7x42 (1 Viewer)

Sancho once coined a phrase about binoculars merely being tubes with glass in the ends.
There's a deeper truth into it, remembering they're afocal systems.

If you look up the sky, then put a 10" wide pipe in front of your face, the sky will not appear darker. And if you take a long black 1" pipe to look through, the brightness of the light in the tunnel is constant all the same.
It might in fact appear brighter than in the first instance, but that's because the eye struggles with the harsh contrast, trying to compenate for the dark pipe wall.

That's essentially everything there's to it. The beam pencil of parallel and diverging light rays isn't somehow compressed, increasing the photon density or speed. It's just a seemingly magnified slice of the reality.
Like David pointed out, it's about modified observation angles.

//L

Edit: Again, the light falling on the iris is "lost" but this does not reduce the brightness. As soon as the exit pupil is smaller than the observer's pupil, brightness will decrease.

Kevin Conville said:

You're missing the fact that with a camera attached all of the light is being focused on the image sensor.

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What is "all of the light" supposed to mean? In my experiment, the exit pupils were larger than the camera's aperture. Although there's some light wasted just outside the camera lens, the brightness was (very) roughly the same as with the camera alone. The exposure values confirm that the size of the objective lens in afocal systems does not affect brightness, as long as the exit pupil is larger or equal to the human pupil size or camera aperture.

Kevin Conville said:

To your last point: OK binoculars are afocal, that's why I said effectively.
Look, the light has to fall on something, right? In a camera it's the focal plane, film or image sensor. Using field optics the light fall on the pupil.

The real point is that the focal plane of the camera doesn't change, the pupil does.

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A camera lens projects a real image on the film/sensor. The light beams are converging.
Since binoculars are afocal, the beams coming out from the exit pupil do not converge. The beam pencil consists of parallel and diverging beams and cannot be projected onto a surface like a paper, a film plane or a sensor.

Thanks to this, the eye can handle the emitted light by again making it converge through the eye's optical system, and be projected on the retina.
We can't observe real images (produced by converging beams) if they aren't projected on a surface. Try to look through a camera lens with your eye somewhere where the sensor would have been.

Through binoculars, the magnification is constant regardless of distance between the eye and the ocular lens. And you cannot project anything coming out from it directly on a surface. If you make a spot metering of the exit pupil's brightness from any given distance, the brightness will be constant. Outside of the spot metering area, other things happen, mostly related to the apparent field of view. So the difference is fundamental.

//L

Kevin Conville said:

So all of the light entering a 42mm objective (normal losses excluded) is brought to a 6mm circle in the case of the 7x and 4.2 in a 10x. Your pupil is dilated to, say 4.2 mm. Are you saying that somehow your eye captures the same amount of light in both despite about 50% (roughly 14 vs 28mm sq.) of the 7x42 isn't reaching the pupil?

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Kevin, that is correct. The light flux for each square millimetre of the exit pupil will be identical for both binoculars. With a 4.2mm pupil, identical levels of light would reach the retina.

Kevin Conville said:

Say you're looking at a boat from shore through 8x35s and 16x70s. Both have 4.375mm exit pupil diameters. Through the 16's the boat fills the image circle. The 8's fill half of that circle. The value of the light through the 16s is better because it's the light of your subject. If the boat is brighter than it's background your pupils will constrict a bit more increasing the effective focal ratio, somewhat improving the optics. If the boat is darker than it's background your pupils will dilate a bit more and allow more light to your eye.

This might also help to explain the mysterious Twilight Factor that used to get a lot of attention.

With the added resolution of magnification comes less noise. Everything else being equal it means less flare, glare, and wasted photons.

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It's true that the metering system of the eye will be responsive to the relative amounts of light and dark in the field of view, and the pupil will respond accordingly, but I don't see what that has to do with the arguement here.

Relative binocular resolution at different effective apertures is an interesting topic, but it doesn't play a part in this brightness story, so I'll shelve that one. I believe the explaination for the Zeiss data that led to the twilight factor calculation, or the parallel work at Leica that Holger explored in his paper, can be found in the neurophysiology of the eye, not binocular optics. Explaining that is well beyond my pay-grade I'm afraid. :eek!:

Cheers,

David

typo said:

Pileatus,

I'm with Lars on this one.

It's a topic I've found challenging to get my head round myself, and have found even more difficult to explain. I'll have another go.

I think it's best to go back to defining magnification. It's a change in the apparent angular size of an object. If a tree is 1 degree tall to the eye then it will be 8 degrees tall with an 8x binocular. It's the light level for the angle that translates to brightness, not total light flux. The angle of the 'light beam' at the exit pupil in this case will be 8x wider than the objective. This means the exit pupil will consequently be 8x smaller. You could imagine the magnification causes a 8 fold reduction in light intensity and the smaller EP an 8 fold increase, cancelling each other out exactly. Together it means that the light level per given angle of view from any point in that exit pupil will be exactly the same as that for the unaided eye. The target will appear equally as bright with or without a binocular (ignoring internal light loss).

Of course that assumes the EP isn't limiting for the eye. If the light level for a particular target means your pupil is 2.5mm unaided, then the light level for a given angle of view will be the same regardless of whether you are using a 5x12.5, a 8x42 or 16x70. On the other hand, in low light with a 5mm pupil diameter, the light level per given angle will be 25% and 76% of the 8x42 levels for the 5x12.5 and 16x70 respectively. However, increased magnification make targets more visible, and it is quite likely that the 16x70 will give the illusion of being brighter than the other two.

Hope that helps.

David

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This is gold. You're doing a great job explaining this more scientifically.

Thanks,

//L

typo said:

Kevin, that is correct. The light flux for each square millimetre of the exit pupil will be identical for both binoculars. With a 4.2mm pupil, identical levels of light would reach the retina.
David

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Hi David
You always explain this well and I think I have understood it but here is another question.

Lets take more or less identical binos, say FL 7x42 and FL 8x42. We all agree that providing your pupil dilates large enough to utilise the whole exit pupil that the 7x42 will be brighter in dim conditions by virtue of the bigger exit pupil.

Is this simply because a greater area of your retina is receiving stimulation (at the identical light flux intensity) and not because this increased area x flux = more photons are being detected?

Lee

Lee,
Providing your pupil will dilate sufficiently, the larger exit pupil will deliver more photons to the retina. The area of the retina is dictated by the AFOV. If you are young enough a 7x42 would be brighter than an 8x42, but it may not be obvious. At light levels that would produce a 6mm pupil, you may still have a higher apparent acuity with the 8x42 (twilight factor 17.1 vs. 18.3) and greater contrast sensitivity (Leica/Holger).

David

(deleted comment)

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I'm running out of explanations, and I recycled most of my arguments twice or more.

- Have you gentlemen considered the significance of my experiment, empirically showing that brightness inside the beam pencil is determined solely by the instrument's transmission rate?

- Have you tried digging into my other thought experiments?

- Do you have empirical evidence for the notion that a big scope is brighter than a smaller binocular when the exit pupil is the same, contradicting mine?

I'm sorry to say that the perceived darkening with the scope eye doesn't qualify as empirical evidence. How did you measure your pupil size while simultaneously looking through the scope? I can think of a couple of explanations to your experience, one of them being confusion with the non-scope eye looking towards the ground and the scope-eye watching the sky with an angled scope.

- Have you considered the fact that your notion regarding increased brightness with bigger apertures would have been tried as a mean to solve the energy crisis? Essentially, it would only take an enormous terrestrial scope directed towards a blue or overcast sky. The light intensity coming out from the small exit pupil of the eyepiece would be sufficient to kill you, be used as a raygun destroying cities or the like. The distance from the eyepiece/outfall would be irrelevant as it's parallel rays.
Similar things can and have been done, but with concave mirrors or convex lenses, both having the distinct disadvantage that they need to focus at one certain distance. Never with afocal instruments, although their independence of distance would be a huge advantage.

Not trying to mock you, just asking you to consider all this.

//L

looksharp65 said:

I'm running out of explanations, and I recycled most of my arguments twice or more.

Not trying to mock you, just asking you to consider all this.

//L

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I guess that the 7x42 will be a bit brighter (31%) than the 8x42 from the beginning because of the larger effective f-stop. But when the eye pupil is the limiting f-stop of the system they will look as bright?

Anything else would also break the existing formula for "relative brightness" in binoculars, though it's a theoretical figure. :smoke:

Vespobuteo said:

I guess that the 7x42 will be a bit brighter (31%) than the 8x42 from the beginning because of the larger effective f-stop. But when the eye pupil is the limiting f-stop of the system they will look as bright?

Anything else would break the existing formula for "relative brightness". :smoke:

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1) Yes, because of the wider exit pupil, more exactly, not constricting the width of the beam pencil below the pupil size.

2) The eye pupil is the true aperture. What's in front of it is basically egal, as long as the beam pencil isn't thinner than the pupil diameter (and, as usual, losses matter - transmission, reflexions and such)

3) :t: The number bears a close relationship to the exit pupil diameter. In this discussion, it's useful, however exit pupil to pupil ratio is sufficient to understand whether or not light loss could occur at any occasion. Having an exit pupil size above you own maximum pupil size ensures you will never see dimmer through the binoculars than beneath them. The ratio should be >1.

//L

Kevin Conville said:

So, why are you biting your tongue? Let's hear it.

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Oh, I had no secret message. Last week, Dennis went bonkers because someone (me) actually agreed with him on something. It's not like I never agree with him; I remember that time about 12 years ago .... (Relax Denny, I'm just having fun.) I had just posted my "garden hose," and could see getting into another "longun," as opposed to a "long gun," standing between 101st Strictly Math brigade and the 81st Physiological Division.

Gratefully, I awoke this morning to see others are sorting it out. :cat:

Bill

Haven't prepared this comment, but would like to admit there's a certain circumstance when all I've written above may not be valid :eek!:

This particular circumstance is when looking at faint stars on the night sky, where, as mentioned, a 10x50 can be fantastic.
I'm not positively certain if and why this would be a valid exception. For terrestrial viewing, I maintain what I wrote.

//L

looksharp65 said:

Haven't prepared this comment, but would like to admit there's a certain circumstance when all I've written above may not be valid :eek!:

This particular circumstance is when looking at faint stars on the night sky, where, as mentioned, a 10x50 can be fantastic.
I'm not positively certain if and why this would be a valid exception. For terrestrial viewing, I maintain what I wrote.

//L

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Lars,

Astronomy is bound by the same rules. There is nothing magic about stars. However, trying to explain how high magnification might make feint stars more visible while simultaneously making them dimmer seems a thankless task.

David

typo said:

Lars,

Astronomy is bound by the same rules. There is nothing magic about stars. However, trying to explain how high magnification might make feint stars more visible while simultaneously making them dimmer seems a thankless task.

David

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'Like trying to explain how some things can be SEEN but not RESOLVED. :cat:

Bill

typo said:

Lars,

Astronomy is bound by the same rules. There is nothing magic about stars. However, trying to explain how high magnification might make feint stars more visible while simultaneously making them dimmer seems a thankless task.

David

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Well.. I could speculate and see what comes out from it. When we observe distant stars, we only see an unfathomably small portion of the light they once emitted. We don't see the actual disk the way we see the Moon, the planets or the Sun.

Light intensity of a light source fades with the square of the distance, so if you illuminate an object with a light source and then double the distance, the illumination of the object will reduce by 75%.

Reflected light is different. If you measure the brightness of a surface to get an exposure value or such, that value is independent from the distance to the object. When trying to shoot the Moon with a small camera, it's always totally over-exposed because the camera takes all of the black sky in consideration when it auto-calculates the exposure.
In reality, since the Moon is illuminated by the Sun, and has a similar reflectiveness as the Earth's surface, the exposure should be set to the same values as during the day - independent of how big or small the Moon will be in the final image.

Despite the fact that we don't see the actual stars, in which case a 10x60 should show more detail than a 7x42, the former does indeed reveal more faint stars. Since it cannot be the function of exit pupil size or magnification, only the true aperture would be left to explain this. Maybe.

Thus, I'm not rejecting the idea that the wide objective collects more photons (and remember, with faint objects the number of photons reaching Earth is limited) but if this is true, there must be another mechanism behind it than the one present when looking at faintly reflected light in the evening.

The photopic, mesopic and scotopic vision play major roles in this, too.

//L

Edit: could it be as simple as the magnification reduces the apparent distance to the faint star? If light intensity reduces with the square of the distance, and here it's the incident light reaching the observer), a distance which is reduced with 90% should "light" quite a few stars previously invisible to the naked eye.
They were under the threshold of perception, and through the binocular, they're above it.
If correct, this might also explain why the combination of [real aperture size + magnification] trumps exit pupil.
It's not a actually flux of photons, more a small number, so a big collecting aperture would be helpful.
I find it interesting to speculate, but I'm also a little bit uneasy with this latest course of the thread.

Lars,

Stars are still bound by the laws of physics. While I've read about some quite impossible claims for 'light grasp', at least one or two astro sites based at educational establishments acknowledge at least that optics can never increase luminosity, and a small exit pupil will decrease apparent brightness while paradoxically making stars more visible. Much the same as terrestrial viewing. They just don't explain how. I'll just mention a couple of things which I think must be contribution factors, but I'm sure it's not the whole story.

Stars with the largest angular diameters would need around 400 fold magnification just to occupy the area of a single foveal receptor in the retina. Sirius is brighter than Betelgeuse for example, but is actually has about a ten fold smaller angular diameter still. Even at ten fold magnification the image it would be so small that the image would frequently fall between receptors in the fovea let alone the lower density rods. Obviously that isn't the case, and other factors must come into play. They appear bigger than other stars so must trigger many receptors. Stars of this magnitude are bright enough to cause retinal glare. The scattered light alone would be sufficient to fire multiple receptors. Feint stars can't do that, and to see the feintest it is necessary to avert one's view to allow detection by the more sensitive rods. However they have a relative low density. It will be necessary to increase the angular size significantly by some means in order to clearly see them. Earth's atmosphere makes a decent start of spreading the leight, so will the aberrations of the eye. A bit of magnification will obviously help further, and a lot of magnification will help a lot. Particularly with diffraction limited optics. It seems the angular expansion of the size of the star is more critical to visualisation in many cases than the decrease in brightness caused by an increasingly small exit pupil.

As I said, just a few elements of an undoubtedly more complex story.

David

typo said:

...

Stars with the largest angular diameters would need around 400 fold magnification just to occupy the area of a single foveal receptor in the retina. Sirius is brighter than Betelgeuse for example, but is actually has about a ten fold smaller angular diameter still. Even at ten fold magnification the image it would be so small that the image would frequently fall between receptors in the fovea let alone the lower density rods. Obviously that isn't the case, and other factors must come into play. They appear bigger than other stars so must trigger many receptors. Stars of this magnitude are bright enough to cause retinal glare. The scattered light alone would be sufficient to fire multiple receptors. Feint stars can't do that, and to see the feintest it is necessary to avert one's view to allow detection by the more sensitive rods. However they have a relative low density. It will be necessary to increase the angular size significantly by some means in order to clearly see them. Earth's atmosphere makes a decent start of spreading the leight, so will the aberrations of the eye. A bit of magnification will obviously help further, and a lot of magnification will help a lot. Particularly with diffraction limited optics. It seems the angular expansion of the size of the star is more critical to visualisation in many cases than the decrease in brightness caused by an increasingly small exit pupil.

As I said, just a few elements of an undoubtedly more complex story.

David

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David,

One of the major factors is saccadel eye movement:

...Main article: Saccade
The eyes are never completely at rest. They make fast random jittering movements even when we are fixated on one point. The reason for this random movement is related to the photoreceptors and the ganglion cells. It appears that a constant visual stimulus can make the photoreceptors or the ganglion cells become unresponsive; on the other hand a changing stimulus will not. Therefore, the random eye movement constantly changes the stimuli that fall on the photoreceptors and the ganglion cells, making the image more clear.[10]

Saccades are the rapid movement of eyes that is used while scanning a visual scene. In our subjective impression, the eyes do not move smoothly across the printed page during reading. Instead, our eyes make short and rapid movements called saccades.[11] During each saccade the eyes move as fast as they can and the speed cannot be consciously controlled in between the stops.[10] The movements are worth a few minutes of arc, moving at regular intervals about three to four per second. One of the main uses for these saccadic eye movements is to be able to scan a greater area with the high-resolution fovea of the eye.[12]

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Ed

Ed, David - I'd better leave that discussion to you and those else who know :t:

//L

Thanks Ed, added to the list.

David

elkcub said:

David,

One of the major factors is saccadel eye movement:


Ed

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And, it's why people can say their view is sharp from "the center to the edge of the field" even in instruments with stout field curvatures. They don't consider the rapid eye movements and the almost instant dioptric accommodations for the differences in focus. :cat:

Bill

dwatsonbirder said:

I'm a huge fan of 7x42 binoculars for many reasons, but I think that Lars has raised an excellent point regarding the exit pupil. Being a whippersnapper myself (30 counts right?) I find that the larger exit pupil of these binoculars is really useful, particularly in dull conditions or in a forested environment. I took my newly acquired 8x32 Nikon HGL's out this afternoon to look for passerines at a local sewage farm. Watching fast moving small birds some 5m above my head was compounded by tricky eye placement, and the slightly duller image of the smaller objective lenses.

Although 8x32's are an excellent all-round binocular, once you've grown accustomed to 7x42's, it is difficult to go back to the less user friendly compact size. I don't personally subscribe to the notion that the perceived difference in magnification between 7x and 8x will make or break the ID of a bird - most birders carry a scope nowadays which is much more useful when ID is critical.

It would be interesting to hear what others preferred 7x binocular is; Personally, I've only tried the classic BGAT Zeiss, and the SLC which I've used heavily for nearly a decade now.

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Trino 7x42 BN and ultravid 7x42 HD But also have a pair of BGAT T&P as well love the old school bins