That is, they serve as light buckets which greatly increase the amount of photons entering your eye, but do not magnify the image at all? I realize that the only likely market for such a device would be limited mostly to amateur astronomers, but if such an optical device is possible you’d think that someone would have built it somewhere? Or is there a technical reason why it wouldn’t work as stated?
They should be optically possible but I have never heard of them. For boats you generally want some magnification but not a lot because too much just makes the image shake too much.
On the other hand you have electronic night vision devices but they are not purely optical.
That’s basically how night vision goggles work (popular with military folks and ghost hunters).
There’s no such thing using just lenses (or mirrors). Binoculars do not make light brighter. They only make the scene larger. If you use them to look at the Moon, any part of the surface of the Moon will be just as bright as with the naked eye.
For stars, your eye can’t resolve them as anything other than a point. So they will get brighter until you magnify them enough for the star’s image to completely cover a single receptor in your eye. After that, they just excite more receptors, they don’t get any brighter with further magnification.
This is (very obviously) wrong. Binoculars have much larger aperture than the eye and gather more light. Anyone who has used binoculars knows this.
I’ve heard that cats eyes glow because they reflect light so the same light can hit the retina 2x (back and forth).
The amount of light that is usable from a binocular (or a telescope, for that matter) is a function of basically three things.
- The diameter of the objective lens. Label this D
- The magnification of the binocular. Label this M
- The diameter of the pupil of your eye at the time of viewing.
Several things are at play here. The objective lens will gather up a big bundle of light, let’s use 50 mm for an example. (This is the usual diameter of a binocular designed for night vision.) So, D = 50
This light will be concentrated into a smaller bundle of light coming out from the eyepiece. Call the diameter of this bundle E. IIRC this is labeled the exit pupil of the system.
Now here the idea that a binocular with a big objective lens and a very low power falls down. In our example, with D = 50mm, if M = 7, then the size of this exiting bundle of rays will be D divided by M, or E = 50/7 = 7 mm, approximately.
This just happens to be the diameter of the pupil of a dark adapted eye (until things like age, cigarettes, and so forth degrade the system), so the eye can take in all of the light coming out of the binocular eyepiece without wasting any. In other words, the view thru the binocular is a bright as it can be.
Now let’s assume that the magnification is only 5. Now the exiting bundle of rays is 50/5 = 10mm, but only 7mm can be taken in by the observer’s eye. This means that roughly half the light is wasted. And the case gets worse as the magnification is reduced.
There is no getting around any of this for an optical system. And as mentioned above, this is why 7X50 binoculars have been standard for night observing just about forever.
Hmm…I’m actually surprised by this. Is that right?
It’s correct, and due to thermodynamic reasons. Binoculars do gather more light, but they also magnify, and the two cancel out.
It’s not possible to have one without the other because you could build a perpetual motion machine otherwise. You could take a low-grade heat source, concentrate it to a high-grade (efficient) source, and use it to drive a generator. The low-grade waste heat could then be concentrated and you could repeat the process.
You have to be careful when talking about astronomy because stars appear as point sources to your eye whether they’re magnified or not. Magnifying a star by 10x gives you 100x the light, assuming sufficient aperture, but since it’s still just hitting a couple of photoreceptors, it appears brighter. The surface brightness of the star is not actually any higher, though.
The ratio of the diameters of the objective and the “exit pupil” (the disk all the light fits through on the way out of the device) equals the magnification ratio. If you want a ratio of one, you have exactly the same size objective as the pupil in your own eye.
On re-reading the thread I am afraid the post I was responding to and my post can be interpreted in different ways and would need explanation.
Let me see if I can do a better job this time around although this is the kind of explanation which benefits immensely from being able to draw a diagram on a bar paper napkin.
Suppose I am looking at some uniform color which entirely covers my field of vision like a completely uniform sky. A certain amount of light enters my eye. Now suppose I use a 5X telescope or binocular, neglecting losses in the glass, the total amount of light reaching my eye is the same. But that light is from an area of the sky or scenery much smaller (1/25 in area) so that area is, in fact, much brighter.
So, if I use binoculars at night I can see lights which I could not see with the naked eye.
This is similar to a photo camera. The longer the focal length of the lens the wider the diameter for the same stop value. So, for a uniform background any 2.8 lens will put the same amount of light on the film but the longer the focal length the more concentrated the origin of that light which in fact makes that area brighter. If I double the focal length I am getting the same amount of light from 1/4 the area which makes that area 4x brighter.
So binoculars are used at night to see things which would not be visible to the naked eye (or to see them better).
I hope I have explained myself better.
Sorta, but you’re using a confusing and probably misleading definition of “brightness”. As you note, if you look at a uniform field of light like the sky through binoculars or a telescope, it doesn’t change appearance. Your eye can’t tell the difference. It’s true that the device is collecting more photons from a given patch of sky, but the total number of photons that enter your eye is about the same.
As ZenBeam and I noted, astronomy is different because stars are point sources (and not uniform fields).
Yes and no.
If you are looking at a uniform field the amount of light which enters the eye is the same but if you are looking at a totally black sky with a star in the center then the longer the focal distance or the magnification, the more light that reaches the eye.
In a totally black night binoculars will allow me to see lights which I cannot see with the naked eye.
Stars: yes (because the stars are point sources). Large galaxies/nebulae/etc.: no (because the surface brightness of the object is unchanged).
Though I should point out that large faint objects may be more easily noticeable than small ones because parts of them will extend into your peripheral vision (which is more sensitive to faint objects). This difference can be avoided if you avert your gaze slightly.
You can take the risky route of using dilating eyedrops to keep your irises fully dilated no matter the lighting conditions or inherent brightness of your feild of vision, then used filters, like sunglasses, to darken to a desired intensity. Of course this isn’t practical (let alone dangerous) for many reasons. And becomes moot if you’re talking about nighttime settings anyhow.
Essentially, you can’t simply amplify the intensity of the photons hitting your eye without optical magnification or film/electronics to perform superhuman focusing, projections and exposures. Night vision goggles use CCDs to amplify the signal, and/or convert infrared radiation into a wavelength our eyes can see.
The “surface brightness” of an object is the same but I am seeing it much bigger and getting more light from it into my eye.
I was responding to “Binoculars do not make light brighter. They only make the scene larger.” That depends on how you understand it. Obviously you can’t make light brighter unless you use some sort of electronic amplifier like night vision devices do, but you can catch more of it and I would call that “brighter”.
If I have a camera with a wider aperture it will catch more light and get a photo where a lens with a smaller aperture cannot.
This is like magnetic flux. If I concentrate it in a smaller area is there more of it? Is it stronger? There is not more of it, there is the same total amount, but there is more per unit of area so you could say it is stronger. You just need to be careful in the phrasing and agree in the interpretation.
I think we all agree and we all know what we mean and we are just going around in circles SDMB-style.
With pure optics, you have to shrink the image, not magnify it to produce a brighter image above what the naked can perform alone.
This doesn’t make sense to me.
The Andromeda Galaxy is 7 times the width of the full moon in the night sky. It is simply too dim to see with the naked eye. However, if you use a wide-field eyepiece in a 16" telescope, you can see it quite clearly. It is obviously much brighter through the eyepiece than it is with the naked eye.
This is essentially the entire purpose of large Dobsonian telescopes for visual observing: they gather more light, resulting in a brighter image at constant magnification.
And by brightness, let me make it clear that I mean “photons per solid angle in the visual field.”
The hiccup here is the notion of a device that can amplify the light’s intensity above what our naked vision is capable of. Sure, we can magnify something and compensate for the dimmed intensity by using larger objective lenses, but you won’t be able to ever achieve a magnification brighter than your naked vision’s “ground-state.”
If you pointed your binoculars at the sun, you’ll damage your retina in no time, not because it’s a brighter image, but because the sun is now exposed across your entire field of vision.
Isn’t that a physiological aspect of our eyes/brain? You’re projecting a very dim image onto far more rods and cones, most of which beyond our center of vision is far more sensitive to low-light. Also, you’re effectively restricting your vision to that object, and cutting off any other sources of light-noise.