Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
|
|
Quote:
The difficulty would be in attributing how much color correction is a result of which changed aspect.
Ed, actually it's not difficult at all. The angular color blur diameter depends only on focal ratio, not on aperture. As Roland says in the link I cited earlier, it's the decrease in Airy disc diameter as aperture increases that makes the RATIO of color blur to Airy disk get worse. But the actual color blur doesn't change.
Milt
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
The angular color blur diameter depends only on focal ratio, not on aperture. As Roland says in the link I cited earlier, it's the decrease in Airy disc diameter as aperture increases that makes the RATIO of color blur to Airy disk get worse. But the actual color blur doesn't change.
"it's the decrease in Airy disc diameter as aperture increases that makes the RATIO of color blur to Airy disk get worse".
While total color blur may be controlled by f ratio, the visual display of that color error is dependant on the size of the Airy disk. As aperture changes that also changes. Therefore, visual color error varies as aperture varies.
If you had an 80mm f/4 aperture with a color blur twice the size of the Airy disk, the color would be prominent. Keep f the same, but now reduce the aperture to 40mm, so you now have an 40mm f/4; yes color blur is still the same size, but now the Airy disk is twice as large, the same size as the color blur. There would therefore be no false color outside the Airy disk.
The visual change in false color in this instance is not dependant on f ratio, since that remains the same. It is dependant on the change in size of aperture.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
gmazza
super member
Reged: 03/10/09
Posts: 102
Loc: RS, Brazil, 29S 51W
|
|
Hi EdZ,
Thank you for a comprehensive response, I'm sorry if I started some statements which drove topics in the wrong direction (actually we have topics in glass features, masks in binoculars, desired magnifications, all at same time) some times people mix the things, I will try to keep my responses and questions more strict on topics.
In that question I also wondered about the glass features of the 3 binoculars compared (The 22X60 has state of art quadriplet lens with fluorite, the WO has FPL-51 ED, the Garret I do not find glass specifications, but the views are praised by everyone here). Last week happened in refractors forum the same, about ED glasses and about the "best 80mm" with crossing ideas. In that forum some people called FPL53 APO's (remember FPL53 Abbe number is about the same as fluorite) as overkill for visual observing, and also that a slower FPL 51 well designed could match a FPL 53. There is even difficult to define what is "ED" in terms of abe number, and if different glasses could be called "ED" it's hard to measure the real benefit.
I see binocular astronomy as just about immersion, we use most heavy binoculars (exclude military) of the bino world. We mount them to scan the sky and enjoy color and correct bright. We could even change focal lenght by masks for specific targets. Equatorial mounted binos for astrophotos are not the bulk reality at this time. Of course is human nature try to find some usefulness to new features as we always want better views. The glass specifications of binoculars are a very important topic to discuss here, but, we can conclude that for this particular niche some glasses are overkill, here the importance of testing.
Also seen some topics of 2005, about TAKs 22X60, and awaited 22X65 from other brands which never come into market, these binos anchieved a superb color performance in small optical designs, but were discontinued and never launched one reason must explain this.
In the "porro binocular" world of nature watching, the shorter focal lenghts probably benefit from all color correction possible, but, even them, there is diversity of opinions.
My final thoughts on this would reiterate the importance of well, arguments reinforced, reviews of each model before any buy.
--------------------
Oberwerk Ultra 10X50
Garrett Signature 15X85
|
milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
|
|
Quote:
If you had an 80mm f/4 aperture with a color blur twice the size of the Airy disk, the color would be prominent.
Ed, actually not. This would be a good example of an ED doublet, and I doubt that anyone would notice color at binocular magnifications. However if this same objective was a standard C-F corrected achromat, the color blur would be a whopping 7.5 times the diameter of the Airy disc and color would become evident on bright stars.
Binoculars are saved by low magnifications and the rapid falloff in eye sensitivity away from green-yellow. In Roger Ceragioli's words:
"so long as a broad range of the brightest wavelengths comes to a true focus or is concentrated into a spot whose size the eye cannot resolve (because of low magnification), the brain will sense this as white".
In other words, the color blur has to get really big, many times larger than the Airy disc, before it is noticed in binoculars. So I stand on my statement that the size of the color blur (set by focal ratio) is the controlling factor, not the size of the Airy disc (set by aperture).
As you have pointed out in this thread, ED glass is not a panacea for binoculars.
Milt
Edited by milt (07/08/09 07:06 PM)
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
Quote:
If you had an 80mm f/4 aperture with a color blur twice the size of the Airy disk, the color would be prominent.
Ed, actually not. This would be a good example of an ED doublet, and I doubt that anyone would notice color at binocular magnifications. However if this same objective was a standard C-F corrected achromat, the color blur would be a whopping 7.5 times the diameter of the Airy disc and color would become evident on bright stars.
Milt
Milt, sorry, I didn't mean that as any specific lens example, only as an example to show the doubling, or halving, of the lens produces such and such an effect... as I think I pointed out in that last post.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
|
|
Ed, just edited my previous post to make more clear what I was saying. This stuff isn't easy... 
Milt
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
|
|
Milt makes a very good point above.
Color Blur or CB value is a ratio of Airy disk size. So a CB=1 is same size as Airy disk. CB=2 is twice the size of the Airy disk.
The forumla for color blur states that for a color free instrument you need to have CB=1, or explained another way, no residual color can fall outside the diameter of the Airy disk. Essentially all the light is contained within the Airy disk.
What's not mentioned in any of this is low power viewing. That's what is brought up by Milt in his post above.
When viewing at low powers, the human eye cannot perceive sizes as small as the Airy disk. Human eye resolution at 30x is not controlled by scope diameter, it is controlled by acuity and hence magnification. The spot size that can be resolved at 30X might be 4 to 5 arcseconds, not the scope Airy disk size of 1 arcsec. So the latitude of how wide the color blur can get before the eye sees false color, when magnification is not high enough to truly resolve the point, is dependant on how large a point the eye makes the object appear to be.
For example, for most people, regardless of scope size and hence it's potential resoltion, (which could be a 5 inch scope with res = 1 arcsec), the eye cannot resolve a 4 arcsec pair at 30x. It will be seen as one. As Milt points out, as long as the color blur is as small as the spot that can be resolved by the eye, (at 30x, perhaps 5 arcseconds), then false color is minimized. Oh, you may see some residual color around the edges, but not nearly as bad as you might think.
It doesn't take too much effort to think thru, as magnification increases, even though it may still be too low to reach the resolution limit of the scope, the spot size resolved by the eye gets smaller. So, for instance at 50x, the eye might be able to resolve 3" and at 75x, the eye might be able to resolve 2". The extent of color blur would need to be progressively smaller in each case to match the spot size, if the observer were to desire a color free image.
But as you can also see, that presents a very wide latitude in CB factor when using powers below that needed for the resolution limit. Low power binoculars are well down the chart. Even high powered BT style instruments are generally used in the wide latitude range, but may approach the resolution limit.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
|
3 registered and 17 anonymous users are browsing this forum.
Moderator: EdZ
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 1596
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
