Are Stan Bray’s Optical centre punch dimensions correct?

[Andrew TinsleyParticipant @andrewtinsley63637]

I thought it about time that I made an optical centre punch, as I am having some difficulty with aging eyes! Checking the MEW index there is an article by Stan Bray in the first MEW issue, reprinted in 25 year anniversary special. I was about to make a start when I noticed that the focal length of the device was approximately 50mm and the radius of curvature of the plano convex lens is given as 25mm.

This set some dim alarm bells ringing, My rusty optics say that the “thick lens” formula was 2F = R, where F is the focal length and R is the radius of curvature. I did a quick check and this is indeed the correct formula (derived with a little approximation!)

The implication is that Stan had used an incorrect formula of F= 2R to derive his dimensions? So has anyone made the optical centre punch and did it work? Alternatively am I making an obvious a misunderstanding of the optical theory?

Andrew.

[FulmenParticipant @fulmen]

I just made two of them: https://www.model-engineer.co.uk/forums/topic/magnetic-optical-punch/

The first was 55mm long and will focus with R=25mm but not with R20. Thee second was 50mm long and works with R20.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hi Fulmen,

Thanks for that very useful information. So it would appear that Stan Bray’s dimensions are essentially correct, as you have shown practically!

It must be my interpretation of the thick lens formula that is faulty. I shall put on my analytical specs and see if I can get to  the bottom of this puzzle.

Thanks again,

Andrew.

 

 

 

[FulmenParticipant @fulmen]

Not quite. The 2F=R would suggest that the 50mm lens shouldn’t work with R20 which it does. One thing lacking in your formula is the refractive index, that might be the difference.

[Author]

Posts