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Chromatic Aberration

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#1 EdZ


    Professor EdZ

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Posted 14 March 2004 - 12:16 PM

The point was raised that chromatic aberration makes it more difficult to reach precise focus. This is absolutely correct. Chromatic Aberration (CA) is the result of the various wavelengths in the light not reaching focus at the same precise point. CA has other significant effects on the image. As CA increases, the light grasp of the instrument is reduced and the contrast in extended images is reduced.

CA is a function of refraction. Light from different wavelengths is refracted differently. Blue, yellow and red wavelengths of light reach focus at slightly different points along the focal length axis of the lens. By using a doublet lens, with each glass having different properties, the focus point for most of the wavelengths can be brought closer together. CA can never be eliminated entirely.

The eyepiece imparts its own CA. However the contribution to the total CA in the system is related to the focal length (FL) of each. Therefore, in a system where the objective has a FL of 500 and the eyepiece has a FL of 25, the simplified relative influence relation is 500/25.

The eyepiece has greater influence on system CA at lower magnifications, such as in binoculars where we might have 300/20 = 15x. CA increases as the focal ratio of the objective is reduced, such as in wide field scopes, (~f5 to 6), and binoculars, (~f3.5 to 5). Therefore, in binoculars we have the worst of both conditions present, both a fast f# objective and low magnification. It would be impossible by looking through the system to determine how much each part is responsible for the whole. However, in products such as the Fujinon 16x70 and the Fujinon 10x70, both having the same objective lens, it may be possible to see a difference in the CA. In a wide field scope, it might be found that a particular eyepiece minimizes CA.

False color, CA, in the image can easily be noticed by adjusting the eyepiece to be slightly inside and then slightly outside of the focus point. The image of very bright objects may appear to show different color surrounding the object at inside of focus than outside of focus. This is simply a function of the various focus points of color along the objective focal point axis.

Only when precisely focused on the object is CA minimized. This helps explain why binoculars when used for terrestrial viewing may show considerably more false color. Only the object at the precise focus distance will have CA minimized. Every other object in the field of view, closer than or further away than the precise focus point, may show greater CA. This may explain why a particular model terrestrial binocular with a low CA seems to have a greater depth of field. When used for astronomy, focus is on infinity and there is no depth of field to compare.

CA affect on light grasp is a result on not being able to focus all the light into the Airy image. The CA light escaping to the fringes does not contribute to the overall image. In a more corrected system, there is greater light in the image. This might explain why some observers say their apochromatic refractor reaches deeper than an achromat of equal diameter. Even though some might not see such a great improvement in the image from the reduction of false color, there is more light being delivered to the image.

In extended object images, any greater degree of false color CA will cause a slight color blurring at the fringes of dark detail and bright background. This will reduce the contrast needed to see such detail.

In typical achromat lenses, most binoculars, there will always be some chromatic aberration. To what degree is dependant on many factors. Some people can ignore the false color and concentrate on the image, others cannot. The best way to judge the detrimental effects of chromatic aberration might be to judge the light grasp and contrast of a given diameter as compared to one of equal size at magnifications not necessarily equal, but nearly so. If the contrast and resolution are there, then any CA you see is not enough to reject the instrument.

See also this post on
Chromatic Aberration on planetary disks


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