinity. For a nearer object he or she would adjust the focusser to place its image in the focal plane of the eyepiece so that the virtual image of this nearer object would also appear at infinity. With most Porro prism binoculars this is achieved by racking out the eyepieces. With modern roof prism binoculars with a fixed distance between objectives and eyepieces it is achieved by a movable focussing lens which reduces the focal length of the objective. On old SLCs, 1st. Gen. ELs and Zeiss SFs this is a converging lens which shifts towards the other objective elements. On EL SVs and most other modern roofs this is a diverging lens that shifts away from the other objective elements.
Users will always adjust the focusser to place the virtual image at their comfortable viewing distance. For a near-sighted user with a -5d prescription this would be 20 cm and rays from any point on the focussed object would diverge from the eyepiece.
Most here are aware that depth of field in binoculars or scopes decreases with the inverse square of the magnification. Similarly focus change in real terms has to be multiplied by the squre of magnification in binocular terms.
Consider viewing an object 4 m distant with naked eyes or corrected vision and then turning your attention to an object 2 m distant. That is a difference of a mere 0,25 dioptres or little more than the tolerance of an eyeglass prescription. Viewed through an 8x binocular though that is a focus travel of 16 dioptres and beyond the accommodation capabilities of any of us here. It might just however be accomplished by 10 year-old Miss C!

John

 

[Alexis Powell]

This post, is not about technique. On the contrary, it is intended to dispel the concept that one should adopt a special technique to relax the eye muscles, or that one should focus from near to far, which could be problematical in some field situations!
Following on from my thread on "Dioptre Setting - Fallacy and Fact", in which I explained and experimentally verified that the chosen object distance for dioptre setting is irrelevant, there are still some doubters, who insist on performing this at extended distances. This is a de facto admission that they have failed to understand the workings of a Keplerian telescope, which I will recap at the bottom of the post to avoid repetition for the cognoscenti and to avoid boring others.
There was also a misleading suggestion that one's own ability to accommodate plays a significant role in focussing a binocular and that the clear results obtained in the above experiment could be attributed to my age and lack of accommodation - pernicious and discriminatory!

Consequently I decided to enlist the help of my neighbour's daughter. Miss C is a smart kid and wants to be a researcher later in life, so she was a willing participant.
I first did a rough check of her visual acuity. She was able to read license plates with either eye at 50 m and could focus on my 1951 USAF glass slide at 10 cm. This corresponds to 10 dioptres of accommodation but she probably falls within the norm for a 10 year-old of between 15 and 33 dioptres, in any event more than that of any active member on Birdforum!

The experiment was repeated with my tripod-mounted Swarovski 10x42 EL SV. Initial tests were performed using both barrels and I was able to mark focus settings with tape on the focus knob with respect to the just visible dioptre scale underneath. There were some variations of around +/-1 dioptre due to reliance on accommodation and unfamiliarity with a binocular. We then went on to repeat the dioptre tests, focussing with the right objective covered and setting the dioptre with the left objective covered.
In between, I set the dioptre knob to a false value and asked Miss C to try to attain the sharpest focus possible. This demanded some rather intense concentration so she took short rests between measurements.

I must admit to being surprised at the results. Miss C consistently set the dioptre to minus 0,5, both on a sign at 80 m distance and on the 1951 USAF glass slide at 4 m!
In binocular terms this was a focus travel of 23,75 dioptres and shows conclusively that object distance is irrelevant to dioptre setting, that any special focussing techniques are superfluous and that accommodation ability is not relevant to achieving precise focus.

Lastly, to recap, a binocular is a passive afocal instrument. The objectives form reversed left to right and inverted images, which are rectified by the prisms.
For an object at infinity the images would fall in the focal plane of the objectives and for closer objects the images would be further from the objectives according to the formula, 1/f =1/d1 + 1/d2, where f is the focal length and d1 and d2 are object and image distances respectively. These are on opposite sides of the objective , so d1 will have a negative value.
These images are viewed through the eyepieces, which can be regarded as (rather complex) loupes. The magnification is the ratio of objective focal length to eyepiece focal length.
Consider a user with normal vision viewing an object at infinity. He or she would adjust the focusser of the binocular to place the image in the focal plane of the eyepiece. Rays from any point on the object would then emerge parallel from the eyepiece, i.e. at infinity. For a nearer object he or she would adjust the focusser to place its image in the focal plane of the eyepiece so that the virtual image of this nearer object would also appear at infinity. With most Porro prism binoculars this is achieved by racking out the eyepieces. With modern roof prism binoculars with a fixed distance between objectives and eyepieces it is achieved by a movable focussing lens which reduces the focal length of the objective. On old SLCs, 1st. Gen. ELs and Zeiss SFs this is a converging lens which shifts towards the other objective elements. On EL SVs and most other modern roofs this is a diverging lens that shifts away from the other objective elements.
Users will always adjust the focusser to place the virtual image at their comfortable viewing distance. For a near-sighted user with a -5d prescription this would be 20 cm and rays from any point on the focussed object would diverge from the eyepiece.
Most here are aware that depth of field in binoculars or scopes decreases with the inverse square of the magnification. Similarly focus change in real terms has to be multiplied by the squre of magnification in binocular terms.
Consider viewing an object 4 m distant with naked eyes or corrected vision and then turning your attention to an object 2 m distant. That is a difference of a mere 0,25 dioptres or little more than the tolerance of an eyeglass prescription. Viewed through an 8x binocular though that is a focus travel of 16 dioptres and beyond the accommodation capabilities of any of us here. It might just however be accomplished by 10 year-old Miss C!

John

All in all great stuff/advice, especially for those with very old eyes (lacking accommodation entirely), those with very young eyes (having much accommodation, and even more importantly, having very elastic and thus quick to tension and rebound lenses), and those whose left and right eyes are very similar in their preferred (or comfortably capable) amount of accommodation. For those folks, setting the diopter is such a trivial procedure they likely wonder why it garners so much discussion on BirdForum and why so many otherwise competent and experienced binocular users seem to have so much trouble with getting a setting that they find consistently satisfactory.

I would caution other folks, especially those with aging eyes (having accommodation but somewhat slow to stabilize lenses) or those who have different amounts of accommodation ability in their left and right eyes [Note, this has nothing to do with need for eyeglasses or amounts of correction for each eye if they do] to take seriously a couple of slight inaccuracies in your description of binocular use.

He or she would adjust the focusser of the binocular to place the image in the focal plane of the eyepiece.

Rather than "in the focal plane", it would be more correct to say that during focusing the user will adjust the focuser to place the real image at or near the focal plane of the eyepiece. The positioning is only approximate because judgement of whether the bin is focused is based on how resolved the image appears, which is a product of both the binocular's optics and the amount of focusing being done by the user's eye.

As a matter of fact, many people with young eyes have a tendency to consistently focus a bit beyond the object and use some close accommodation of their eyes to correct the overall focus of the optical system to perfection. Doing so can sometimes produce a sharper view because when the eye (ciliary) muscles contract to accomplish close focus (through lens rebound), the pupil also reflexively contracts somewhat, creating a smaller aperture which can increase depth of field and also reduce aberrations from irregularities in the cornea and lens. When I was younger, these effects lead me to select eyeglasses which were, by the measure of having relaxed eyes near infinity viewing distance, too strong a prescription for my nearsightedness, because it gave me somewhat sharper vision in practice. My ophthalmologist said that doing as such (i.e. choosing a prescription that was too strong) was not uncommon in young people obsessed with achieving highest visual acuity. As I got older, I had to learn to "stare" (as Bill Cook is fond of saying) in order to resist choosing too high a prescription lest my eyes would become very tired by the end of the day.

What I describe in the above paragraph leads me to my second point.

Users will always adjust the focusser to place the virtual image at their comfortable viewing distance.

Maybe, but, as I've outlined above, maybe not. A binocular user concerned to achieve maximum sharpness of the view that they are getting as they focus their bin or adjust its diopter may not set it at "their comfortable viewing distance" but rather, at the distance that leads to greatest sharpness of the view, which may be equivalent to focusing on a point farther than infinity (or the relaxed focus point of their eye, whatever it might be) such that they stimulate eye close focus and, in turn, pupil contraction.

If a user has significant differences in accommodation ability between their eyes, and if they adjust the overall focus with one eye and the diopter with the other (i.e. separately, and covering each eye in turn while the other side of the bin is adjusted), they are likely to dial in proportionally different amounts of focus. In my case, my left eye has always been capable of more accommodation than my right, so I tend to favor (or tolerate without noticing) a stronger amount of instrument far focus with my left eye than my right when looking one eye at a time through a binocular or scope. For this reason, as previously noted, "staring" (i.e. relaxing the eyes) while keeping both eyes open when setting the diopter will lead to a more consistently satisfactory diopter setting for those who have aging eye lenses and significant differences in accommodation ability between their eyes. A number of other issues related to accommodation (discussed in other threads) come into play in practice (and, as e.g. described above, having little to nothing to do with the optics of the binocular itself) when considering how to keep one's eyes balanced and from tiring during a long day of birding, most of which I find are most easily accomplished by keeping one's attention at infinity when adjusting bins, but the points that I've made here are, I think, the most relevant for an experienced user of binoculars for birding (and esp. experienced users of binoculars regularly at very close distances, such as when butterflying) to consider.

--AP

 

[Maljunulo]

This stuff is fascinating.

 

[Tringa45]

Alexis,
As one of the doubters who provoked this post through your failure to understand the workings of a Keplerian telescope and spreading false information, you are now attempting to hijack this thread by quoting me out of context and dispensing your own advice.

John

 

[William Lewis]

I don't even know what perfidious means but I don't mess about with dipter settings. They can change between binocualars though, my 8x32 srga's are at zero and my habicht 7x42's live just to the left of the + marking. I do adjust ipd slightly when close focusing, more with the 8x32's than 7x42's.

 

[Maljunulo]

I’d like to see some literature citations on the business of the effects of accommodation on focusing of optics.

Has this been investigated systematically?

My ability to accommodate is exactly zero, since my lenses are both implants. I, therefor, attribute any differential diopter setting to differences in rhe depth of my individual eyeballs.

 

[Tringa45]

I’d like to see some literature citations on the business of the effects of accommodation on focusing of optics.

Has this been investigated systematically?

I know of none, so that is what I was trying to show. My own accommodation is very limited, Miss C's is probably better than that of anyone here.
If you have a tripod and bino adapter available you could try repeating the experiment yourself. As many binoculars have a scale on the dioptre, performing the test as dioptre setting will not only show discrepancies in your left and right eyes, but also discrepancies from measurement to measurement.

John

 

[Troubador]

Alexis,
As one of the doubters who provoked this post through your failure to understand the workings of a Keplerian telescope and spreading false information, you are now attempting to hijack this thread by quoting me out of context and dispensing your own advice.

John

Disagreements and enthusiastic debates are the life-blood of forums. Do not let this debate degenerate into personal attacks or comments.

Lee
MODERATOR

 

[Tringa45]

On reflection, perhaps I should not have been so surprised at a 10 year-old's ability to achieve repeatably accurate diopter settings near and far.
When conducting an eye test, patients of all ages are required to make judgements on spherical and cylindrical correction in increments of 1/4 diopter.
Binastro here even insists on lengthy tests to an accuracy of 1/8 dioptre.
I think that we can conclude that no special techniques are necessary for accurate focussing and dioptre setting, only that the use of a support in setting the latter could be beneficial.
In any event I had fun doing the tests and, according to her mum, so did Miss C.

John

 

[Hauksen]

Hi John,

I must admit to being surprised at the results. Miss C consistently set the dioptre to minus 0,5, both on a sign at 80 m distance and on the 1951 USAF glass slide at 4 m!
In binocular terms this was a focus travel of 23,75 dioptres and shows conclusively that object distance is irrelevant to dioptre setting, that any special focussing techniques are superfluous and that accommodation ability is not relevant to achieving precise focus.

Thanks, that's an interesting result indeed! I suppose your surprise is owed to the fact that she did set the dioptre consistently, not so much that it's the same regardless of distance?

I recently handed my Papilios to a 10-year-old, and while she was curios about the focussing wheel and tried it out to see the effect, it turned out that she was able to switch from viewing details on the opposite bank of the Rhine (300 m) to the closest distance (less than a meter) without actually adjusting the focus in between, at all. As you previously had told me about just the same indifference on part of (I guess) Miss C, I took that as a confirmation of your observation!

Accordingly, I find it fascinating that despite this very wide accommodation range, Miss C still had a definite preference for a certain accommodation "working point" which she could achieve repeatably.

Regards,

Henning

 

[forent]

(...) I think that we can conclude that no special techniques are necessary for accurate focussing and dioptre setting, only that the use of a support in setting the latter could be beneficial.
In any event I had fun doing the tests and, according to her mum, so did Miss C.
John

Concerning Miss C: I confidently expect that she has luckily experienced the magic of a binocular! In the 70s when I was a boy my grandfather inducted me in birdwatching: We looked at the feeders in his ample garden with his venerable pre-war Dr. Hensoldt 7x56 Nacht-Dialyt. Oh so many birds came in, so many species were plain common back then! I was infected instantly and forever.

Concerning diopter setting: Without scientific backbround and only by my anectdotical experience with the very same bino but aging eyes I can confirm Tringa45's statement that that the chosen object distance for diopter setting is irrelevant and so is c.p. the age of the user: I received my Leica Trinovid 8x42 BA in 1993 when I was 25 years old. I have always been near-sighted (1993: L -1.0, R -4.5 dpt; 2022: L -1.5, R -5.0 dpt). The L-R difference of 3.5 dpt has been pretty constant all along. I've used the Trinovid till this day fifty-fifty with and without spectacles, so ever and anon I had to set the diopter. In general I just use the scale but frequently I set the diopter very careful first for the left, then the right eye to obtain the best result and to detect possible changes. In all those nearly 30 years and independent of whether I focused on close or distant objects and other parameters I received (within a really small margin of error) the very same value on the diopter scale! And sadly, my senescent, presbyopic eyes accomodate far worse than 10 or even 30 years before. But the diopter value remains the same, old or young, summer or winter and especially: near or far.

Thanks, John, for explaining the reasons to me!

 

[Alexis Powell]

...Accordingly, I find it fascinating that despite this very wide accommodation range, Miss C still had a definite preference for a certain accommodation "working point" which she could achieve repeatably...

I also find this interesting because it is not universal in my experience. As some will know from past threads, I've long had an interest in the fit of binoculars to children (adequately minimum IPD often being the deal breaker) and, for example, always outfitted my own kids with binoculars when they were growing up, starting when they were around age 3 or 4. On many occasions, I tried to indirectly check their vision while we were playing around with optics (especially tripod mounted scopes), testing whether they set the focus the same as I would (with my eyeglasses corrected vision). My daughter was quite consistent in choice of focus adjustment for a given target at given distance, but my son was not. He tolerated focus both in front of and behind the best overall setting despite having excellent naked-eye acuity for both far and near. I had his eyes checked with a doctor and everything was supposedly normal. Later though, by the time he was about 6, he consistently focused the scope to (variably) farther focus than normal. I had his eyes tested, and found he had become nearsighted (and needed eyeglasses).

--AP

 

[Hauksen]

Hi again,

I suppose your surprise is owed to the fact that she did set the dioptre consistently, not so much that it's the same regardless of distance?

If my above message makes less than perfect sense, that might be because I got confused by the ambiguity in the use of "dioptre" in the English language - my apologies!

John, I would love to hear if Miss C did in fact repeatably set the same focussing wheel position for the same distance, in case you checked that (which you'd probably have mentioned, but I thought I'd ask anyway .

Regards,

Henning

 

[Hauksen]

Hi Alexis,

On many occasions, I tried to indirectly check their vision while we were playing around with optics (especially tripod mounted scopes), testing whether they set the focus the same as I would (with my eyeglasses corrected vision). My daughter was quite consistent in choice of focus adjustment for a given target at given distance, but my son was not. He tolerated focus both in front of and behind the best overall setting despite having excellent naked-eye acuity for both far and near.

Thanks for sharing that observation, that's something I was really curious about!

(And just what I had mistakenly thought John had pointed out - thanks for fixing my world so quickly! ;-)

Regards,

Henning

 

[Maljunulo]

Are we confusing visual acuity with range of accommodation?

 

[Hauksen]

Hi,

Are we confusing visual acuity with range of accommodation?

Where would we?

Regards,

Henning

 

[Maljunulo]

When you mentioned your son’s (implied) wide range of accommodation together with his visual acuity, I assumed you saw some sort of correlation.

I think I must have read too much into the statement, and assumed something which was not there.

Richard

 

[Hauksen]

Hi Richard,

When you mentioned your son’s (implied) wide range of accommodation together with his visual acuity, I assumed you saw some sort of correlation.

Ah, that was Alexis' statement, but I as understand him, the interesting aspect for our discussion was in fact range of accommodation. I hope we're all on the same page regarding that aspect!

Regards,

Henning

 

[Alexis Powell]

When you mentioned your son’s (implied) wide range of accommodation together with his visual acuity, I assumed you saw some sort of correlation.

I think I must have read too much into the statement, and assumed something which was not there.

Richard

They are two different things of course. I mentioned it because I thought a reader might suspect that if my son didn't focus the scope consistently that it might mean that he had limited vision (e.g. poor acuity). His acuity was and still is normal, though he has become nearsighted.

--AP

 

[WJC]

Hi John,



Thanks, that's an interesting result indeed! I suppose your surprise is owed to the fact that she did set the dioptre consistently, not so much that it's the same regardless of distance?

I recently handed my Papilios to a 10-year-old, and while she was curios about the focussing wheel and tried it out to see the effect, it turned out that she was able to switch from viewing details on the opposite bank of the Rhine (300 m) to the closest distance (less than a meter) without actually adjusting the focus in between, at all. As you previously had told me about just the same indifference on part of (I guess) Miss C, I took that as a confirmation of your observation!

Accordingly, I find it fascinating that despite this very wide accommodation range, Miss C still had a definite preference for a certain accommodation "working point" which she could achieve repeatably.

Regards,

Henning

It is certainly not difficult to believe that a 10-year-old could see images on the near and far side of the river, considering they could probably have TWICE the accommodation of a person of 45 years (See graph below). Also, I used the term "stare" because dealing with the series of scientific/medical occurrences involved is beyond my capacity to explain and beyond* the capacity of many others to appreciate.

* "If you can't explain it simply, you don't understand it well enough." — Albert Einstein

Thus, "STARE" works for me.