Re: [ATM] Reverse engineering a lens.

["Thomas Janstrom" <tomjan@ozemail.com.au>]

Hi Richard,

I’m afraid I made a bit of a mistake there. I used the outer diameter as
measured edge to edge (138.45mm), but measured the edge thickness at the
edge of the bevel (which would make the effective diameter 137.45mm). I
know this will change things a bit.

As for the spherometer, well it spans 92.4mm which is just as well as
all these lenses have central perforations, readout is good to 0.005mm.

As far as the wavelength used I wouldn’t have a clue, I’ve got OSLO set
to 587.56nm if that’s any help. ;)

Any suggestions on how to measure the focal length of a negative
meniscus lens? I’m at a loss on this one, I’m thinking seeing as it is
dispersive in one direction shouldn’t it have a focal point in the
other?

Clear Skies, Thomas Janstrom.
www.tjanstrom.com
www.norsewines.com.au 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Richard F.L.R. Snashall
Sent: Tuesday, 27 September 2005 6:50 PM
To: atm@atmlist.net
Subject: Re: [ATM] Reverse engineering a lens.

Thomas Janstrom wrote:

>Hi Richard etal,
>
>Well I now have the lenses on hand now and I can give the ROCs of all
>surfaces, however I have found a problem with some of the supplied data
>for the negative meniscus, to be detailed with the measured data below.
>So here goes.
>
>Lens one (now know as L1898, seeing as this was scribbled on the paper
>it was wrapped in.)
>Double convex, not iso. 
>Diameter 138.45mm (.5mm bevel on both sides)
>FL 525mm
>ET 5.0mm
>CT 13.595mm (OSLO LT derived)
>ROC surface one 454.247mm
>ROC surface two -679.712mm
>OSLO LT's best guess at the glass is Schott ZKN7 (matches stated FL to
>with in .125mm) 
>
>
With D = 138.45 mm, R1 = 454.247 mm, R2 = -679.712 mm, and
t(edge) = 5.00 mm, I come up with t = 13.84 mm.  How does
the bevel affect your edge thickness measurement?

Those are pretty accurate radii. To get that accurate, you'd
need full diameter sagitta measurements (assuming you're using
a spherometer) on the order of  0.01 micron. What diameters and
accuracy do you have?

With t = 13.84 mm and the f.l. = 525.0 mm, I get a refractive
index at the measured wavelength of 1.520808.  One quick question
is, "at what wavelength"?  I've seen some companies use the d-line,
some use the e-line;  Newport Glass, for example, actually
supplies their data for the C-line.

>
>So there we have it the "ground truth data". Now although I'm writing
>this on the morning of the 26th I won't get to send it for a couple of
>days as my phone line had a meeting with a grass fire and came off
>second best....
>
>Clear Skies, Thomas Janstrom.
>www.tjanstrom.com
>www.norsewines.com.au 
>
>
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