# Photographing a light source: why does it not make whole image equally bright?

1. Nov 10, 2012

### tris_d

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The above image gets brightest in the middle, but photons get equally emitted in all the directions, so what is it that keeps them form making the whole image equally bright? Is it angle of impact? Could it be the rate by which photons hit parts of the image further from the middle is somehow slower? Maybe dispersion, absorption along longer distance? What else could it be?

2. Nov 10, 2012

### Staff: Mentor

Is there an optical system here, or are you just shining light onto a piece of paper? I assume there is one, but I want to be sure.

3. Nov 10, 2012

### Staff: Mentor

If you're using a camera, what kind of camera is it? How are you focusing it? Low-quality cameras often have the image brightness decrease with distance from the center. This is called "vignetting."

4. Nov 10, 2012

### tris_d

Shining light onto a piece of paper. Where the light is not directional like flashlight, but is some kind of point source that shines light in all direction. I am asking this in relation to stars we are talking about in another thread.

5. Nov 10, 2012

### tris_d

I was thinking the simplest case scenario (shining light onto a piece of paper), but if there are any other peculiarities like the one you just mentioned, especially in relation to photographing stars, then please tell me about it.

Last edited: Nov 10, 2012
6. Nov 10, 2012

### Staff: Mentor

Ah. When someone says "photograph," many people tend to think a camera is involved.

The intensity of light from a point source decreases with the square of the distance. Assuming the source is opposite the center of the paper, how does the distance from the source, at the edges of the paper, compare with the distance at the center?

7. Nov 10, 2012

### tris_d

The distance towards the edges is longer. And if I imagine light source as a shower I can see how water drops would spread out more over longer distance, but it's hard to imagine light can become sparse like that. It seems that would indicate photons do not get emitted from the same point on the source one right after another forming continuous rays, but rather get spit out from the same point in the same direction only every now and then.

8. Nov 10, 2012

### sophiecentaur

You seem to have explained well enough (although with no Maths) why you wouldn't expect the illumination to be uniform.
You only have to think of a (near) infinite sheet beneath a point source to see why illumination could not be the same right out to the edges. The only time you would get uniform illumination ( from an isotropic source) would be if it were at the centre of a spherical screen. The distance in every direction would be the same.

9. Nov 10, 2012

### Staff: Mentor

Perhaps it would be better to think of light as a classical EM wave and ignore photons for the moment. I guarantee you that light is not emitted as little particles that then travel in straight lines. The reality is far more complicated.

10. Nov 10, 2012

### tris_d

Ok. Let me put it this way: why are not rays infinitely dense? If you look at the picture below at any two rays, why are there not infinitely many rays between them? Is it due to Planck scale, or something like that?

11. Nov 10, 2012

### Staff: Mentor

Because each emitter can only output a finite amount of radiation. As the wavefront expands the energy is spread out more and more. The picture merely illustrates this using a few rays, as drawing hundreds or thousands wouldn't be possible. If you want to represent each ray as being one photon, then any real source would not put out an infinite number of photons.

12. Nov 10, 2012

### Staff: Mentor

Light rays are a conceptual device, not physical objects. The intensity of the light falling on a surface (typically measured in watts per square meter) is proportional to the number of rays per unit area at that surface, but you're free to make the proportionality constant whatever you like, so long as you're consistent about it.

13. Nov 10, 2012

### Staff: Mentor

For clarity's sake, please note that the "pieces of paper" in this image are not flat.

14. Nov 10, 2012

### tris_d

OK, great. Thank you all. So the light coming from some star, why does it not spread out and produce blurred blob on a photo? They kind of appear "sharp", like it's directional light coming from a flash light where all the rays are parallel.

15. Nov 10, 2012

### Staff: Mentor

Optics! Lenses and mirrors are curved and focus the light down to a point. The size of this point varies with the wavelength of the light and the diameter of the aperture.

http://en.wikipedia.org/wiki/Airy_disk
http://en.wikipedia.org/wiki/Lens_(optics)

Little known fact: The apparent size of a star in a photograph has absolutely no relation to the actual size of the star. Brighter stars make a bigger, brighter airy disk than smaller stars. Of course this only applies to stars other than the Sun, as far away stars can be considered "point sources". This means that the actual disk of the star is far smaller than the size of the airy disk.

16. Nov 19, 2012

### fargoth

Yes, unless you use a really big lens \ mirror, and then you do get an image, and not just an airy disc (although, in real life this would apply to distinguishing between galaxies, stars would probably still be limited to airy disc size unless you got some VERY VERY big telescope) .
But another little known fact is that lots of the "stars" we see looking at the sky are actually galaxies :)
And yet another nice photography fact - if you want cool shaped stars, use a none circular aperture stop, the shape of the star is the square of the Fourier of the aperture.
So a rectangular aperture would look like a cross, and a pentagonal aperture would produce a star shaped star :)

17. Nov 19, 2012

### Staff: Mentor

Well, not with our eyes. Only about 2-3 galaxies are visible to the naked eye, along with another couple of dwarf galaxies.