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Message: <blockquote data-quote="Dorian Gray" data-source="post: 1106753" data-attributes="member: 61072">Objectives, prisms and eyepieces are all definitely potential sources of CA, but I sadly share your uncertainty about the specifics. But any aberrations in the image projected by the objective must be there for good: information has been lost, and the best one can hope for is that the rest of the system perfectly reproduces what is left.Chromatic aberrations fall into two types, as you mentioned: longitudinal CA (also known as axial colour, in which different wavelengths are focused at different planes) and transverse CA (also known as lateral colour, in which magnification varies by wavelength). Binoculars suffer from both to varying degrees. Axial colour affects the entire image, while lateral colour gets progressively worse with increasing image height (i.e. off-axis).Axial colour is characterised by colour fringes around high contrast objects of any orientation, and the colour of the fringes can often be induced to visibly change by very slightly adjusting the focus. Due to its nature it is reduced by stopping down the optical system, which increases the depth of field, masking any colours that are slightly out of focus due to the aberration. Binoculars are effectively stopped down when the viewer's pupil is smaller than the exit pupil, as long as the pupil is perfectly centred within the exit pupil. In practice this is difficult to achieve, though using a 50 mm binocular in bright sunlight, for example, will almost certainly reduce visible aberrations to some extent.Lateral colour is also characterised by colour fringes around high contrast objects, of course, but as it is the result of the magnification varying by wavelength, it is non-existent on-axis (because a point in the centre of the field is mapped to the centre of the image regardless of magnification), and becomes progressively more visible away from the centre of the image. It reveals itself as fringes of two different colours, one on either side of the high-contrast structure. It is only visible around structures with a tangential component in their orientation; sagittal structures (i.e., those with a radial, spoke-like orientation) are immune to lateral colour. The interesting thing about lateral colour is that stopping down the system does nothing to reduce this aberration, because increased depth of field cannot help when light of different wavelengths is all perfectly focused on the image plane in the first place.One might think that axial colour would be a much greater problem than lateral colour when designing a binocular objective, but this is complicated by the public demand for compact binoculars, which necessitates using a telephoto design for the objective. Telephotos, like retrofocus lenses, are markedly asymmetrical and consequently suffer from greater lateral colour problems than a simple lens (such as a flint-crown achromat) of the same focal length. But they are physically shorter than their focal length, and that is all-important.Eyepiece design is also complicated in today's market, which at once demands compact size and long eye-relief, design features that are mutually opposing. A binocular's power is given by dividing its objective focal length by its eyepiece focal length, but designers have tended towards using short focal lengths (i.e. fast systems with small f-numbers, which also works against high image quality) to satisfy the people's desire for small size. This means the eyepiece design has a short focal length, which results in a short eye-relief unless the eyepiece design too is made asymmetric, which again introduces lateral colour. It's maybe useful to note that the Nikon HG L binoculars have long eye-relief despite their relatively compact dimensions, indicating a very strongly telephoto objective and/or an asymmetric eyepiece design, both of which scenarios introduce large design problems of lateral colour.My understanding of dispersion, which is the cause of colour fringing, is that it occurs where light traverses a boundary from one material to another of a different refractive index. Therefore it stands to reason that dispersion would not occur within the prisms, at the surfaces of total internal reflection, but only at the entry and exit faces of the prisms. Further, if the incident rays of light are limited to those close to the normal of the prism (perpendicular to the prism's entry face), as would be the case in a binocular of narrow field of view and/or large f-number (or long focal length, when talking about binoculars of a given objective diameter), then the dispersion caused by the prism would be minimised.Interestingly, Cabela's website <a href="http://www.cabelas.com/prod-1/0049755712893a.shtml" target="_blank">states</a> that Kowa use SK15 glass for the prisms of the Genesis/Prominar XD 44. Here is how that compares to common prism glasses, according to <a href="http://www.schott.com/advanced_optics/english/download/minikatalog_e_v1.7_170108.pdf" target="_blank">Schott's 2007 optical glass catalogue</a> (PDF file):Refractive index:N-BK7 - 1.51680N-BAK4 - 1.56883N-SK15 - 1.62296Abbe number:N-BK7 - 64.17N-BAK4 - 55.98N-SK15 - 58.02(I think the "N" means the glasses are environmentally friendly replacements for the old leaded versions.)Cabela's says that the "SK15 roof prisms have a high refractive index". Now, I can't speak for anyone else, but I'm not terribly fussy about the refractive index of my prisms! <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" /> As long as the refractive index is sufficient to cause total internal reflection for all light within the light-cone from the objective, as will be the case in any competently designed binocular, the refractive index doesn't matter. BAK4 glass allows a steeper light cone (i.e. a faster objective, which in turn permits a more compact binocular) than BK7, but otherwise BK7 is superior due to its higher Abbe number (lower dispersion). SK15 seems to strike a decent balance: a higher refractive index than BK7 or even BAK4, permitting a more compact binocular, but lower dispersion than BAK4. I suspect the high Abbe number is why Kowa chose to use it, because the Kowa Prominar XD isn't unusually compact (though compact enough to prevent the use of BK7). I don't know what glass Leica, Swarovski, etc., use in their binocular prisms.And so we see once again that everything about short, compact binoculars militates against high image quality. But it's what the people want, and I can't say I'm any different!</blockquote>

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