We haven't seen much of our resident optician Looksharp65 recently, so here are a few observations open to correction.
Use with binoculars:-
If one cannot see clearly at moderate distances with the naked eyes, due either to near-sightedness, advanced presbyopia or astigmatism, then one is going to need correction when birding, and if wearing glasses the eye relief of the binoculars should ideally be sufficient to see the field stop.
Correction for far-sightedness with positive lenses effectively reduces eye relief, whereas negative lenses for the near-sighted increase it.
Swarovski's specifications for eye relief seem to be fairly consistent, so being far-sighted I am content with about 18 mm by their standards. Two or three mm less would probably suffice for the near-sighted.
Frames:-
Titanium is an ideal material for spectacle frames. Being strong, the frames can be made thin without intruding into the peripheral vision. It is also light in weight and hypoallergenic. The high yield strength though makes it difficult to adjust, so the frame should be a comfortable fit from the start.
Lenses should not be too large. Negative lenses that correct for near-sightedness will have a certain minimal thickness at the optical centre and get progressively thicker towards the edge, so weight will increase exponentially with increasing diameter.
Positive lenses that correct for far-sightedness require a minimal edge thickness, so the thickness of the blanks at the optical centre will also have to be adapted to the lens diameter.
The ideal lens shape would be circular if the lens spacing corresponded with the wearer's IPD. Rectangular shapes that have been fashionable in recent years make inefficient use of lens blanks and are unsuitable for multifocal lenses. The so-called "panto" form is a good compromise.
Varifocals and Bifocals:-
Apart from cost, bifocals have a number of advantages. When used with binoculars one can avoid the near section intruding into the view and for reading the usable width is greater than for varifocals. The borderline also provides a visual check of correct allignment. My specs once came back from the opticians with one bifocal lens visibly askew. An assembly error of that kind would have been a lot harder to detect with varifocals and would have caused much frustration.
The disadvantages with bifocals are theoretically the intermediate distances. My eye doctor always prescribes an addition of 2,5 dioptres, which corresponds to a reading distance of 40 cm. I, however always request a +2 dioptre addition (50 cm) and this works well for reading and also for a laptop. The worst case would be +1 dioptre, i.e. 1 m distance, but in good light, objects at this distance are acceptably sharp, top or bottom.
Lens materials:-
There are still some advocates of borosilicate glass but for the comparatively weak lenses used for eyeglasses no-one would ever detect any optical advantages. It does have a comparatively high Abbe no. (low dispersion) but transmits quite a lot of UVA and even some UVB, see here:-https://entokey.com/spectacle-lens-materials/
Scratch resistance is not really a valid argument as modern coatings provide similar or better scratch resistance than the glass substrate and their renunciation would entail reflective losses of 4% at each surface, i.e. an 8% transmission loss!
For moderate prescriptions CR-39 is an ideal plastic material. It's cheap, hard and has low dispersion (CA) and good UV absorption. There's just one minor caveat (see "pitfalls" below).
Cleaning:-
Careful cleaning involves no effort. Just a drop of liquid soap between thumb and forefinger under running water. If the lenses have hydrophobic coatings the water droplets can be dabbed off with a soft tissue (Kleenex or similar). I always have a box in the bathroom solely for this purpose. If out birding in cold weather teardrops can fall on the lenses, particularly if using an angled scope, and then a lens cleang tissue can be useful.
Pitfalls:-
A short while ago my everyday glasses fell onto gravel and sufferd a minor scratch. After six years of daily cleaning they had been in absolute pristine condition, so I took them to the opticians for a new set of CR-39 lenses with a minor change in prescription.
For everyday I was then using a second pair I had bought some years ago. This is a rather expensive and very minimalist titanium frame with contoured nose bridge, lacking nose pads and sleeves on the temples. I chose the frameless version, which was about €100 cheaper than the full frame -big mistake!
Frameless designs require great precision in assembly and the chain of opticians with whom I had previously been very satisfied botched it completely. They do guarantee customer satisfaction or your money back and at the third attempt eventually got it right.
While taking off some headphones, one of the temples got caught and a lens snapped. Another BF member had pointed out that CR-39 is unsuitable for frameless designs because of insufficient strength, so these went in for some new Trivex lenses.
The bill for otherwise identical bifocal lenses was €157 for CR-39 in full frame and €426 for Trivex in frameless!
John
Use with binoculars:-
If one cannot see clearly at moderate distances with the naked eyes, due either to near-sightedness, advanced presbyopia or astigmatism, then one is going to need correction when birding, and if wearing glasses the eye relief of the binoculars should ideally be sufficient to see the field stop.
Correction for far-sightedness with positive lenses effectively reduces eye relief, whereas negative lenses for the near-sighted increase it.
Swarovski's specifications for eye relief seem to be fairly consistent, so being far-sighted I am content with about 18 mm by their standards. Two or three mm less would probably suffice for the near-sighted.
Frames:-
Titanium is an ideal material for spectacle frames. Being strong, the frames can be made thin without intruding into the peripheral vision. It is also light in weight and hypoallergenic. The high yield strength though makes it difficult to adjust, so the frame should be a comfortable fit from the start.
Lenses should not be too large. Negative lenses that correct for near-sightedness will have a certain minimal thickness at the optical centre and get progressively thicker towards the edge, so weight will increase exponentially with increasing diameter.
Positive lenses that correct for far-sightedness require a minimal edge thickness, so the thickness of the blanks at the optical centre will also have to be adapted to the lens diameter.
The ideal lens shape would be circular if the lens spacing corresponded with the wearer's IPD. Rectangular shapes that have been fashionable in recent years make inefficient use of lens blanks and are unsuitable for multifocal lenses. The so-called "panto" form is a good compromise.
Varifocals and Bifocals:-
Apart from cost, bifocals have a number of advantages. When used with binoculars one can avoid the near section intruding into the view and for reading the usable width is greater than for varifocals. The borderline also provides a visual check of correct allignment. My specs once came back from the opticians with one bifocal lens visibly askew. An assembly error of that kind would have been a lot harder to detect with varifocals and would have caused much frustration.
The disadvantages with bifocals are theoretically the intermediate distances. My eye doctor always prescribes an addition of 2,5 dioptres, which corresponds to a reading distance of 40 cm. I, however always request a +2 dioptre addition (50 cm) and this works well for reading and also for a laptop. The worst case would be +1 dioptre, i.e. 1 m distance, but in good light, objects at this distance are acceptably sharp, top or bottom.
Lens materials:-
There are still some advocates of borosilicate glass but for the comparatively weak lenses used for eyeglasses no-one would ever detect any optical advantages. It does have a comparatively high Abbe no. (low dispersion) but transmits quite a lot of UVA and even some UVB, see here:-https://entokey.com/spectacle-lens-materials/
Scratch resistance is not really a valid argument as modern coatings provide similar or better scratch resistance than the glass substrate and their renunciation would entail reflective losses of 4% at each surface, i.e. an 8% transmission loss!
For moderate prescriptions CR-39 is an ideal plastic material. It's cheap, hard and has low dispersion (CA) and good UV absorption. There's just one minor caveat (see "pitfalls" below).
Cleaning:-
Careful cleaning involves no effort. Just a drop of liquid soap between thumb and forefinger under running water. If the lenses have hydrophobic coatings the water droplets can be dabbed off with a soft tissue (Kleenex or similar). I always have a box in the bathroom solely for this purpose. If out birding in cold weather teardrops can fall on the lenses, particularly if using an angled scope, and then a lens cleang tissue can be useful.
Pitfalls:-
A short while ago my everyday glasses fell onto gravel and sufferd a minor scratch. After six years of daily cleaning they had been in absolute pristine condition, so I took them to the opticians for a new set of CR-39 lenses with a minor change in prescription.
For everyday I was then using a second pair I had bought some years ago. This is a rather expensive and very minimalist titanium frame with contoured nose bridge, lacking nose pads and sleeves on the temples. I chose the frameless version, which was about €100 cheaper than the full frame -big mistake!
Frameless designs require great precision in assembly and the chain of opticians with whom I had previously been very satisfied botched it completely. They do guarantee customer satisfaction or your money back and at the third attempt eventually got it right.
While taking off some headphones, one of the temples got caught and a lens snapped. Another BF member had pointed out that CR-39 is unsuitable for frameless designs because of insufficient strength, so these went in for some new Trivex lenses.
The bill for otherwise identical bifocal lenses was €157 for CR-39 in full frame and €426 for Trivex in frameless!
John
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