View Full Version : strange lens phenomenon

30-Sep-2018, 00:25
sorry if this is in the wrong place but this seemed most appropriate

I wear glasses and without them nothing is in focus from my nose to infinity
they are fixed focus not vary focals or the like

yesterday i noticed a neon sign reflected on the inside surface of my glasses and it was tack sharp

at first I thought i could focus onto that surface that would be about 10mm from my eye but holding a phone up without my glasses did not work
then I thought that i am focussing at infinity via the reflex of the lens but taking my glasses off and looking at the sign it was very blurred

maybe iím focusing on the inside of the front surface so focussing thru the lens ?
but then surely i would be then focusing back thru the lens so effectively looking thru the lens twice
which would at least double the image size or something but the sign was the same size as it was when i looked at it straight with my glasses

anybody know what is going on ?



Jeff Donald
30-Sep-2018, 14:50
Probably this,


30-Sep-2018, 15:46
You really want to see the ground glass up close. Have you tried a loupe?

4-Oct-2018, 11:35
hi Jeff and Ic-racer
I should clarify

when I say glasses I'm referring to eyeglasses aka spectacles and not the ground glass focusing screen


Colin Robertson
4-Oct-2018, 11:45
The inside surface of your spectacle lens is not flat, it will be contoured to your prescription. Is it focusing the reflection to accomodate your eyesight?
See if a flat piece of glass (maybe a filter) does the same thing and report back- that would really be revealing.