I wish I hadn't started reading this thread.

I got left behind with the academic stuff, I didn't really want to get my books out but I probably will now!!

However, it got me thinking. At least 3 or 4 times a week someone comes to me and asks
"Clive, what's the index of this lens"

My answer is nearly always right.

After establishing the Rx and lens style it's then a combination of weight, feel, smell, chromatic aberration at edges, colour of material, sound when gently dropped and just a general instinct of having lived with this stuff for 12 hours a day, 6 days a week, 49 weeks a year and 35 years.

I'm sure I'm not the only one who can do this!