On Wed, 03 Feb 2021 09:13:09 +0000, Martin Brown wrote:

On 03/02/2021 05:52, jon wrote:

About four years ago, I had an eye test and got a pair of specs with
photochromic, varifocal lenses. I subsequently had a cataract operation
on one eye and had to buy another lens. Sometime later, I had another
cataract operation on the other eye, this required another lens and the
optician said I would need to replace both lenses because they could
not guarantee matching the other lens. Anyway I elected to go with just
one replacement and it was true, they didn't match. I was surprised at
the random nature of preparing lenses, in science the processes should
be repeatable to a very high degree.

Photochromic varifocal lenses are virtually impossible to match since
one of the pair will have been exposed to a few years of sunlight whilst
the other is fresh stock material (which varies a bit batch to batch).

Same problem with matching car paints. The pigments age under UV so a
perfect match from new will diverge as UV in sunlight fades the car.

I understand Martin, but at £100 a lens I would expect a bit of time
matching the tint. A cabinet maker would spend time and have remnants of
wood to match a colour and texture.