[rollei_list] Re: Lens recommendation
- From: CarlosMFreaza <cmfreaza@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Tue, 3 Aug 2010 09:38:54 -0300
Thanks Richard for the explanation and the difference about
longitudinal chromatic aberration and lateral color aberration
specially, and then the "Apochromatic" DIN standard is/was for the
lateral color aberration while the traditional Abbe definition is for
the longitudinal color aberration.
And then if the lateral color aberration is proportional to the focal
length and a lens reduces this proportion to a minute percentage of
its focal length, this lens is "apochromatic" from this point of view,
it does not mean it is also apochromatic for the longitudinal color
aberration, it could be or not to be and vice-versa. It becomes also
clear that the lateral color aberration is more significant for long
tele-lenses as the Schneider brochure states.
Carlos
2010/8/2 Richard Knoppow <dickburk@xxxxxxxxxxxxx>:
> ---
>
> Well, lateral color is a separate aberration from
> longitudinal chromatic. Usually what we think of for color
> correction is longitudinal color, that is the difference in
> focus point for various colors. Its corrected by using glass
> of differing dispersions in positive and negative surfaces.
> One of the reasons for the importance of high index-low
> dispersion glass is that it makes possible better chromatic
> correction. Lateral color is a difference in the image size
> or magnification with color. The focal points for the colors
> may be the same, that is, images of different colors are all
> sharp at the same plane, but are of different size. That
> leads to fringing. Sometimes one can tell which aberration
> results in fringing by seeing how it changes with focus.
> Since with longitudinal chromatic the colors do not come to
> focus at the same point the color of the fringe will change
> with focus being red when blue is focused and red when blue
> is focused. With lateral color generally the fringes stay
> the same as focus is changed.
> Longitudinal color is corrected as above by proper
> choice of the glass type and power of the surfaces. Lateral
> color is corrected by the geometry of the lens, either by
> symmetry or by some other method of insuring that the
> magnification is the same for a range of colors.
> Historically, the terms achromatic and apochromatic
> have been applied to the longitudinal aberration and not the
> lateral aberration. Here again, the shape of the curve
> showing the position of the focal plane Vs: color will
> immediately tell you which sort of lens you have. A
> non-corrected lens will have a line which is constantly
> sloping, an achromat will have an approximately parabolic
> curve which goes through the center line at two points, an
> apochromatic lens will have an S curve which crosses the
> center line at three points, a superachromat will have a
> wavy line which crosses the center at four or more points.
> Chromatic aberration gets even more complicated because
> the deviation of the glass is not constant. Glass also has
> what is called partial dispersion where the rate of change
> in index of refraction becomes very great as it approaches
> the cut-off point of the glass. Correcting color near the
> limit of cut-off requires very careful choice of glass with
> different dispersion but similar partial dispersion.
> Dispersion should probably be defined here. All glass
> bends light that enters it at any angle to perpendicular.
> The ray of light is deviated by an amount which is
> proportional to the index of refraction. This is the
> difference in the velocity of propagation of light through
> the glass in comparison to its speed in a vacuum. All would
> be fine except that the velocity of propagation and, hence,
> the index of refraction change with the wavelength. This
> change in index is called dispersion and is the reason that
> a prism will produce a rainbow from a beam of white light.
> The dispersion depends on the nature of the glass. When the
> glass catalogue gives a value for index its an average or,
> more often, the index for a particular wavelength usually at
> some specified spectral radiation line of a gas since those
> are very easy to duplicate. The dispersion is the ratio of
> the index for the reference color to the range of indices
> over some specified range of colors, again usually defined
> in terms of the spectral lines of a gas. Dispersion is often
> given as an Abbe number or v number, which is the reciprocal of the
> ratio of indices. The dispersion of optical glass is usually called "partial
> dispersion" and its value is given in the glass catalogues. While the index
> of refraction usually increases with frequency (reciprocal of wavelength) so
> that it is greater for blue than for red light there are materials which
> exhibit anomalous dispersion where the index actually decreases.
> A good discussion of glass characteristics can be found in a brochure
> published by Hoya, one of the major makers of optical glass, at:
> http://www.hoyaoptics.com/pdf/OpticalGlass.pdf
---
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