Aperture and Field of View

[Strange]

16 minutes ago, Dalo said:

when using the filter the beams are not visible anymore, only the light sources.

What is the difference between a beam being visible and the light source being visible? The light source is only visible because there is a beam of light from it. 

And why would the presence of a filter change whether the beam was visible? It could only make it dimmer.

(Are we back to polarisation, by any chance?)

16 minutes ago, Dalo said:

That does not mean that an empirical confirmation would be superfluous.

Indeed. Feel free to do that.

23 minutes ago, Dalo said:

when using the filter the beams are not visible anymore, only the light sources.

Just in case (and feel free to be offended by this) ... you do realise that the reason you can see the beams from the lasers in the video you posted is because they are shining on (and reflected from) the white surface that the experiment is being done on? You can't see laser beams in the air. But you knew that, of course...

Edited November 30, 2017 by Strange

[Dalo]

27 minutes ago, Klaynos said:

Are you suggesting that the filter completely removed the lasers so they are not dominating the response of the sensor and then you are illuminating the scene with some other light (e.g. room lights) and can image the front of the laser sources?

No, when you use a gray filter the beams are mostly absorbed and are not visible anymore. Just like when you take a picture of the sun using a so-called sun-filter

Edited November 30, 2017 by Dalo

[Strange]

4 minutes ago, Dalo said:

No, when you use a gray filter the beams are mostly absorbed and are not visible anymore. Just like when you take a picture of the sun using a so-called sun-filter

The beams will be partly absorbed and hence dimmer, and so will the image formed from them. It won't change the path the beams take and so they will still be blocked by the diaphragm.

An image of the sun with or without a filter will be identical other than brightness. The filter is only used to reduce the brightness (so the film doesn't catch fire!)

BTW that is a very odd picture of the Sun. Where did you get it from?

[Dalo]

30 minutes ago, Strange said:

Just in case (and feel free to be offended by this) ... you do realise that the reason you can see the beams from the lasers in the video you posted is because they are shining on (and reflected from) the white surface that the experiment is being done on? You can't see laser beams in the air. But you knew that, of course...

yes I did. But what I mean goes beyond that: the beams are not projected on the screen anymore. Just like in the case of the picture of the sun with a filter. The sky is dark, and only the sun is visible.

[Strange]

Just now, Dalo said:

Just like in the case of the picture of the sun with a filter. The sky is dark, and only the sun is visible.

OK. Maybe we are getting somewhere now. This relates to the difference between your use of 5 lasers and photographing a general scene.

So you are comparing the loss of the background with the addition of the filter, to the loss of some of the lasers? 

This is not a valid comparison.

In the case of the Sun you have one bright source surrounded by an illuminated sky. Both the Sun and the sky radiate light in all directions. (Unlike the lasers.) The Sun is many times brighter than the sky (a few million times brighter, I think). This means that if you put a filter in the way, the sky will rapidly drop below the level of detectability, while the Sun is still visible. (This would be equally true if you achieved the same exposure by using a very short exposure time and a very narrow aperture - it is not related to the filter itself, just the relative brightness of the Sun and the background.)

In the case of your 5 lasers, there is almost no other source (I am assuming we are doing the experiment in the dark, as in the video you posted). Therefore the only beams are the (unidirectional) beams from each laser, creating the images of those sources. The brightness of these will all be changed equally by the filter. So there is nothing that will disappear or appear. It s just the brightness of the image that will change.

[Dalo]

29 minutes ago, Strange said:

OK. Maybe we are getting somewhere now. This relates to the difference between your use of 5 lasers and photographing a general scene.

So you are comparing the loss of the background with the addition of the filter, to the loss of some of the lasers? 

This is not a valid comparison.

In the case of the Sun you have one bright source surrounded by an illuminated sky. Both the Sun and the sky radiate light in all directions. (Unlike the lasers.) The Sun is many times brighter than the sky (a few million times brighter, I think). This means that if you put a filter in the way, the sky will rapidly drop below the level of detectability, while the Sun is still visible. (This would be equally true if you achieved the same exposure by using a very short exposure time and a very narrow aperture - it is not related to the filter itself, just the relative brightness of the Sun and the background.)

In the case of your 5 lasers, there is almost no other source (I am assuming we are doing the experiment in the dark, as in the video you posted). Therefore the only beams are the (unidirectional) beams from each laser, creating the images of those sources. The brightness of these will all be changed equally by the filter. So there is nothing that will disappear or appear. It s just the brightness of the image that will change.

This is more of an analogy. And the difference is what makes the experiment meaningful in my eyes.

By reducing the intensity of the light, the laser lamp (the light source) itself becomes visible, just like the sun through a filter. Remember that when I used the term mini suns for the first time, I assumed that they propagated light in only one direction. This is I think the fifth or sixth time that you, and others, have tried to remind me that light is propagated in all directions, which is an undeniable fact.

That is why I am using unidirectional lasers (or lamp with slits).

Please stop repeating the same objections over and over again. It looked like my setup was finally clear to you. It is obvious that you do not agree with my prediction. You think an experiment is superfluous. What more could you possibly say that would make me change my mind?

I will tell you, hoping that it will all save us time and energy, and protect us from frustration:

only an experiment that would prove me wrong will convince me. 

There is no theory that, at least in principle, could not be falsified by an empirical experiment, how trivial it may seem.

Edited November 30, 2017 by Dalo

[Strange]

2 minutes ago, Dalo said:

By reducing the intensity of the light, the laser lamp (the light source) itself becomes visible, just like the sun through a filter.

Both the laser and the Sun are always visible. The filter just makes them dimmer. 

3 minutes ago, Dalo said:

only an experiment that would prove me wrong will convince me. 

Great. Let us know the results.

[Dalo]

Taken in the afternoon between 3 and 4 with filter cap normally used with a telescope to observe the sun. As you can see, the sky is completely dark, all the sun rays having been absorbed, leaving only the sun itself.

Edited December 1, 2017 by Dalo

[Strange]

Just now, Dalo said:

Taken in the afternoon between 3 and 4 with filter cap normally used with a telescope to observe the sun. As you can see, the sky is completely dark, all the sun rays having been absorbed, leaving on the sun itself.

The "sky" rays have been (partially) absorbed, so the sky appears black. (Because of the dynamic range of the sensor.)

The sun rays have also been (partially) absorbed, to exactly the same extent, so the sun is darker than it would have been without the filter.

If "all the sun rays" had been absorbed then you wouldn't be able to see the sun, would you?

If you took the same picture without the filter, but with the exposure adjusted to get the same brightness image of the Sun, then the sky would still appear black. (Because of the dynamic range of the sensor.)  --- But do not do this as you are likely to damage the camera.

[swansont]

21 hours ago, Dalo said:

I do not see why

Changing two things at once means you don't know what caused the difference in the result.

21 hours ago, Dalo said:

edit: if you still think that a filter should have no effect on the number of sources projected on the screen, with the condition as stated concerning the position of the filter (before the beams enter the lens), then you prediction is exactly the opposite of mine. You may be already convinced that you are right, I would like to see it confirmed by an experiment.

When you put on a pair of sunglasses, things don't disappear.

[Dalo]

My point is: when beams are blocked, we get to see the light source, here the sun, in my experiment the (laser) lamps. But please let's not restart the discussion. The authenticity of my previous picture of the sun had raised doubts which I could not take away, not having noted the conditions under which it was taken. So, today, I took another picture of the sun with the same filter.

1 minute ago, swansont said:

When you put on a pair of sunglasses, things don't disappear.

I am sad to say that I believe you.

Edited December 1, 2017 by Dalo

[Strange]

1 minute ago, Dalo said:

My point is: when beams are blocked, we get to see the light source, here the sun, in my experiment the (laser) lamps.

You can always see the light source. It just becomes dimmer (and safer in the case of lasers or the sun) with a filter. But the filter doesn't make it visible. 

And if you block all the rays, then you can't see the light source at all. This is why you can't see then sun when you are down a coal mine. Or at night.

[Dalo]

1 minute ago, Strange said:

And if you block all the rays, then you can't see the light source at all. This is why you can't see then sun when you are down a coal mine. Or at night.

wrong arguments since in those cases the sun is not in the line of sight as the lamps would be.

[Strange]

Just now, Dalo said:

wrong arguments since in those cases the sun is not in the line of sight as the lamps would be.

It would be if you look towards it. But, fair enough. I was just trying to draw a distinction between a filter that just absorbs some rays (so you can still see the source, just as you can without the filter) and something that absorbs all the rays (so you can no longer see the source).

[Klaynos]

21 hours ago, pzkpfw said:

This whole 5 laser thing still seems like a pointless diversion, as I think Dalo has misunderstood post #2 (which he seemed to accept).

Dalo, the diagram showed how light from one point on the source travels multiple paths to the lens. So the diaphram blocks some of the light from points on the source, but not all. Note how in post #2 the diaphram does not cut off the head of the stick figure; just some of the light from the head. I've slightly expanded the diagram, possibly wasting my time.

Your five lasers do not replicate what happens for general photo taking, as we don't get single sources of light from the source we are photographing. e.g. there won't be a single ray of light from the head of the stick figure to the lens.

 

Dalo, what don't you understand about the quoted post and image?

[Strange]

58 minutes ago, swansont said:

When you put on a pair of sunglasses, things don't disappear.

Neither do they appear!

Dalo, which parts of the experiment are important? Could we do without the lens and the sensor/film? Could we reduce the setup to two lasers and a partial obstruction:

lasers     aperture      filter
              |
O------------ |             :
              |             :              (> observer
O---------------------------:-------
                            :
                            :

Will the observer see one or two light sources? Will this change if the filter is removed?

[pzkpfw]

I had a moment, Dalo, so took your diagram to the next logical step - diaphragm closed a bit.

Please explain what your filter does to make how I've shown the blue rays wrong. Even better, draw it yourself.

(This is basically the same thing Strange asks above.)

 

[studiot]

On ‎11‎/‎30‎/‎2017 at 8:34 PM, Dalo said:

When the diaphragm is completely open, five beams pass through the aperture, three when it is closed.

My prediction is, when the (neutral) filter is used, five light sources will be projected on the screen, even if the aperture would only let three beams through.

I am not a talented artist, and the way the beams are refracted is certainly not realistic.

 

 

Do not be modest about your diagram,. It is not only better than no diagram, it is perfectly fit for purpose ie adequate.

 

So +1 

[John Cuthber]

Why isn't the screen drawn at the focus?

[studiot]

1 hour ago, John Cuthber said:

Why isn't the screen drawn at the focus?

Because Dalo want's to display an image on the screen, not burn a hole in it ??

 

[Dalo]

4 hours ago, pzkpfw said:

I had a moment, Dalo, so took your diagram to the next logical step - diaphragm closed a bit.

Please explain what your filter does to make how I've shown the blue rays wrong. Even better, draw it yourself.

(This is basically the same thing Strange asks above.)

 

You agree with Strange that only three light source would be visible, just like only three beams pass the diaphragm. You probably also would find an empirical confirmation superfluous. I do not. For arguments see the whole discussion with Strange which I thought was finished.

1 hour ago, studiot said:

Because Dalo want's to display an image on the screen, not burn a hole in it ??

 

+1

1) Take my diagram but this time it is not important whether the filter is present or not.

2) Let us turn all lasers off.

3) I suppose that everybody will agree with me that, even if the diaphragm is put at the same position as in in my diagram, all five lamps will be visible from the screen position, whatever the aperture?

4) Now turn all lasers off. Only three beams will make it through and two will be blocked if the diaphragm is closed.

Let me know where I went wrong.

Edited December 2, 2017 by Dalo

[pzkpfw]

58 minutes ago, Dalo said:

For arguments see the whole discussion with Strange which I thought was finished.

Your argument never made sense. It amounted to "A, then magic, then C". Please look at the diagram, and tell me why what I drew for the blue lines would be wrong.

58 minutes ago, Dalo said:

1) Take my diagram but this time it is not important whether the filter is present or not.

2) Let us turn all lasers off.

3) I suppose that everybody will agree with me that, even if the diaphragm is put at the same position as in in my diagram, all five lamps will be visible from the screen position, whatever the aperture?

4) Now turn all lasers off. Only three beams will make it through and two will be blocked if the diaphragm is closed.

Let me know where I went wrong.

Assuming you mean "on" in step 2).

3) No! The aperture size will affect which beams go through! (Note, we're talking lasers here, not simply an image as in the diagram shown in post #2).

3) edit: also now "lamps"? Eh? Lasers or lamps? Which is it? See comment at bottom ...

4) What? With the lasers off, what beams are there?

I think maybe in all of these posts, you've got some extra information about the scenario that you are assuming but not actually mentioning.

Edited December 2, 2017 by pzkpfw

[Dalo]

13 minutes ago, pzkpfw said:

Assuming you mean "on" in step 2).

nope. I do mean off.

but in step 4 it should be on.

Edited December 2, 2017 by Dalo

[pzkpfw]

5 minutes ago, Dalo said:

nope. I do mean off.

Weird. Why then do you say turn off the lasers in step 4? If they are already off?

In any case, at step 3, if the overall scene is lit by some other source, then the physical laser devices will be visible. Just like the head and feet of the stick figure in post #2. (Because you're not talking about unidirectional beams of laser light, you're just talking about a normally lit scene that happens to contain 5 devices, that happen to be laser light emitters).

But then, in step 4 (off lasers, still off), what beams are you talking about?

 

Edited December 2, 2017 by pzkpfw

[Dalo]

6 minutes ago, pzkpfw said:

Weird. Why then do you say turn off the lasers in step 4? If they are already off?

In any case, at step 3, if the overall scene is lit by some other source, then the physical laser devices will be visible. Just like the head and feet of the stick figure in post #2. (Because you're not talking about unidirectional beams of laser light, you're just talking about a normally lit scene that happens to contain 5 devices, that happen to be laser light emitters).

But then, in step 4 (off lasers, still off), what beams are you talking about?

 

you are right of course. It was a typo in step 4.