Quote:
Originally Posted by bojan
Gets,
Where do opticians buy the material for prescribed glasses?
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Bojan,
Suppliers like Hoya and Zeiss have catalogs of standard circular lens blanks spanning the whole range of whatever an optician might require. These are circular, approx 75mm diameter, big enough to accommodate being cut down to suit any frame. What the cheap opticians do is buy the appopriate blanks, cut to size and fit to the frame. The inter-pupil-distance (IPD) is also taken into account to determine where the lens centres should be as this varies quite widely from person to person.
The cheap $25 specs from chemists only deal with diopters (plus or minus) and optically are quite poor; since in daylight the iris of the eye is around 1mm aperture and working at f/15, there is no real need to attempt to correct for anything else.
However your post implies you want to correct for astigmatism, that's a whole different ball-game ...
a) chemists dont stock anything that corrects for astigmatism, and
b) the likes of Hoya or Zeiss will not sell lens blanks to an individual, only a qualified registered optician they recognise.
That leaves the practical alternatives as:
1. Cannibalise the lenses from a pair of old specs you have that have the required astigmatism correction, or
2. Buy a Televue Dioptrix - the only commercial astigmatism corrector you (or I) can actually buy.
NB I have "been there done this before" asked my optometrists before (they know I am into astro) and was given some blanks to play with from Hoya, and have discussed what's actually available from them - at the time I was aiming to make a really big RDF instead of the miserable little things we see. The blanks I got were OK for specs, but totally hopeless for astro. I ended up using a glass meniscus lens instead, from AliExpress.