Binocular ramblings (1 Viewer)

[Kevin Purcell]

Hello Kevin,

Very interesting but contrary to the standard method of computing the twilight factor, which is the square root of the multiple of magnification and aperture. It also seems to be contrary to what hunters seem to find.
However, I will write that Twilight Factor may be too simple to describe twilight perception. Zeiss derived this term for users at a much higher latititude, where twilight is more extended than in areas most people inhabit in the continental USA.
As the binoculars you mentioned were three 8x32's and a 6x30, are you referring specifically to the Yosemite?
As for the light output of Porro's vs. roof prism binoculars, it takes a lot of high priced engineering to overcome the inherent faults of the Schmidt-Pechan prisms.

Happy bird watching,
Arthur :scribe:

Yes, just considering lower magnification (x6) bins compared to the others.

I now what the twilight factor says but that's not what I saw both brightness and acuity seemed better. I speculate that's because its putting more light on the fovea.

But ....

The only way to test it though is to control for all the tens of other variables here. The differences were small between the two (2!) data points of the DX (x8) and Yosemite (x6). So I'm not going to claim anything new.

BTW, is there a decent cite for why we should believe the Twilight Factor numbers? What's the model behind it? Why should it work?

The dimmness of the roofs were a much bigger surprise.

I'd love to encourage other people to try twilight testing. Do you see differences in your bins?

Thanks,
Kevin

 

[Pinewood]

After some thought, I realize that twilight factor cannot account for differences in light transmission. The roof prism binoculars were rated, by you, as dim, probably for loss of light on the mirrored surfaces, while the Yosemite, a Porro, has no mirrored surfaces.
An 8x32 has a twilight factor of 16 while the Yosemite 6x30 has a twilight factor of 13.4. If you assume, just as an example, 50% more light output, the equivalent aperture would be 37.5 mm, giving a twilight factor of 15. Should the Yosemite have fewer air to glass surfaces or better coatings, differences would disappear.
Comparing twilight factors of binoculars with differing coatings and constructions would sees riddled with pitfalls.
Therefore twilight factors may be suited for comparison within the same line of binoculars, like the 8x30 and 6x30 Yosemites.
Finally, twilight factor does not trump important contributors to image quality. Zeiss puts it this way:
The twilight factor is only one parameter among many, it does not say anything about the image quality which is a determining factor in detail recognition in twilight (twilight performance)! Twilight performance is mainly determined by as high a transmission as possible in the right spectral range, as low a straylight portion as possible, as high contrast as possible and as high a resolution as possible. Only if all these requirements are met at the same time - and only then - can the twilight factor be used a measure of the twilight performance in binocular viewing.
http://www.zeiss.com/c1256bcf0020be5f/Contents-Frame/2f74e36875a2227785256df1006e6bbf
Even Zeiss understands the limitations of using twilight factor. If you don't have light transmission, high resolution, contrast and blocking of stray light, you don't have twilight performance; you just don't have that swing; you don't have a thing!
Look at: http://www.zeiss.com/c1256bcf0020be5f/Contents-Frame/7c0c4ce74a32e9ed852571cb00499727
Also look at the binocular material by Roy L. Bishop in the annual Handbook of the Royal Astronomical Society of Canada, but remember he wrote about the needs of astronomers.

Happy bird watching,
Arthur :egghead:

 

[Kevin Purcell]

[...]Should the Yosemite have fewer air to glass surfaces or better coatings, differences would disappear.
Comparing twilight factors of binoculars with differing coatings and constructions would seems riddled with pitfalls.

Yes, I assumed that all coatings were similar.

Obviously it's difficult to count the number of glass/air transitions without pulling the bins (how many eyepiece elements each on has) but at this price point I do not expect major differences in designs or coatings.

Between roofs and porro designs there are two potentially significant differences:

1. the three reflections from mirrored surfaces in Schmidt-Pechan roof prism

2. an extra pair of glass/air transitions from the internal focuser (not a moving eyepiece). With typical multicoatings that might loose perhaps 0.5%, maybe 1% for a poor multilayer or for a doublet. It was probably worse in the past (perhaps 2% with a single MgF layer). Not much compared to the mirrored surfaces.

FYI, bins that use a Abbe-Koenig roof prism rely on total internal reflection not mirrored surfaces so they don't have this problem though they need phase correction.

Therefore twilight factors may be suited for comparison within the same line of binoculars, like the 8x30 and 6x30 Yosemites.

Yes, twilight factor (if it's useful) would only be useful with everything else controlled for: same coating, same number of glass/air transitions. Bins in the same range are the only ones that meet this requirement (even the next range up you might not be getting the same design, coatings, etc).

So aside from understanding twilight factor model it doesn't seem particularly useful.

And all this for an extra 10 or 15 minutes of birding in twilight

 

[Pinewood]

And all this for an extra 10 or 15 minutes of birding in twilight

Kevin,

I do have a 7x42 dialyt, which uses Abbe-Koenig prisms and does not have internal focussing. With its well known resolution and contrast, it is often useful in twilight. It also delivers more twilight detail, eight minutes after sunset, than a Leica 7x42 BGA, both having 17.15 twilight factor. Of course, the Leica has all the light consuming features you have enumerated.

Happy bird watching,
Arthur

 

[Tero]

Kevin, I forgot where it was, but you had asked about porros with ED glass. Here is one
http://www.eagleoptics.com/index.asp?pid=2452

 

[Kevin Purcell]

Kevin,

I do have a 7x42 dialyt, which use Abbe-Koenig prisms and does not have internal focussing. With its well known resolution and contrast, it is often useful in twilight. It also delivers more twilight detail, eight minutes after sunset, than a Leica 7x42 BGA, both having 17.15 twilight factor. Of course, the Leica has all the light consuming features you have enumerated.

Happy bird watching,
Arthur

Of course!

Abbe-Koenig prisms use TIR not mirrored surfaces so they're as good a win as porros in this case (though they do need phase correction unlike the porros). One can see why these were so popular for so long.

Does/did any one else other than Zeiss make bins with Abbe-Koenig prisms?

 

[Kevin Purcell]

Kevin, I forgot where it was, but you had asked about porros with ED glass. Here is one
http://www.eagleoptics.com/index.asp?pid=2452

Thanks, Tero.

Yeah, that's the only other one I know of and it's ER is "17mm" which lot of people point out is really 12mm. No so good for the astigmatic myope

Plus the mechanics (the flexible bridge) seem to be not so good. An internal focuser would fix that.

Any others?

 

[Tero]

Not aware of any. I am a little surprised there are no astronomy porros with ED glass. Weight is no issue, they mount them anyway.

You could take two ED scopes and duct tape them together.

 

[Pinewood]

Not aware of any. I am a little surprised there are no astronomy porros with ED glass. Weight is no issue, they mount them anyway.

You could take two ED scopes and duct tape them together.

Hello Tero,

Amateur astronomers spend money on aperture not high priced glass. They are interested in "light buckets."

Happy bird watching,
Arthur :brains:

 

[Alexis Powell]

Amateur astronomers spend money on aperture not high priced glass. They are interested in "light buckets."

In my experience, a more accurate characterization would be that astronomy enthusiasts are not interested in exotic glass to the exclusion of aperture. They're willing to spend plenty on high priced glass if it is of practical importance and worth the cost. With respect to binocular viewing, light buckets and high priced glass, my dad's favorite set-up is a carefully tweaked 16'' Dobsonian mated to a Denkmeier binocular viewer and twin Televue Naglers.

--AP

 

[Kevin Purcell]

Amateur astronomers spend money on aperture not high priced glass. They are interested in "light buckets."

I beg to differ planetary astronomers spend a lot of money on smaller (80mm and up) ED and Apo refractors for the same reason birders do: color control and contrast. The contrast between the bands on the gas giants is a lot less than most feathers on a bird.

Even "large diffuse object" observer split into two camps: light bucket reflectors and compact wide-angle refractors.

Tero, I have seen ED astro bins out there (don't have time to find the references) but they are out there. But they're astro bins: large objective, large(r) magnifications and they're heavy. I've found with my cheaper astro bins that they work fine in the dark but they have (bad!) stray light issues in the day. Sort of vampire bins.

 

[bkrownd]

Amateur astronomers spend money on aperture not high priced glass. They are interested in "light buckets."

Oh, to have some binoculars that could be described as "light buckets"... :-O

 

[NWBirder]

So much for my optics budget. Spent 160 on an iPod. A bit late for birds songs but...

My curiosity now came back, can I find a decent 8x42 with 400ft fov. The Diamondback is the only candidate, and I had it once and sent back. I have some 8x32s with 400ft. fov.

The more binoculars I have seen through over the years, the pickier I become on the edge sharpness. For most of the inexpensive binoculars (diamondback or excursion), the wide FOV is done by compromising the edge resolution. It is worse for 10x than 8x. 8x typically has a reasonable on-axis sharpness that can be extended to 70% of the field of view. Beyond that, just try to take whatever available as extra bonus.

It seems that both inexpensive and super expensive binoculars have wider FOV than mid-priced ones (those between $300 to $800), just my observation.

 

[elkcub]

Sorry to have missed this interesting discussion about twilight factor. There have been several threads on the subject, and some well known optics enthusiasts don't think it's a term worth retaining (private emails). A year ago I posted (#6):
http://www.birdforum.net/showthread.php?t=77086

I believe the twilight factor (TF) metric is intended to reflect the human's ability to see detail at twilight, namely, at a time when the retina is somewhere between light and dark adapted. This is called the mesotopic region, but unfortunately it has a very wide and non-stationary range of capability.

To me, it makes most sense to consider TF when the exit pupil is held constant. This would mean that the predicted order of improved visibility would be 6x24 < 7x28 < 8x32 < 10x40 under twilight conditions, even though these configurations all have an EP=4. Similarly, 8x20 < 10x25 (EP=2.5), and 7x35 < 8x40 < 10x50 (EP=5). This assumes that all other factors are equal, such as roof/Porro design, coatings, light leakage, etc.

I don't know the underlying basis of the formula, which weights objective size and magnification, but in this narrow context the metric seems to provide some valid guidance. For example, I do find a 10x40 better than an 8x32 at twilight, as well as a 10x25 better than an 8x20. Unfortunately, it's also very easy to slip into thinking of the metric as a comparative brightness measure, which it really was not intended to be.

Ed

Surveyor then summarized it nicely: "...As long as exit pupil (brightness) is the same, increased power = increased [visual] resolution." On a similar note for astronomy applications, as long as the exit pupil is the same, increased objective (aperture) = increased [optical] resolution.

I recently found a paragraph on twilight factor in Eagle Optics' 1995 catalog, which says much the same with regard to spotting scopes.

So, is TF worth retaining?

Ed

 

[elkcub]

...Also look at the binocular material by Roy L. Bishop in the annual Handbook of the Royal Astronomical Society of Canada, but remember he wrote about the needs of astronomers.

Arthur,

I came across this url: http://www.wwnorton.com/college/astronomy/astro21/sandt/powerbinocs.html. It's pretty clear that Bishop is talking about exactly the same thing. Note in the graph he uses exit pupil as a primary parameter. Taking the square root of a product is simply a way to scale large numbers. He just uses the product and calls it the "visibility factor," which is what it is.
QED

It all ties together.

Thanks,
Ed

 

[Tero]

Looking back, I have gone through a few dozen binoculars.* They certainly have been field tested, with birds. If they do not get used, there is a problem. Bulk, inconvenience, optics that just don't work with me. For instance the Zeiss Conquest 10x40 were never a favorite. I do not know if the Pentax 10x43 will be, I will spend a winter with it. here are my best two optics, if I keep the porros, from an image in another thread. The porros are 8x, the roofs 10x.

http://www.birdforum.net/attachment.php?attachmentid=159436&d=1221097525

*at least a dozen were under 40 dollars each
had a barska 10x50 porro I gave away as dim..could have kept it for tests

 

[Tero]

Now that I have had a dozen binoculars, I come back to the same thing. I really do not like 30 oz or more, some 800g. Even with 24-25oz, I carry it part of the time in my hand. So the best I have come up with so far is still the 8x32 roof prisms at around 20 oz.

But the slung over the shoulder style works half way OK for 31oz, Kevin style.

 

[FrankD]

It is funny you should say that Tero. I have found pretty much just the opposite for myself. I prefer the heavier binoculars. In fact, out of all the models I currently plan on keeping the lightest one is about 27 ounces. I do have the lighter 8x32 SE but other than that I do truly prefer the heavier full-sized 8 and 7x42s.

 

[Tero]

It takes a bit of a commitment to like the high end stuff. People who paid over $1200 and like the optics at hand tend to stick with it, no matter what issues they have otherwise. Some do select 8x32 models at the high end and are perfectly happy, others go for the 30 oz stuff.

When I have the binoculars in hand, I prefer 25 oz. When the same binocular is around my neck, I prefer it to be 20oz. Can you engineers do that for me?

 

[Boomer]

...When I have the binoculars in hand, I prefer 25 oz. When the same binocular is around my neck, I prefer it to be 20oz. Can you engineers do that for me?

Tero,
LOL! :-O Thanks for the great post...and for starting this thread. Like you, I also look forward to the day when 'you engineers' will finally be able to design bins that will indeed sastisfy our 'personal dream bin' needs. Best Wishes :t:.

Ron Davidson