Brightness and light pass through (1 Viewer)

I spent many hours with the Vortex Viper 10x42 and side-by-sided with the Pentax DCF SP 10x43. You birders have been very kind and given me good explanations about "pin cushion" in another thread.

OK so you pick up a binocular and look through it and you go wow this image is bright I like this better than that one over there. Big deal.

I was comparing the aforemention bins with a friend and that was his impression of the Vortex. "It's really brighter" he said. I kept looking. There was some kind of brightness impression. However, what I noticed, looking here and there, was that the color saturation in the pentax appeared to be deeper. There was more of a "presence" effect when looking at a branch. I'm not sure I have the vocabular to describe it other than to say that there was a deeper sensation of color but not necessarily what one would call bright. Contrast maybe?

Anyhow if we're talking light pass-through this is something that I, as a man who has spent many hours chasing limiting magnitude stars in telescopes, feel compfortable with. If I want to know how much light is getting through I go out and I look at fields of stars and pick out the faintest stars and I watch them come and go. Because faint stars come and go when they are at the instrument limit, as the seeing goes bad their light gets spread over a larger area and they disappear and they reappear when their light gets concentrated again. And there's other tricks like averted vision, when do you need it, when do you not.

On the moon it quickly became clear that DCF SPs were showing more detail in the rays on the maria and the color correction in focus showed better in the DCF SPs as well. On Orion's belt (I was propped against a tree with bins steadied on knees etc.) it was clear that the three 8th magnitude stars indicated in the attached map were more readily viewable in the Pentax than the Vortex. As far as I'm concerned these tests settle my mind on optical quality between the two instruments (I had 8x43 Leupolds along for the ride). A binocular should be putting light through and the Pentax is delivering more than the Vortex; contrast is close but superior in the Pentax.

So when all that is done I'm not sure what we mean by brightness. I don't think brightness and color saturation are the same thing (maybe going out on a limb there) but it seems to me that if you have very good color saturation that the view might actually be darker than in a "bright" field, darker because, say, trees and bushes make for darker hues which, if well saturated, might not strike one as bright. But i'm talking out the side of my mouth on that one. It just seems to me that in daytime observing you've got massive giant bucketfuls of light everywhere and the issue in performance is not light pass through.

As for my own test I'm glad that the moon was at 85% and dimming the sky a bit. The reason is that my Uranometria only goes to ninth magnitude and change and my sky map program is not working. FWIW I think both Pentax 43 DCF SP and Vortex Viper 42mm could easily go beyond 10th magnitude under a moonless transparent sky.

As to the aperture difference, the simplest formula I've seen is posted by Celestron on its web site for approximating limiting magnitude. It is 7.5+((log aperture in cm)*5). This is a conservative number that does not take into account observer experience or sky conditions but on the whole, I think it works reasonably. So let's use that:

43mm=10.67 limiting magnitude
42mm=10.61 limiting magnitude

So probably for a scope hound maybe around 11 to 11.5 mag with these bins "often enough but not always."

Now, based on the star test I would put the light pull-through difference between the Vortex and the Pentax at 1/4 to 1/2 magnitude. If we take the lower number, that implies that with Vortex coatings and design you would need a 47mm Vortex to give comparable light throughput to a 43mm Pentax.

And thus much for light throughput and coatings. Now it may not matter. There may be some way that Vortex feels better. This certainly is the case with Saturn in my C14. Everyone loves it because it looks creamy. Roland christen of Astrophysics says it is creamy because in the outer and inner zones of the optic it is not fully color correct and some violet is subtracted from the image, making for a creamy Saturn. My Tak FS128 does the same thing, he says. I like the effect, my friends like the effect, my dob-owning freinds who have truer-color whiter Saturns like the effect, one even bought a filter to try to duplicate the effect. But when all is said and done there is a tad less light coming through.

YMMV, as they like to say, and I will be wanting to repeat these tests, but that's my story for now and I"m stickin' to it. Until I change my mind.

thanks for reading all this,

Greg N

p.s. in the attached map I'm saying that all three test stars were fundamentally visible in the Pentax, marginal and not easy to see in the Vortex much of the time, and difficult-to-simply-not-there in the Leupold.

Greg,
Thanks for your story. Others here have opined that a darker view carries with it the impression of higher contrast. Some say subtle differences in color transmission come into play. I can offer that my brightest binocular, a Fujinon porro, also strikes my eyes a delivering the highest contrast of my small but good collection. This may be influenced by all kinds of things, but the 95% transmission of that bino certainly means there isn't much light to go astray an contriubute to contrast-robbing, insidious, creeping broad glare. That said my low-80s transmitting Trinovid isn't far behind.

Also, the formula you give for limiting magnitude gets the aperture dependence right, but it doesn't recognize the large role of magnification in seeing deeper, and doesn't recognize differences in sky quality, eye sensitivity, nor observer experience.

Just how increasing magnification helps is still not very well understood, at least not by me, but the effect has been remarked on many times. It darkens the background sky, and spreads the size of the star, which we can see is finite rather than an ideal point. This works with small galaxies too. And, yeah, small faint birds!
Ron

Several points.

the formula referenced for limiting magnitude is scope LM, meaning it is intended to represent the LM if the scope were used at optimum magnification, which is generally about a 1mm exit pupil, or in this case at about 40x.

I've lost count of the number of readings I've recorded determining Binocular LM with different instruments under different skies (approx 10,000), but I do know this, a 10x42 under a mag 5 sky will see no deeper than about mag 9.5. Under the best skies I get from home about mag 5.7, it would get no deeper than about mag 10. The deepest I've ever reached with a 10x42, both the Nikon SE10x42 and Celestron Regal LX10x42 was mag 9.8.

Far more important than coatings in total light throughput, is total illumination. It is not uncommon to see binoculars that have only the central 10%-20% of the objective providing 100% illumination on the exit pupil. After that, it drops off considerably such that by the time you move out to a point on the lens 75% to the edge, that part of the lens is providing only 50% illumination. That would be a fairly normal binocular. Most of the roofs I've tested do not meet that total. The best binoculars I've seen illuminate the exit pupil with 100% of the rays from all of the central 40% and even 50% of the diameter of the objective. A binocular like that will show far more light throughput.

edz

Hello everyone: thanks for the comments. Yes the celestron LM formula is only indicative; but let's put it this way, when I want to have an easy night not trying to hard I usually subtract 2 from the formula's results; if I want to work a little I subtract 1, and if I want to push to the edge I work at the formula's predictions.

That's primarily because surface brightness is a function of area. I find galaxies in the high 14s and low 15s very difficult in my C14 (formula predicts 15.25) but my faintest star on a very good night, using a photometrically verified field, was mag 16.2. One can go deeper with stars than with faint fuzzies. I think that since I was working at mag 8 with the moon at 85% I could probably easily get into tenth magnitude stars with no moon at my club's dark sky site. I don't know about hand-held, though. It wouldn't surprise me to get to 11th mag with either of these binoculars but especially the Pentax.

I am aware of the illumination drop off issue as a function of Newtonian telescope designs and on our Yahoo C14 group one of our local gurus did a series of calculations on SCT field vignetting and illumination drop off that was interesting. But I was unaware that it was at play in binocular optics. Is that due to prism size or what?

Anyhow I had my astro buddy friend over tonight and we did some comparative testing with the Vortex Viper and the Pentax 43 SP DFC. We were in the dark and he was unfamiliar with either bino so he managed to use them without knowing which was which. He very quickly (relative to me) selected the Pentax as being brighter and better corrected which is also my judgment; he's got decades of experience and knows good optics, so anyhow, in the circle in which I operate I think Pentax is going to get the edge. Our tests also included the full moon (tonight!).

But I will say something else. I think one would need several years of serious telescope time to discriminate between the Vortex and the Pentax on astronomical objects. You have to know what level of detail you're looking for as well as the usual suspects like CA. I don't know whether it is true that optics which "prove themselves" in astronomical applications will do as well in daytime; many Televue eyepieces are not recommended for daytime use. I've been watching beavers under dark trees at dusk and generally comparing these two bins on many different targets. I've been enjoying a few birds (redwinged blackbirds, large numbers of geese and ducks, chickadees, American bluebird, jays, etc.) and I think the daytime performance of the binocular is better as well, and I really like that I can pan the opposite shore of the river, and the shores of some jetties, without getting barrel distortioned up the wazoo.

So for me it's sort of a no-brainer at this point. When the Pentax was running $150 to $200 more than the Vortex it was, I think, appropriately priced from the point of view of optical capability. But with the Pentaxes running at $450 shipped for the 10x43 SP and as low as $385 on the 10x50 I would think that one would run, not walk, to pick up one of these bins. Maybe if ED glass becomes more generally available this level of performance will be "so yesterday" but hell these are good optics at a great price. The Vortex ain't half bad either. I've greatly enjoyed it this past year but will put it on the market at some point in the near future. Maybe I should take it to Israel. I can't figure out which bino to take to Israel: the ones I really enjoy using or the one I don't mind losing.

Anyhow I like binoculars because you can use 'em when the sky is clouded over and even when it's raining, my astro scopes are too cumbersome for that. So I expect I'll try a few brands besides pentax and vortex but I don't know if I'm going to do serious buying in the "Alpha" binoculars. As with eyepieces, I expect there is a learning curve: I was very happy with my first cheap eyepieces and only gradually learned their deficiences, upgrading to medium level and loving those till I finally found myself at the high end and with perfectly good reasons for being there, it seemed to me. So maybe it'll work that way with binoculars.

regards
Greg N

After that, it drops off considerably such that by the time you move out to a point on the lens 75% to the edge, that part of the lens is providing only 50% illumination. That would be a fairly normal binocular.

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Thanks for that. I have always noticed it, never could put it into words.