small exit pupils in low light (1 Viewer)

Howdy,

Lately, going out for nighthawks in evening dusk/twilight, I have much enjoyed using my 8x30 Fujinon FMTR-SX. Not being purely bird-obsessed, I direct my gaze not only into the sky for birds but also at darkening terrestial scenes, under bushes etc., the darkest places I can find. I gotta' tell you folks, I can flat see stuff in near dark better with this little bugger than with my binoculars with big exit pupils up to 7mm.

Why, I asked myself, can I see so well in twilight through a binocular with a perfectly miserable "twilight factor"? I have a guess, open to your customary kind criticism naturally. It is simply that it is a very good little binocular. I'll try to explain.

Henry Link recently found his binocular of this model to be better than 1/4 wave aberration stopped to 23mm (corresponding to a typical daytime eye pupil). I haven't tested mine but it must be similar, as it is easily the sharpest binocular in my collection. This level of aberration control is merely the worst that would be tolerated from a telescope, but as binoculars have gotten fancier and more complex, it is almost never encountered there. That's a pity.

As darkness falls, our eye pupils expand, which is sort of a good thing, as they let in more light. But it's also sort of bad in that normal visual acuity, at its best around 1 minute of arc at 2.5mm eye opening, worsens dreadfully above 4mm, due to typical imperfections in the cornea and eye lens. To the binocular viewer using a 7x50, 8x56 or such, this is compounded by the errors in binoculars at full exit pupil resulting from overly tight objective lens mounts, spherical aberration from prisms, etc. etc. (Why bother to make the binocular good at full aperture, when the eye at full aperture sucks in the first place?) What is cool (to me at least) is that this loss in sharpness with fading light is completely absent in a binocular with a small exit pupil, provided it is of good optical quality at full aperture.

The little Fujinon apparently apparently fits this description. Of course what it lacks is a factor of two or three in light throughput compared to an honest large exit pupil "low light" binocular. But vision's response to light variation is logarithmic, or less than that factor of two or three in this case, more like a factor of 1.5 in how much the image "looks dimmer". I am finding that full sharpness, and I mean just like in daylight, combined with noticeably but not drastically reduced apparent brightness, is not a bad compromise. That's an understatement actually, as I have come to prefer the sharper although dimmer low light view (I have a Fujinon 7x50 FMTR-SX and a 10x56 Zeiss FL, both arguably excellent in low light, to compare with).

By many accounts, binoculars of optical quality that would be taken for granted in even a cheap telescope are rare. The justification seems to be that low power brings forgiveness for optical shortcomings. Of course life is hard for the binocular maker, what with complex optical chains, ergonomics, ruggedness, collimation and price point to worry about. But, I'm finding that small but really good is all I really need.

Ron

Hi Ron,
What was the size of your two eye pupils during these dusk observations?

Binastro,
Is there a device that would enable self measurement?

A wife, a ruler, low room lighting and maybe a magnifying glass.
Yes, I know she is the ruler, but I mean a thing that measures distances in addition to the wife.

Alternatively a Konica/Minolta Z6 or maybe Z5 camera and a lot of careful planning..

P.S.
I actually use pinholes and Waterhouse stops of varying sizes and out of focus distant street lights to get a good estimate.
This is because my eyes don't focus at infinity.
Photos with the Z6 in total darkness give fine results, but this is not recommended in the instruction book.
The flash may go down to 1/30,000 second and it focuses without a visible help light and gives accurately exposed images that can be measured.
This method needs careful tests and you must know what you are doing.

Hi Ron,

I've seen this same effect too. The first time was 30 years ago when I was comparing a cheap Nikon 8x23 in low light to the pride of my little collection at the time, a CZJ 8x50 Nobilem Super. I was shocked to see the Nikon image looked obviously sharper. I finally figured out that my own aberrated eyesight at a large pupil dilation was to blame by stopping down the Nobilem to 23mm.

That 8x30 Fujinon is a gem.

Henry

The-Wanderer said:

Binastro,
Is there a device that would enable self measurement?

Click to expand...

There is an easy technique (no wife required). You need an artificial star and some masking tape.

A good star for this purpose can be made by punching a tiny pinhole in aluminum foil and stretching it over a flash light lens. Place the flashlight within the dark scene you are observing at a distance of about 3-4m. The star point should not be bright because you don't want it to affect your pupil dilation.

Next, stretch two pieces of masking tape over one of the objective lens trim rings, parallel to each other with a slit separating them of perhaps 30mm.

Now set the focus of the binocular to infinity and examine the artificial star with one eye through the side with the tape. The star will be so far out of focus that it will appear as a focused image of the objective lens. You should be able to see the slit separating the pieces of tape. Now change the gap between the pieces of tape until they just barely disappear and you see a full circle. That circle will be one of two things: the entire objective lens if the exit pupil is smaller than your eye's pupil or it will be the diameter of your dilated eye multiplied by the magnification of the binocular if the exit pupil is larger than your eye's pupil. Measure the width of the slit, divide it be the binocular magnification and you have your pupil dilation for that light level.

Incidentally, that is why I love 12x45 binoculars for astronomy.
It stops my eyes down to 3.75mm, same as 8x30.

A friend was having bad trouble with her eyes using a 10x50 binocular.
I gave her a selected 12x45 Russian binocular, and she quickly rang me thanking me for giving her back her eyesight.
Her optician really did not understand this.

The problem is that there are so few 12x45 wide angle binoculars.

Howdy Ron :hi:

I think we here on the Forum have danced in and around this topic obliquely and directly, periodically --- usually with more questions being raised quicker than answers found :eek!: o This is something that perplexes me, and quite interests me ..... :cat:

So is the choice between a bit sharper but dim view (and therefore not much use for seeing anything), and a softer but slightly relatively brighter view (again not much use for seeing anything :h?: :-O

I was going to pounce on you and mercilessly rip your arguments to shreds by claiming that you "cheated" by pitting the Porro 'clarity' of the 8x30 unfairly against mere underling S-P prisms, but I see you've comprehensively shot that down by pulling out your 100% reflecting prism big guns --- so drat ! :scribe:

Conventional TWF wisdom holds that the big 10x56 FL should be much better with a 50% greater TWF number. There's no Relative Brightness edge for the little 8x30 either with less than half the RB of the big jigger. Is that 4mm dilation figure a guestimate, or based in some study or calculation? (If my brain wasn't so foggy, I'm thinking there must be plots of environmental luminance vs visual acuity, and then modified by/ and correlated to proportional brightness increase by illuminated pupil diameter, and actual detail comprehension due to increased magnification, and all with reference to objective diameter :brains: ..... phew! such 5th dimensional numerical wranglings are beyond me :bounce: at the moment though

Also, does that ~4mm figure vary with age? and is there some sort of whipper snapper to old f*rt, linear change, exponential deterioration, or inflection point? ....... *cheeky wink* hahaha )

I can readily notice a 5mm EP limitation around dusk compared to a 5.3mm one which is most annoying at that beautiful time of light, though as the light drops even further, things start to even out again I think?? ......

Final thought for now -- have you measured the resolution of each of these bins in best light to know what they are capable of, and how they rank compared to each other?

Thanks for your thought provoking musings .....

Chosun :gh:

Binastro,

I showed my wife your post and she intends to run our democracy as she always has.

Henry,

did you mean 30mm or 3mm?

30mm, remember the tape is at the objective end of the binocular. For an 8x binocular a 30mm wide slit in front of the objective lens will be 3.75mm at the exit pupil. The starting point is somewhat arbitrary. The space between the pieces of tape just needs to be narrow enough so that once it's divided by the magnification at the exit pupil it will be smaller than your eye's pupil. Then you gradually increase the width until the edges of the tape dissappear.

Sorry Wanderer,
I don't know what you mean.
8x30 binocular has an exit pupil of 3.75mm if not vignetted.

Chosun,
When younger I preferred 12x50 for astronomy, but now 12x45 or even 10x30.
It is possible as a youngster 10x50 was good.
So I think it is age related.

I am afraid the so called theory about binocular or other optics performance, must be trumped by actual observational experience.
Observations come first.
Theories are made up to fit them and become so called theory or science.
Empirical wins the Golds, as do GB at the moment compared with Australia, although theory might say otherwise.
Large country, super fit outdoors types, versus crammed in, polluted, clouded out folks.

Binastro said:

..... Chosun,
When younger I preferred 12x50 for astronomy, but now 12x45 or even 10x30.
It is possible as a youngster 10x50 was good.
So I think it is age related.

I am afraid the so called theory about binocular or other optics performance, must be trumped by actual observational experience.
Observations come first.
Theories are made up to fit them and become so called theory or science.
Empirical wins the Golds, as do GB at the moment compared with Australia, although theory might say otherwise.
Large country, super fit outdoors types, versus crammed in, polluted, clouded out folks.

Click to expand...

Oooooooooooooooooooooooo ! |8.| ..... that's harsh Bin --- just because you've finally slightly dipped an oar in front ! ......... and so you should with near 3x the population and a helluva lot of our wealth for toil! :king:
Super fit outdoors types we may be, though I think we have underperformed so far due to Generation 'soft' - where it's all about the 'experience', and the 'journey', and everyone is 'valued' for who they are (regardless of results, or performance), and everyone is a 'winner' ......... Are you kidding me! :eek!: ---- Someone needs to take these kiddies aside and give 'em a good ol' bl**dy slap up the side of the head ! and tell them the Olympics is about Gold, Silver, Bronze - nothing else ! 1st, 2nd, 3rd ..... and everyone else is a loser :storm:

Empire, Empirical or Imperial ?! :-O
While individual observations are important, there's far too much variability and poorly understood inherent, and other influencing factors to base design decisions on -- consumer choice decisions yes, but large scale population studies, experiments, and meta-analyses are what's really called for ...... :brains:

This astronomy thing interests me, and I think may go some way to possibly informing us :t: Thinking of large astro binoculars, or telescopes, so necessarily below 4mm EP ..... Given that light gathering, and objective diameter for a given best quality, largely comes down to a matter of money, or limits thereto, it then becomes an equation for a given objective diameter that is affordable of:- magnification vs exit pupil .......

Question: At what point does increasing magnification become pointless for actually seeing more far far away stuff better, due to an exit pupil that has become too small? Is there wide consensus on this point? or is it subject to individual variability? Also, is this point reached because the diminishing EP loses too much brightness, or do alignment and viewing ease factors trump that? :cat:

How transferable do you think this astronomy experience is to terrestrial binocular viewing in deep shade, and around dusk, twilight?, and then beyond into moonlight? Many thanks :t:


Chosun :gh:

Astronomical telescopes are nowadays not expensive, say 10 inch Dobsonians or 4 inch refractors or 5/6 inch Maksutovs (Russian ones better than Chinese)
It depends on what object is being observed.
For Saturn/Jupiter I used 265x on my 12.5 inch compound scope.
Paul Doherty, with superb planetary vision used 190x on his 12 inch Newtonian.
That meant greater surface brightness and more detail.
I was not good for faint contrast, but good for faint moons/stars.
But for close Jupiter detail I used 400x and 600x plus to view Jupiter's moons. I tried 1100x but above 750x empty magnification even on best nights.
Very few telescopes used above 1000x even 60 inch because of our atmosphere.

Mars 400x to 500x common.
Small top long focus 4 inch refractors can easily take 300x on planets or double stars and 400x plus for testing.

Stars are different being point sources. High mags can be used to see fainter stars as the background darkens and contrast increases.

William Herschel used very high mags and found Uranus and many other things. He used tiny spherical glass ball eyepieces sometimes with tiny fields.
Dawes used typically 65x per inch for Mars, Jupiter, Jupiter's moons and double stars. Using ~6inch refractors.
O'Meara used about 25x per inch on 24 inch Hawai scope breathing pure oxygen at 14,000 ft.

Unfortunately astro observations have little relevance to wide field binocular observations.

Bin, so mag rules unfettered up to atmospheric imposed limits, with EP's well below our sharpest daytime acuity (~2.5mm) , and damn the difficulty of view, and loss of brightness, since contrast is as high as it gets because the background (space) is er, pretty um, well - dark !! ) :brains: 3 :king:

But how to relate this to lower light levels in terrestrial viewing, and on through the purkinje shift ........ it seems that some complex multivariate equation (with the addition of a contrast function too) like I suggested is required, and then model this wrt age ...... :cat:

Btw, it seems the carbon fibre wheel is turning, and you are due to edge ahead on the tally again ...... :loveme:


Chosun :gh:

Posted a few hours back in the thread "Zeiss Conquest 10x42". Here "another thread" is this one.

adhoc said:

Post #43, The-Wanderer. Sorry I can respond only now. In the meantime first Typo, then others in another thread, have done so. During that time I sought for facts from a few sources, including my eye doctor in a phone call of about a minute as he drove to work in the morning (he is personally known), but without much clarity.

FWIW, it seems that a pupil whose dark-adaptation is limited by age is dilated beyond that limit by the eye drops. These may fail to do that if dilation is limited by diabetes.

About measuring methods I learnt a lot from the responses here! My own is to peer at the bathroom mirror at night and shine a dim light at an angle quickly. This takes some practice, with the small LED in a cell phone dimmed more easily than a flashlight, and precision is poor as I just guess to the nearest mm looking at a scale immediately.

Click to expand...

Chosun,
I have spoken privately to acknowledged world leading astro observers with superb vision, both acuity and faint vision, and they confess that as they get older their eyesight gets worse. Experience is built up over years, but can only go so far.
I had good unaided eyes faint vision, but others had much better telescope faint vision than I did. I cannot explain this, but it is how it is.
It takes two or three years for a dedicated astro observer to begin to get experience and skills as an observer.

Birdwatchers have skills I cannot fathom at bird recognition.

The atmospheric problem was dealt with by the military who kept adaptive optics a secret for years.

Old timers would be amazed at the digital imagers skills, using tens of thousands of video frames stacked, using the best maybe 10,000 frames. It really is Frankenstein like.
Yet observers skills built up over years are useful even here. I often spot superb digital images that are in a way fake. Detail is not real, just artifacts, and claimed images are clearly not possible. Moons are presented that are in fact stars and so on. Planets at impossible angles.

I very recently pointed out that a cover photo on a well known magazine that included night sky was not real, 'fake'.
I was told by the editor I was the only reader to notice this.
The number of fake scenes in movies is enormous.

Ron

Really interesting stuff, thanks for posting.

Lee

Binastro said:

Chosun,
I have spoken privately to acknowledged world leading astro observers with superb vision, both acuity and faint vision, and they confess that as they get older their eyesight gets worse. Experience is built up over years, but can only go so far.
I had good unaided eyes faint vision, but others had much better telescope faint vision than I did. I cannot explain this, but it is how it is.
It takes two or three years for a dedicated astro observer to begin to get experience and skills as an observer.

Birdwatchers have skills I cannot fathom at bird recognition.

The atmospheric problem was dealt with by the military who kept adaptive optics a secret for years.

Old timers would be amazed at the digital imagers skills, using tens of thousands of video frames stacked, using the best maybe 10,000 frames. It really is Frankenstein like.
Yet observers skills built up over years are useful even here. I often spot superb digital images that are in a way fake. Detail is not real, just artifacts, and claimed images are clearly not possible. Moons are presented that are in fact stars and so on. Planets at impossible angles.

I very recently pointed out that a cover photo on a well known magazine that included night sky was not real, 'fake'.
I was told by the editor I was the only reader to notice this.
The number of fake scenes in movies is enormous.

Click to expand...

Yes Bin, I recall watching a documentary on the construction and operating principles of the (quasi-binocular like) Keck Telescopes (I'm also getting recollection of the standard mirror LBT in Arizona - but that's just making me hungry! :eat: :-O

Very very exciting times, with the James Webb, and the European Extremely Large Telescope both coming on line (soonish) over the near future :t:


Chosun :gh:

I think that astro viewing with a telescope does overlap low light binocular use in what I'd call the medium power range, where exit pupils between 1 and 4 mm are used. That's the range that looks really sharp to me. Surface brightness gains at greater apertures rarely seem to buy anything except in the VERY darkest sky, and the whole reason for doing that is to see nebula which are hazy in the first place and don't need resolution, so the eye's large-pupil acuity degradation isn't noticed. Below 1mm, no additional detail is resolved by the scope due to the diffraction limit (I'm assuming perfect atmospheric steadiness here), and the view becomes soft, but tiny features like planetary details often become easier to see just by being made larger, although there is an accompanying contrast loss, so you can lose that way too. Planetary observers often suffer the soft view to see all the telescope can reveal without having to bust their eyes on features displayed at 1 arc minute size.

Ron

I would agree with most of that Ron.
But where you have a faint moon near to say Saturn or its rings, by really piling on the magnification the surface brightness of the planet or rings reduces, AND the distance of the moon from the surface detail increases.
If the Seeing is good enough the faint or very faint moon is revealed.
Another way is to use an occulting bar in the eyepiece or put the surface detail just outside the eyepiece field, but high magnification helps here also.
There are other situations where exit pupils of less than 1mm help if your eyes work well at these magnifications.

For instance with Saturn's edge on rings which are almost or actually invisible.
The moons are strung out along the edge on rings and additionally the ring gaps become star like objects.

It also definitely helps in double star observations to use high powers.

With Mars, which has a high surface brightness, 1mm exit pupils are just too large to reveal all the detail.
With a good 4 inch refractor 100x is just too low as is 200x with a top quality 8 inch Maksutov.
Observers use double that, sometimes more.
The view with Horace Dall's 8 inch Maksutov through the glass of his attic window at an angle at 400x was something that had to be seen to be believed.