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B Binoculars and light amplification

  1. Feb 21, 2016 #1
    http://www.rocketmime.com/astronomy/Telescope/Magnification.html

    im studying from above how image is magnified in a binocular or telescope.
    In binocular say 7X. can you say light is amplified or jus magnified? if amplified, is it 7 times? if just magnified and not amplified. how come using binocular on the sun can blind person faster than naked eye?

    also for example you are viewing buildings at daytime and suddenly you see the sun reflected from the building glasses... is the effect like looking at the sun directly.. or how much reduction if not same exposure?
     
  2. jcsd
  3. Feb 21, 2016 #2
    Hi @cube137 It's definitely magnified. If you think back to the way you can burn things with a magnifying glass, you're taking all of the solar energy that would normally hit the ground over the area of the magnifying glass, and focusing it into a tiny area. Same goes for looking at the sun through a telescope; if you stare at the sun unaided with just the area of your pupil letting in light you'll go blind. Pump in rays from the area of the telescope lens through your pupil and you'll go blind more quickly!

    I think (open to challenge) if you want amplification, you would need to be adding additional light from another source - think of the analogy of how a guitar amp works; takes in a small voltage and amplifies it to a larger one, but you feed it additional voltage from an external source.

    Regarding your question on reflected light then there's a number of factors in play. If you ignore any loss through the reflected surface, from the lenses themselves, etc then there's no difference to looking at the sun direct - just like shining a torch in your eyes directly or into a mirror first. However, the other factors will undoubtedly have a reducing effect - not all light may get reflected; the lenses may have coatings which reduces throughput, etc.
     
  4. Feb 21, 2016 #3
    thanks for sharing.. but isnt magnify and amplify same meaning?
     
  5. Feb 21, 2016 #4
    Pretty sure the answer is no. Any use of the word magnify I can think of involves looking at the same "stuff" more closely, or in the case of lenses taking the same amount of light and concentrating it into a smaller area. By contrast, I can't think of any use of the work Amplify that doesn't involve adding more "stuff" to the original - e.g. amplify sound = plug in a powered amplifier; or amplify light you need to add more light

    There's plenty of references out there to the difference between magnify and amplify - here's a couple of examples :
    1. Magnification is increasing the size of something; amplification is increasing the contrast of something. If you were to amplify an image, the contrast between lights and darks would be increased, allowing subtle details to be seen. To magnify it would instead make it larger, allowing smaller details to be seen. Amplifying a sound makes it louder because it increases the contrast between the highs and lows of the waves.

    2. Magnify is to make something look bigger . Amplify is to make the amount (volume) bigger.
     
  6. Feb 21, 2016 #5
    are you familiar with magnification principles of binoculars or telescopes in the link i shared in the first image? if you do.. well in binocular or telescope.. do you consider the brightness as many times increased or same brightness as original view?
     
  7. Feb 21, 2016 #6

    sophiecentaur

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    Both terms are in common use by non-technical people so you can't rely on what you read in many places. I think it's pretty sure to say that 'amplification' needs an 'amplifier', which uses a source signal (input light / sound/ etc) to control the power from another energy source and produce a more Powerful output signal.
    So a simple lever can only be a Force Magnifier because the energy got out, by raising a weight, say, can only be less than the energy put in. But a servo system can be a Force Amplifier because it uses electrical (or other) power to do the work.
    I would beware using Dictionary and Encyclopaedia entries as arbiters of such things and rely more on a number of 'learned' articles. The more you know about a topic, the more errors you find in 'journalistic' sources. (Same goes for News Reporting)
     
  8. Feb 21, 2016 #7
    I am familiar with the principle. The brightness is increased - think back to the magnifying glass burning something. However, I still maintain that this is magnification and not amplification as you're not adding anything new into the mix, just focussing what's already there to start with.
     
  9. Feb 21, 2016 #8

    sophiecentaur

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    The 50mm (say) telescope gathers much more light energy than your 2mm (say) pupil so this can produce a brighter image than with the naked eye without using an 'amplifier'.
     
  10. Feb 21, 2016 #9
    for a binocular of 50mm objective size and 7 X magnification.. how many times is the brightness increased? 7 times too.. what ? how do you compute for it? thanks
     
  11. Feb 21, 2016 #10
    I was asking all this because what if you were viewing the green leaves with very strong sunlight at 8X. I heard blue light can cause macular degeneration. If you use naked eye. Let's say you only view the scenery with many shadows and some with sunlight.. but say only 7 degrees of the field of view has strong sunlight.. now when you aim your 8X binocular with 7 degrees field of view.. you would see entirely the green leaves with strong sunlight.. won't this produce strong blue light that can harm the eyes (versus viewing with the eyes only where majority of the scene are dark or without sunlight). I know the arguments that if the naked eye scene and binocular scene are equally bright.. the binocular glassses may even attenuate the blue light making one with binocular safer.... but I'm talking of the bright sunlight on the leaves at only 7 degree field of view.. this would produce 7x8=56 degree apparent field where the entirely is leaves with strong sunlight and more blue light.
     
  12. Feb 21, 2016 #11

    sophiecentaur

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    Remember, there are two things at work; magnification and aperture (objective diameter, in this case). A wide objective lens will gather more light and can reduce diffraction effects ( which may or may not be relevant, depending on other aberrations. Light power reaching the eye will be proportional to lens area but, afaik, will also be proportional to the field and the two can cancel each other out.
    I know nothing of how blue light can affect macular degen; does it depend only on incident energy density or total input to the eye?
     
  13. Feb 21, 2016 #12
    http://www.reviewofoptometry.com/continuing_education/tabviewtest/lessonid/109744/
    https://www.macular.org/ultra-violet-and-blue-light

    What do you mean by "Light power reaching the eye will be proportional to lens area but, afaik, will also be proportional to the field and the two can cancel each other out."? I'm familiar with the basic of how telescope magnification work.. but rephrase it.. thanks.
     
  14. Feb 21, 2016 #13

    davenn

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    the larger the lens or mirror the more light that is gathered

    what field ??

    that statement is in contradiction to everything else you have posted in the thread so far

    your complete answer was given to you in post #2 and backed up in post #4



    Dave
     
  15. Feb 21, 2016 #14
    Magnify and amplify are words in common usage. When they are used in physics they are usually defined in a specific way.
    In optics magnification means size of object/size of image (standard text book definition)so there is no confusion with amplification or any other term.
    Note....'size' may be linear or angular. So magnification of X7 could mean that the image is 7 times longer than the object or that the angle subtended at the eye by the image is 7X the angle subtended at the eye by the object.
     
  16. Feb 21, 2016 #15
    that quotation about "field" came from sophiecentaur.. I was asking her (or him?) what he meant by it when she said "but, afaik, will also be proportional to the field and the two can cancel each other out".. well sophie?
     
  17. Feb 21, 2016 #16
    I asked all this because I'd like to know the following regard blue light magnification.

    Supposed you are in a forest in a shade in overcast sky.. and you saw a tree with sunshine shining on it and you aim your 8X binocular into it.. in your eyepiece, the entire leaves in the tree would be exposed to sunshine.. would this transmit more blue light to your eyes than when you don't use the binocular and the sunshine in the leaves is just 7 degrees in your field of vision with the rest without sunshine?
     
  18. Feb 21, 2016 #17

    sophiecentaur

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    I as referring to field of view - i.e. how many degrees wide. Sloppy of me, without a definition. Sorry. I did think that Field was a well known term, aamof.
    I'm a him with a PF profile. The name is historical and too late to change, I think.
    My point was that the field will define the portion of the scene that's admitted to the sensor array / film / retina. That tells you how much of the energy source is available. The area of the objective governs how much of the energy from each point on the source its gathered. I think that means that doubling the magnification (linear) will reduce the useful energy admitted to the sensor, which requires doubling of the objective diameter in order to obtain the same brightness of image. I think that's the right argument.
     
  19. Feb 21, 2016 #18

    davenn

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    ahhh ok, not sure what he is getting at there either ??

    well considering that is the leaves are green, as is common ( I haven't seen many blue leaved trees ), there is going to be little or no blue light coming off them


    Dave
     
  20. Feb 21, 2016 #19

    sophiecentaur

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    I think a more 'ideal' scenario than 'the leaves' would be better to discuss this problem with. A black screen with a white patch in the middle would be more appropriate, I reckon. We could discuss colours later on.
     
  21. Feb 21, 2016 #20

    Merlin3189

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    You've got it right with the 7 x 50 binoculars. The image is magnified 10x, it looks 10x as big. The diameter of the objective lens is 50mm and its area is about 2000mm2 which is bigger than the pupil of your eye (at up to about 50mm2) So it allows through at least 40x more light than your unaided pupil would and therefore has more light available to make a brighter image.
    The magnification works against this concentration of light, since as the image is made bigger, the light is spread over a larger area. If the image is 10x larger, then its area is 100x greater. So with the numbers above, you actually get a dimmer image with the binoculars 40% as bright as with your unaided eye. However I took a maximum value for your pupil and a more realistic area for your pupil area might be 5mm diameter, about 20 mm2, meaning the 50 mm diameter binocular objective would collect about 100x more light than your pupil and, after allowing for magnification, the image would have the same brightness.
    There is yet another link in the chain known as the exit pupil of the telescope, which represents the area of pupil needed to accept all the light leaving it. This exit pupil is calculated by dividing the objective diameter by the magnification, so the 7 x 50 binoculars have an exit pupil of about 7mm. All the light from the binocular will enter your eye only if your pupil is at least 7mm diameter, otherwise some will be blocked.
    After all that the small amounts lost by reflections at lens (and prism) surfaces probably don't make much difference to the image brightness, though they may affect the quality by lowering contrast and maybe producing spurious image effects.

    Now I'm no expert on this and had to do a lot of checking before replying. I find the conclusions difficult to swallow, because I feel I do see things more brightly through my 10x50 binoculars or my 70x112 telescope even though their calculated brightnesses are only around 100%.
    I wonder whether this is a property of the retina & brain, whereby 100x as much light spread over 100x the area is more detectable, even though physically it is the same brightness? (That is my speculation.)
    Or maybe because we are talking about point sources (at least, I am, talking about stars) magnification of a point does not produce a bigger point: the light entering my unaided eye gets spread over an Airy disc and the light from the telescope is still imaged on my retina to a similarly sized Airy disc - only the spaces between the stars being magnified? (Also speculation. Only just thought of that one!)

    Edit. PS I'm sure we have astronomers on PF. I hope one of them will shed a bit of light on this.
     
  22. Feb 21, 2016 #21

    sophiecentaur

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    Try the astronomy forum. People don't always stray out of their favourite playgrounds.
     
  23. Feb 21, 2016 #22
    My question is this. Since the objective lens collect 40 times more light and focus this to a point. Why doesn't our retina detect this single point as 40 times brighter.. or does it? Because it seems our eyes detect the brightness as the same as the original view. But there is 40 times more brightness. Note I'm talking of a single angle.. so you can't reason as magnification got bigger it spreads to larger area.. this is for total of all angles.. i'm talking of just one angle.. one ray and one focus that is 40 times brighter and hitting one retina...
     
  24. Feb 21, 2016 #23

    Merlin3189

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    You may well have a point as far as a single point is concerned. That was what I just bumped into in my last paragraph.
    But we have to be careful here! if a point is really a point, something infinitely small, then how can we have any light from it at all?! Say the object emits so much light per square metre, how much light does it emit from a point of zero diameter and area 0 m2?
    On the other hand, if the point does have a finite size, however small, then the light it emits is focused to another point of finite size, determined by the magnification of the lens system. So the light may be less or more bright per unit area.

    The point I was thinking of in my final comment, was that for a very tiny source like a star light years away, the light is focused to a point which should be very small, but is limited by diffraction to a size much bigger than it should be. When this image is magnified the image point should still be very small and is still limited by diffraction to a similar size. In that case the greater amount of light collected by the telescope is indeed focussed to the same area that the smaller amount collected by our eye would be. Then the image is brighter.

    When we abandon very tiny sources like distant stars and start looking at a leaf on a tree for example, we now get images whose sizes are determined by the magnification of the lenses, not just diffraction. There will be a total amount of light from a given area of the object and this will be concentrated or spread over the area of the corresponding area of the image
     
  25. Feb 21, 2016 #24
    Ok. Supposed there was a black screen with a white patch in the middle which sophie suggest to use as initial example. Or supposed let's use the example of black screen with a source of blue light at the center that span 7 degrees. Using the 8X binocular, the eyepiece (apparent) field of view would be 7x8=56 degrees. So this means the blue light is indeed magnified to your eyes (meaning your retina receive more blue light overall) using the binocular versus just staring at the background using naked eye?
     
  26. Feb 21, 2016 #25

    sophiecentaur

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    But, as Merlin has been saying (and me, also) there is no such thing as a single point when you are dealing with power flow. An object consists of infinitesimal elements, each of which produces an infinitesimal amount of energy - that is an energy flux density. In the case of astronomy, all the star images are the size of the diffraction pattern and they are still not absolute points, as far as the energy coming out of your binos or into your eye. A cm2 or a 1° subtended diameter of image will send light in a cone that fills your pupil and produce a given signal level from each of a number of retinal cells over an area of your retina. Your binos have a larger objective lens and will collect more light but that light is spread over a bigger area of retina (a magnified image). If all the light entering the objective got into your pupil then the increase in brightness would be in the ratio of the areas of lens and pupil (but, as Merlin says) it doesn't all get there. IF it did, you would find only one viewing position where you could actually see anything. So you have some proportion of all the gathered light entering your pupil (making things brighter) but it forms a bigger image so the energy flux density is lower so it won't be as much brighter than you might have assumed.
    I guess the design of binos and spotting telescopes gives a is based on the fact that most scenes are bright enough to allow you to see things and the exit aperture is made big enough for comfortable viewing. That could account for Merlin's experience with his equipment. I would imagine that telescope systems designed for photographing faint astronomical objects may be arranged differently, to get the image as bright as possible at the expense of needing to get the camera alligned more precisely (? Could that be right, Merlin?)
     
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