Telescope Reviews: Contrast

In essence, adding magnification decreases exit pupil and reduces brightness, but it does not increase contrast. It only gives the perception of increased contrast on stars because the faint stars in the view becomes easier to see with added magnification. What happens when you add magnification to your existing aperture is the sky background, which is an extended object, gets darkened. But all other extended objects would get equally darker. However, the object has some perceptible light gradient difference (contrast) with the background and its contrast difference with the sky background is maintained, not increased or decreased. As the object gets larger the perception is it has become easier to see. While it may possibly be made easier to see with the optimum magnification, this is improperly interpreted as increased contrast.

Post in the above noted thread that includes links to some of the best written works on contrast.

Thread on CONTRAST with links explaining Optimum Detection Magnification

And posts in this thread that include even more works on ODM and contrast
Exit Pupil Overrated

Please refer to these two written works by Bill Ferris which provide some of the best and easiest to understand explanations of contrast that you are likely to find anywhere.
An Explanation of Contrast Ratio by Bill Ferris

An offsite paper by Bill Ferris explaining contrast threshold
Lowering the Threshold

What other quality aspects affect contrast?
Among other things, contrast is reduced by reflected internal light (which also is a result of poor or no coatings), improper or no baffles which permits stray light to interfer, poorly polished glass surfaces which produce scatter, poorly figured lenses which produce poor resolution and reduce the amount of light in the Airy disk, chromatic aberration which reduces the concentration of all colors of light in the Airy disk and spherical aberration which increases the size of the point image instead of concentrating the light to a point.

Are coatings more important for large exit pupils?
this thread title belies the content of the thread. The discussion revolves around contrast, but rather than just discussing the importance of coatings, it contains discussion of how contrast is not only affected by lesser quality coatings, but also by: sky conditions bright or dark therefore apparent contrast, larger aperture therefore larger exit pupil, smaller aperture therefore lower light gathering, lens aberrations, eye pupils, masking the aperture, lower magnification therefore larger exit pupil, higher magnification therefore smaller exit pupil but also larger image scale.

Users comment on contrast.

Please note, some of these following explanations apply to stars and this is improperly described as contrast. In some cases below this is explained.

Exit pupil is also a factor in contrast. I find that 3-4mm works best for my skies (also good for compensating for my astigmatism), but at 5mm the contrast starts to fade, at 6mm the skies begin to wash out and fainter stars disappear, and at 7mm, the skies look brighter through the bins than naked eye, if that's possible.

Performance of 8.5x44 vs 9x63 vs 10x50

A higher contrast, sharper image from a smaller exit pupil can exceed an image from a larger exit pupil if it's not from an equally high quality binocular.

What do you give up to get 7mm exit pupils? Image size. But if the image is broad and diffuse, do you need a larger image scale? Maybe not.

The binocular that would be best for broad diffuse extended objects is not the same binocular that would be best for most other objects. For most everything else, a higher magnification (that produces a smaller exit pupil) provides a larger image and a bit deeper limiting magnitude that allows seeing more.

Think of the sky as the backdrop in a portrait. Your subject must stand out from the backdrop to be seen. (There must be contrast.) Consider it to be a grey scale. You all understand the affect of dark sky background. Higher magnifications in a telescope reduce the extended light of the background until it approaches a completely dark sky, to the point you can no longer see the field stop. Lower magnification with a larger exit pupil delivers more light, not only from your subject, but also from the background. This can be beneficial or detrimental, depends on the condition. Generally, this makes it much more difficult to see most objects, but in dark conditions may allow you to see nebulous objects.

Exit Pupil, Large or Small

There is no question that a larger aperture gathers more light. But it gathers not only the point source light, but also the sky background light. Under a dark sky, with far less sky background light, the performance curves are much further apart. Under a bright sky, the performance curves are almost equal.
But now on to (C), the most significant point Fiske highlights here. Let me repeat it.
(C)They (comparisons of performance as determined by point source LM) do not reliably represent performance on dim, extended objects. Try doing the 10x50mm versus 10x70mm comparison on a faint open cluster like NGC 7789 in Cassiopiea and see which instrument comes out on top.

This point cannot go overlooked. The difficulty is now involved in assessing which is better when viewing extended objects. This can be a subjective call. Point source LM is not subjective at all. However, it is not a good indicator of performance of contrast on extended objects.

There are a wide ranging class of objects that would fit the criteria "extended" in binoculars. Of course, all the typical extended nebula (Neb) and galaxies (Glx), but also some open clusters (OC). OC, if very dense may be resolved in scopes but may not be resolved in binoculars, and especially if comprised of predominantly faint stars. NGC7789 probably fits this criteria pretty well.

The difficulty comes in objectively assessing the performance on objects where you don't get a maximum or a count or something objective. This doesn't make it impossible to judge the differences in contrast, just more difficult.

Comment about Exit Pupil Size

"Contrast Enhancement via Magnification--As the magnifying power of an eyepiece increases, the amount of light reaching the eye decreases. However, a modest increase in magnification is often found to enhance the contrast between stars and the surrounding sky, and this effect can sometimes be exploited when making estimates of relative magnitude in moderately light polluted skies. It is frequently found, for example, that 10x-50mm binoculars are preferable to 7x-50mm binoculars in less than totally dark skies. The same holds true for a telescope, and you may find that an increase from a low power to a medium power eyepiece, say, from 20x to 40x, will provide a more favorable viewing situation under marginal conditions."
-- Sommers-Bausch Observatory -- University of Colorado:

"The glow of the background sky is a diffuse source, which will be spread out by higher magnification, reducing its brightness. Although high magnification doesn't increase the brightness of faint stars, it improves their contrast (ed. note apparent brightness, this article inproperly refers to this as contrast) against the sky, making them more visible!"

On the night sky, apparent contrast has *EVERYTHING* to do with magnification, light pollution, seeing conditions, surface light scatter, internal light scatter, quality & thoroughness of coatings, baffling, quality of optics, etc. In short, perceived contrast is a direct result of the summation of the whole instrument, the observing site and conditions and the observer.

edz

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Edited by EdZ (08/27/07 11:53 AM)

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