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Steiner Peregine 8.5X44 XP |
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Bird Watcher
Optics Master Joined: August/30/2006 Status: Offline Points: 1523 |
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Interesting!
One company uses the word index to describe their glass. Another company uses the word index to describe their matching lens coatings. Ain't advertising grand? (confusing?) Do they say what they mean, or, do they mean what they say? |
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Roy Finn
MODERATOR Steiner Junkie Joined: April/05/2004 Location: United States Status: Offline Points: 4856 |
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I wonder what happens when you combine a scope with "matched indexes", "extended twilight lens systems", sealed in space age gases with diamond like coatings and throw in some "total light throughput"??? I guess them deer's just walk right in to ya and surrender....
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Bird Watcher
Optics Master Joined: August/30/2006 Status: Offline Points: 1523 |
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Only if you are wearing the proper "scent".
Don't forget the HD (High Definition) camo clothing while you are at it. Are Americans the only ones who have to put up with this nonsense, or is it a worldwide phenomenon |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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OK, before we get too carried away here. When talking about optics, index refers to one optical parameter: "index of refraction". Index of refraction is the ratio between the speed of light in the vacuum and the speed of light in the material we are looking at here. Index of refraction, as the name suggests, defines the angle of refraction of glass. It also defines which fraction of the light gets reflected from the boundary between two materials of different indices of refraction. For two materials with indices of refraction n1 and n2, the reflected fraction of light (R) will be equal to [(n1-n2)/(n1+n2)]^2. For a common example of light being reflected at the interface between air (index of refraction = 1.00001 or so) and glass (index of refraction varies for different glass melts, but let’s assume it is 1.5 for the time being), R=0.04, or 4% of the light gets reflected (ignoring angular considerations for the time being). In the example above, if you deposit a thin layer of additional material with n=1.25 (or anywhere in between the refractive indices of air and glass) onto a glass surface, you will end up with two interfaces that reflect light: air to “thin deposited layer” and “thin deposited layer to glass”. However, since the differences between indices of refraction at each interface are smaller, despite the fact that there are two reflections now, the total amount of light reflected is actually smaller. On top of that, by matching the thickness of the deposited layer to the wavelength of incident light (more exactly, to the quarter of the wavelength of the incident light in the particular material), you can interferometrically cancel out the reflected light. By adding additional thin layers of material of varying indices of refraction, you can minimize the reflection even further. Pretty straight forward so far, right? When you are doing this in practice, things get a little more complicated. The index of refraction of glass is different for different wavelengths of light (different colors). The variation of index of refraction with wavelength of light is called dispersion (think ED glass, for example). From a coating standpoint, different colors are a major pain: to optimize for a particular color, you need coating layers of different thickness, and of very uniform thicknesses. There is something called “Abbe number” which is a measure of glass dispersion for visible light. For example, here is a picture of the
transmission/reflection profile of cerulin-plus coatings used by Optolyth (I
was just reviewing a couple of very nice Optolyth spotters). You always have to compromise with
coatings. Optolyth made sure there is
good transmission of blue light and when you play with their scope in poor
light, it shows. They sacrificed some transmission between blue and green to get a good peak in the blue/violet region that is important in low light. Back to Leupold. I have talked to several different Leupold reps. They always give me the same speech on how their “index matched” system has the coatings with the refraction indices matched to the type of glass used and glass with the refraction indices matched to the visible light spectrum. Well, anti-reflection coatings, by definition have to be matched to the glass they are deposited on, and glass has to be selected appropriately for the light. Every manufacturer in the world does it. It is very good marketing and when they say all this stuff about matching indices, they are not lying. However, the only thing Leupold invented with regard to this whole index matching thing is the marketing slogan. They improved their coatings compared to Multicoat4 they used before, and simply came up with a new name for their new coatings. The rest is bologne. ILya |
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Roy Finn
MODERATOR Steiner Junkie Joined: April/05/2004 Location: United States Status: Offline Points: 4856 |
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Thanks ILya. Not sure if you have an answer for this but I'll through it out just the same. Would you say that the thickness of the material applied is more important than the type of material?
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13181 |
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Both are important.
If material is of the wrong type you will not get the desired effect (for example, if the index of refraction of the material is actually greater than that of the glass in question, you will have more reflection, not less). However, if the material is of the wrong thickness, you will only get a part of the desired effect: the itnerferometric component (destructive interference) will not work. ILya |
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Roy Finn
MODERATOR Steiner Junkie Joined: April/05/2004 Location: United States Status: Offline Points: 4856 |
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Thanks ILya.
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RUBLE
Optics GrassHopper Joined: July/12/2007 Location: United States Status: Offline Points: 1 |
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HI, new to this site but have been a bird watcher for some time now. I bought a pair of Zeiss 8x56 FL-T last week and can tell you that after looking throught every high and mid end binoculars, they won hands down! Some had what i though was better ergonomics but none were better, if you wear eyeglass, than the Zeiss. I just love these binoculars!
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IT AN'T COLD UNTIL IT HITS -60F
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RifleDude
MODERATOR EVIL OPPRESSOR Joined: October/13/2006 Location: Texas Status: Offline Points: 16337 |
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Welcome, Ruble! The Zeiss Victory FL is an awesome binocular for sure. You made an excellent choice that will provide you with many years of incredible viewing.
Koshkin, Thanks for the excellent information on coatings, refraction indices, and glass. Edited by RifleDude |
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Ted
Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle. |
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