>>> I read somewhere that the Star in the Pentax lenses stands for
>>> APO, so it should apply to the apochromatic lenses. But I may be
>>> wrong...
>> I don't know the exact techanical differences, but Pentax &
>> Nikkor use ED, while Sigma use APO.
> The lens designation ED is not a definitive indication of lens
> performance it only indicates that the lens design utilizes glass
> with extraordinary dispersion characteristics. The lens
> designation APO is an indication of measured optical behaviour.
> By definition an APO labelled lens will provide coincident focus
> on a plane perpendicular to the lens axis at at least three
> frequencies in the visible spectra. Unfortunately few "APO"
> labelled lenses actually achieve this and the lens designation
> APO is no guarantee that the lens may not have other significant
> optical aberrations.
Here's a quote from "Alexander" (I believe it was from a post to the
now defunct "X-Star" group [is that the correct name? - was it a
Yahoo club or group?] and not to the PDML) -
> In a brochure from Pentax Germany (Technik-Information 8/88, p.15)
> the A* designation and the difference between A* and A* ED lenses
> is explained as follows (rough translation from German)
> "Lenses designated such as e.g. A*1,4/85 or A*2.8/300 ED contain
> 'special glasses'. Such glasses are characterised by a high
> refractive index and a low dispersion. They reduce the residual
> secondary spectrum[.....]. Lenses additionally designated with
> 'ED' are so-called Super-Achromats. They also contain special
> glasses; 'ED' stands for 'extra low dispersion'."
> According to this definition, all star lenses have low dispersion
> ('ED') glasses. The additional 'ED' designation refers to a higher
> degree of correction of chromatic aberation compared to the
> 'normal' star lenses, rather than it seems to refer to a
> particular sort of low dispersion (i.e. 'special') glass. 'ED',
> means that the lens is a Super Achromat.
> As far as my knowledge goes, a Super Achromat (i.e. as this term
> is used by Zeiss) is corrected from blue to infrared, or for four
> wavelengths - blue (about 430 nanometer), bluegreen (about 500nm),
> yellow (about 640nm), red (about 710nm) and infrared (about
> 900nm).
> Due to dispersion (means different refractive indexes for
> different wavelenths in a glass) a single lens does not focus the
> three (visible) colors at one point. A lens system is correced
> such that at least two colors (or more) focus at the same point.
> According to the amount of the "residual secondary spectrum" (i.e.
> the remaining "apochromatic error") a lens system can be ranked
> from low to high as achromat, semi-apochromat, apochromat and
> super-achromat.
> I don't think there is a strict definition for semi-apochromat and
> apochromat-lenses. Approximately, Apo- means that a lens system
> focuses three wavelengths at the same point (or may be "close to
> one point"). A Super Achromat focuses four wavelengths (or more)
> at the same point. (please correct if there is something wrong
> with this, I am no expert).
> I think this is an important information (particularly for Astro-
> and infrared photography). As far as I know the A*200mmED macro is
> the only Super Achromat (Tele) macro lens for 35mm systems.
Fred
