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Have Nikon given up competing with the big three? (1 Viewer)

Dorian,

Regarding this quote I have a few questions you may be able to answer.

A binocular with a short focal length objective uses a short focal length eyepiece to produce the same magnification (e.g. 8x) as one with a longer objective focal length and longer ocular focal length. In the short focal length binocular the image projected by the objective is smaller, so all aberrations are smaller at that point in the light path, but that is exactly cancelled out by the increased magnification of the short focal length eyepiece. The extent of visible chromatic aberration therefore shouldn't vary much with focal length, but objectives with a low f-number (fast systems) and/or eyepieces with a wide field of view are more difficult to design and manufacture to a high optical quality.

To reduce chromatic aberration, especially in high-power binoculars of sensible size, you really need glass with anomalous partial dispersion.
Click to expand...

As far as I know, the objective, inverting prism, and eyepiece are each potential sources of CA, but I have only a sketchy idea how they combine to produce what we see in the final image. Do you know how this works? Are their effects additive? Multiplicative? Do such aberration effects reinforce or cancel each other? What you mentioned above seems to addresses only CA produced by the objective.

It may be incorrect, but the design structure of the typical objective seems to me to favor control of lateral CA, and the design of the typical eyepiece to favor control of axial CA. My suspicion is, therefore, that low-dispersion (ED) glass might be most effective for control of axial CA when used in the objective, and allow for better control of lateral CA when used in the eyepiece.

I haven't got the foggiest idea how much CA is produced by the prism system, or how relatively important it might be. If porros show less CA than roofs, however (I'm not sure I believe this), then the inverting system design might be a culprit, in addition to the Abbe number of the prism glass.

If you have insights or references they would be appreciated.

Elk
PS. I'm not even sure that asking these questions is the right way to conceptualize the CA issue, particularly if binocular design is a holistic, integrated optimization using trial-and-error or heuristic computer modeling.
 
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elkcub said:
Dorian,

If porros have less CA than roofs, however (I'm not sure I believe this), then the inverting system itself might be a culprit, as well as the Abbe number of the prism glass.


Elk
Click to expand...

Elk,

Don't forget that roof prisms cause interference problems, which are corrected by phase coating. Whether or not it affects CA, I would not be surprised if phase coating had a downside.

Happy bird watching,
Arthur
 
Pinewood said:
Elk,

Don't forget that roof prisms cause interference problems, which are corrected by phase coating. Whether or not it affects CA, I would not be surprised if phase coating had a downside.

Happy bird watching,
Arthur
Click to expand...

Hmmm. Or, it could be the number of reflecting surfaces in the system, each producing additional dispersion.

I need help. :-C

Elk
 
Yeah, it's the Nikon bycicle-handle bar style applied to the Swarovski open hinge approach. Could be attractive to birders with a knack for hybridization.

Renze
 
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elkcub said:
As far as I know, the objective, inverting prism, and eyepiece are each potential sources of CA, but I have only a sketchy idea how they combine to produce what we see in the final image. Do you know how this works? Are their effects additive? Multiplicative? Do such aberration effects reinforce or cancel each other? What you mentioned above seems to addresses only CA produced by the objective.
Click to expand...
Objectives, prisms and eyepieces are all definitely potential sources of CA, but I sadly share your uncertainty about the specifics. But any aberrations in the image projected by the objective must be there for good: information has been lost, and the best one can hope for is that the rest of the system perfectly reproduces what is left.

Chromatic aberrations fall into two types, as you mentioned: longitudinal CA (also known as axial colour, in which different wavelengths are focused at different planes) and transverse CA (also known as lateral colour, in which magnification varies by wavelength). Binoculars suffer from both to varying degrees. Axial colour affects the entire image, while lateral colour gets progressively worse with increasing image height (i.e. off-axis).

Axial colour is characterised by colour fringes around high contrast objects of any orientation, and the colour of the fringes can often be induced to visibly change by very slightly adjusting the focus. Due to its nature it is reduced by stopping down the optical system, which increases the depth of field, masking any colours that are slightly out of focus due to the aberration. Binoculars are effectively stopped down when the viewer's pupil is smaller than the exit pupil, as long as the pupil is perfectly centred within the exit pupil. In practice this is difficult to achieve, though using a 50 mm binocular in bright sunlight, for example, will almost certainly reduce visible aberrations to some extent.

Lateral colour is also characterised by colour fringes around high contrast objects, of course, but as it is the result of the magnification varying by wavelength, it is non-existent on-axis (because a point in the centre of the field is mapped to the centre of the image regardless of magnification), and becomes progressively more visible away from the centre of the image. It reveals itself as fringes of two different colours, one on either side of the high-contrast structure. It is only visible around structures with a tangential component in their orientation; sagittal structures (i.e., those with a radial, spoke-like orientation) are immune to lateral colour. The interesting thing about lateral colour is that stopping down the system does nothing to reduce this aberration, because increased depth of field cannot help when light of different wavelengths is all perfectly focused on the image plane in the first place.

One might think that axial colour would be a much greater problem than lateral colour when designing a binocular objective, but this is complicated by the public demand for compact binoculars, which necessitates using a telephoto design for the objective. Telephotos, like retrofocus lenses, are markedly asymmetrical and consequently suffer from greater lateral colour problems than a simple lens (such as a flint-crown achromat) of the same focal length. But they are physically shorter than their focal length, and that is all-important.

Eyepiece design is also complicated in today's market, which at once demands compact size and long eye-relief, design features that are mutually opposing. A binocular's power is given by dividing its objective focal length by its eyepiece focal length, but designers have tended towards using short focal lengths (i.e. fast systems with small f-numbers, which also works against high image quality) to satisfy the people's desire for small size. This means the eyepiece design has a short focal length, which results in a short eye-relief unless the eyepiece design too is made asymmetric, which again introduces lateral colour. It's maybe useful to note that the Nikon HG L binoculars have long eye-relief despite their relatively compact dimensions, indicating a very strongly telephoto objective and/or an asymmetric eyepiece design, both of which scenarios introduce large design problems of lateral colour.

My understanding of dispersion, which is the cause of colour fringing, is that it occurs where light traverses a boundary from one material to another of a different refractive index. Therefore it stands to reason that dispersion would not occur within the prisms, at the surfaces of total internal reflection, but only at the entry and exit faces of the prisms. Further, if the incident rays of light are limited to those close to the normal of the prism (perpendicular to the prism's entry face), as would be the case in a binocular of narrow field of view and/or large f-number (or long focal length, when talking about binoculars of a given objective diameter), then the dispersion caused by the prism would be minimised.

Interestingly, Cabela's website states that Kowa use SK15 glass for the prisms of the Genesis/Prominar XD 44. Here is how that compares to common prism glasses, according to Schott's 2007 optical glass catalogue (PDF file):

Refractive index:
N-BK7 - 1.51680
N-BAK4 - 1.56883
N-SK15 - 1.62296

Abbe number:
N-BK7 - 64.17
N-BAK4 - 55.98
N-SK15 - 58.02

(I think the "N" means the glasses are environmentally friendly replacements for the old leaded versions.)

Cabela's says that the "SK15 roof prisms have a high refractive index". Now, I can't speak for anyone else, but I'm not terribly fussy about the refractive index of my prisms! ;) As long as the refractive index is sufficient to cause total internal reflection for all light within the light-cone from the objective, as will be the case in any competently designed binocular, the refractive index doesn't matter. BAK4 glass allows a steeper light cone (i.e. a faster objective, which in turn permits a more compact binocular) than BK7, but otherwise BK7 is superior due to its higher Abbe number (lower dispersion). SK15 seems to strike a decent balance: a higher refractive index than BK7 or even BAK4, permitting a more compact binocular, but lower dispersion than BAK4. I suspect the high Abbe number is why Kowa chose to use it, because the Kowa Prominar XD isn't unusually compact (though compact enough to prevent the use of BK7). I don't know what glass Leica, Swarovski, etc., use in their binocular prisms.

And so we see once again that everything about short, compact binoculars militates against high image quality. But it's what the people want, and I can't say I'm any different!
 
Just to let you I was one of/the person whose focusing mechanism froze up on their LX's on here. It took a little while longer than I may have hoped but I just got my replacement LX L's. I have been borrowing a mates old 8.5x42 EL's for the duration of my Nikons being out of sink and I have to say I'm absolutely thrilled to give them back. Maybe it's just my eyes being accustomed to the LX's but I didn't find the Swarovski remotely as enjoyable to use - hard to describe the difference but the image on the EL's to my mind seemed less contrasty and not quite so crisp. Also couple in the fact that they feel clunky in the hand (to me) and that these were the old EL's with the slow focusing mechanism. For that reason alone I find it hard to believe that these bins every got picked up quite so much as they were by birders (at least here they dominate the market). I am sure they were good for hunters or birders scanning slow moving or stationary birds but for passerines forget it by the time you'd focussed the bird was gone. Anyway ecstatic to have my bins back - going to have to sneak out to use them at some point today. I wonder what their new bins will be like.

Luke
 
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Dorian Gray said:
Objectives, prisms and eyepieces are all definitely potential sources of CA, but I sadly share your uncertainty about the specifics. But any aberrations in the image projected by the objective must be there for good: information has been lost, and the best one can hope for is that the rest of the system perfectly reproduces what is left.
Click to expand...

Your post is very informative, but as far as I know the eyepiece can compensate for aberrations in the objective. The information is still there. Indeed in many compound microscopes it is inadvisable to mix and match objectives and eyepieces from different manufacturers. Nikon puts all of the corrections in the objective. Olympus doesn't. Whether or not that is so for binoculars, I have no idea. I suspect I know who here can answer that one.

On a related note, many software packages can automatically correct for CA, partially or wholly, in an image given the nature of the lens. In fact quite a lot of aberrations can be corrected for in software if you know details of the lens.
 
Leif said:
On a related note, many software packages can automatically correct for CA, partially or wholly, in an image given the nature of the lens. In fact quite a lot of aberrations can be corrected for in software if you know details of the lens.
Click to expand...

But how do I get many software packages into my brain when I use a specific pair of binoculars?
And what if that software package is not "supported" by my operating system?

T
 
ThoLa said:
But how do I get many software packages into my brain when I use a specific pair of binoculars?
And what if that software package is not "supported" by my operating system?

T
Click to expand...

I did add "On a related note" to suggest that this was not directly related to the main point i.e. binoculars. I was adding to the point that the information from the objective is not necessarily lost.
 
Here are some specs for the 8x42 EDG. Prism uses dielectric mirror coating. Body is magnesium.

FOV - 7.7 degrees (61.6 AFOV)

Eye Relief - 19.3mm

Close Focus - 3m

Weight - 835 g

minimum advertised price US$1899.00 (street price should be a bit lower)
 
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Dorian,

My understanding of dispersion, which is the cause of colour fringing, is that it occurs where light traverses a boundary from one material to another of a different refractive index. Therefore it stands to reason that dispersion would not occur within the prisms, at the surfaces of total internal reflection, but only at the entry and exit faces of the prisms. Further, if the incident rays of light are limited to those close to the normal of the prism (perpendicular to the prism's entry face), as would be the case in a binocular of narrow field of view and/or large f-number (or long focal length, when talking about binoculars of a given objective diameter), then the dispersion caused by the prism would be minimised.
Click to expand...

Yes, I appreciate that. I guess I was thinking of the effects of prism tolerance errors causing dispersal, which I imagine would become more dramatic with increased light path length. But, that's not a design issue, in any case, so perhaps it's better to drop prism CA effects, or consider them merged at the objective's image plane. The underlying question, then, is how do the two remaining sources of lateral CA combine in the final image on the retina? (I'm leaving out the considerable CA in the eye itself, which nature seems to have adapted to nicely.)

What about this relationship?

CA(tot) = m*CA(obj) + CA(eyepiece)

Elk
 
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I think the next generation is upon us. After that, will it be IS without compromising all the other advances? I think the immediate Nikon promise looks very good indeed. I very much appreciate the improvements due to the new glass, and find this discussion most interesting.
David
 
I don't know guys... but if Nikon is aiming at the $1600-$1900 market, consumers will surely have to refer to the top level as the playing field of the "Big Four"

Bins look good to me. Like an open bridge LXL with some EII/SE magnesium thrown in. Like the visual nod to the EII/SE (which are very nice looking).

So... an open bridge, great styling, LXL smooth focusing, center diopter and FL/HD type view and (I predict), industry leading FOV. Looks like Nikon could be at the front of the pack once more.

Cheers
 

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More curious marketing by Nikon, though....they seemed to have sneaked this one up on us without advance warning (compare to Leica's fanfares long in advance of the new HD, which is dead late hitting the shops anyway). Then it turns up being marketed as "Hunting Optics" on some obscure hunting site....are hunters a bigger tarket market than birders?
 
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