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
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
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