[rollei_list] Re: 2.8F models...the 3.5/75 six elements (again), the answ...

  • From: Jerry Lehrer <jerryleh@xxxxxxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Thu, 05 Jan 2006 10:59:23 -0800

Richard,

Didn't an old friend of yours. Izzy Newton, discover the differential
chromatic dispersion of a prism a few centuries back?

Jerry

Richard Knoppow wrote:

> ----- Original Message -----
> From: "Jerry Lehrer" <jerryleh@xxxxxxxxxxx>
> To: <rollei_list@xxxxxxxxxxxxx>
> Sent: Wednesday, January 04, 2006 7:51 PM
> Subject: [rollei_list] Re: 2.8F models...the 3.5/75 six
> elements (again), the answ...
>
> > Richard,
> >
> > Now we know how to make a Movado!
> >
> > Jerry
>
>    Well, maybe a Timex. I also forgot to say that all the
> data is for a standardized focal length of 100.0mm and the
> radii and distances are also in mm.
>   This would have been obvious if I could have copied and
> pasted the optical data. If there is a way of doing this in
> Lensview I haven't figured it out.
>    Also, even where the dispersion of the glass is shown the
> partial dispersions are not. Partial dispersion is important
> in calculating or controlling the chromatic correction.
>    Briefly, the Index of Refraction is an avarage value,
> usually given for green light, sometimes yellow sodium
> light. Glass has the property that its index changes with
> the wavelength of light generally becoming higher as the
> wavelength shortens. In other words, a glass lens or prism
> bends blue light more than red light. The difference between
> two specified wavelengths is called the dispersion and is
> usually stated as an inverse number, a method originated by
> Ernst Abbe and called the V-number or Abbey number. Now,
> things are actually not this simple because the rate at
> which the index changes with wavelength is not constant.
> Especially near the cut off of transmission of the glass the
> index begins to rise rapidly. So one can have the condition
> where an achromatized lens has considerable blue fringing
> because the two types of glass don't match there even though
> they may match very well elsewhere in the transmission band
> of the lens. This is called "Anomolous Dispersion" and is
> given in complete glass specs.
>    If you look at the transmission band for glass you will
> find that most glass has several "windows" where it
> transmits light with intervals where it is opaque. At the
> margins of these bands the speed of light in the glass
> changes very rapidly. Since the speed of light can not
> exceed the speed of light in a vacuum some of the values
> become undefined at the transition points. Near these points
> the rate of change is quite non linear although it is
> continuous. By finding glass types with differing overall
> index but similar dispersion curves the disigner can reduce
> chromatic aberration significantly. Since other aberrations
> are also affected by the chromatic correction the overall
> performance of the lens can be improved. Modern glass types
> offer a much wider range of average Index to V-number but
> even if one has excellent high index, low dispersion glass
> for positive elements and low index, high dispersion glass
> for the negative elements the achromatism will not be
> perfect unless the anomolous dispersions match closely.
>
> You can now build a chronograph.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
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