| EdZ |
| (Professor EdZ) |
| 07/02/09 09:26 AM |
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Re: ED glass in binoculars
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Now, regarding the ED/nonED question, I think we cannot really do the topic justice without entering the discussion of color sensitivity among various observers. Simply put, how much do differences of individual color perception affect the decision to purchase an ED or nonED model?
I addressed that issue in my report. It's not simply human perception (which by the way we cannot quantify, so I won't bother discussing), but it is color bias in the correction of the instrument. How many users hate the Fujinon FMT-SX becuase of the broad blue false color fringe, and yet how many does it not bother at all? That individual color perception is not relegated to ED/nonED choices. But as I reported above:
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So, having two binoculars of apparently the same quality, why is it one binocular can look so much better than another? Refer to the portion above about bias. Color correction of two different achromat scopes (two different binoculars) can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.
This brings up a good point. This issue on bias and the following I answered above just this moring on lateral color
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Every wavelength of light has a different focal length. Since a lens presents rays across not a flat plane at the focal point, but a curve, if it were corrected to have CA minimized at the center, it will still present the focal point of the same wavelength rays at a different position to the eyepiece as you move further off-axis. An eyepiece cannot focus across all the points of a curve at the same time, so while focused on any given point, some other point must be out of focus. The presence of lateral color is showing the amount of off-axis curve that cannot be accomodated by the eyepiece, focal plane junction.
I'll assume it is for this reason, often the CA correction in a doublet is corrected at the position 70% off axis. This at least helps minimize this effect. Perhaps a lens design that moves the CA correction away from the 70% off-axis position inward is one that shows considerably greater lateral color. But this is now considerably off-topic to the discussion of ED glass. This is lens design.
Take a moment and think about both these issues. They can both be accommodated very well in either an ED binocular or a non-ED binocular. BUT more importantly, neither one really is dependantt on ED glass. So here are two examples of how someone can perceive that the binocular marketed as ED glass so much improves the view, yet the reason for the improved view might have nothing to do with ED.
There are some not so expensive glasses that can be employed in design to market as what may be called ED binoculars. Not all of them really give all the improvements that may be perceived by the user. Few users really expend the effort to actually quantify what causes results. If you read clearly what I summarized in my very first post, you may notice that one of my points is that ED glass may not be responsible for all that is perceived. Isolating cause and effect is not so easy. Perception can often be mis-perception.
edz