Circles of Confusion - photon level - Is this ~ correct ?

[DeF02003]

HI All:

Newbie question. [ This may have been posted already but I can't find it. ]
Any thoughts, clarifications, etc are VERY welcome.

THANKS!

D

Say that I want to photograph an image of trees , grass, sky, etc. .
From the objects in this scene there are billions?? of ‘streams of photons’ emanating in many directions.
Each stream is like a movie showing me THAT photons contribution to the whole scene.
I adjust the exposure levels of my DSLR camera such that:
The foreground tree, grass etc. is out of focus.
The Depth of Field tree, grass is [ obviously ] in focus.
The background trees, grass are out of focus.
I take the picture of this scene.
Some of these streams of photons are in focus, some streams are out of focus.
The shutter screens open and close admitting 1 - n number of photons from the scene.
This ‘block’ of in and out of focus streams of photons, pass to the sensor.
For streams that are in focus:
In each individual stream, photon 1 to photon n become lined up [ focused ] one after the other.
Many photons from the same stream, impact one sensor pixel and only that pixel.
The camera has a multi-million pixel sensor.
This impact [ ‘filling in’ ] of pixels by photon streams is repeated millions of times in that camera’s sensor.
These streams build the in focus part of the exposure.
For streams that are out of focus:
Foreground out of focus streams:
These photons, headed for their focal point never reach it since the focal point is past the sensor.
Therefore, photons can be parallel/several photons wide in these, out of focus, streams.
[ Picture these streams as the cross section of a cone, not the tip of the cone. ]
This type of stream impacts in more than one pixel on the sensor
This causes a blurring of that photon [ light ] source as it is represented in the picture taken.

Background out of focus streams:
These photons do reach their focal point however, this focal point is in front of the sensor.
Passing the focal point, these photons spread out in an ever widening cone shape toward the sensor.
Photons in these out of focus streams, are also parallel/several photons wide.
As these photons impact the sensor, it’s in the shape of a cross section of a cone.
This stream of photons impact more than one pixel as well.
This causes a blurring of that photon [ light ] source as it is represented in the picture taken.
These streams of photons, build the out of focus portions of the exposure.

[Jimr1961]

If it isn't, it ought to be

[tao2]

Sometimes ye fish...other times ye dig for bait...

or...




or.....



or....







It is Friday...

[dje]

Hi David

For many things in science, models are used to explain observed behaviour and then to predict behaviour under certain circumstances. The photon concept is one of these which is useful for characterising the energy in light waves (eg sensor behaviour) but there are other models too.

Light waves are part of the electromagnetic wave spectrum and for certain things, wave behaviour is a useful approach for analysis. Also, in optics, ray tracing is another very useful technique and it is this technique which is probably most useful for things like lens behaviour.

Personally I can't see that trying to use photons to consider the issue you raise to be of much use for lens focussing but then I'm not a quantum mechanics kind of guy !

Dave

[TonyW]

David, the short answer to your question is yes, that's correct.

When talking about things like refraction, diffraction and sometimes reflection, it's easier, at least for me, to think in terms of the wave model of light rather than photons.

[george013]

I don't understand anything of it. It seems that energie and lightrays are mixed up.

George

[pnodrog]

A stream of photons is far to narrow to impact on more than one pixel. It will be other streams that reach an adjacent pixel.

[IzzieK]

A photon checks into a hotel and the bellhop asks him if he has any luggage.
The photon replies, "No, I'm travelling light."

Now that statement above is more simpler to me...Quantum physics ain't my thing...photography is.

Thanks Boab....

[JohnRostron]

As others have said, you have the basic idea right, but your use of photons and streams should be replaced by the (non-PC) rays. The factor you seem to have omitted is the aperture. The size of the circle of confusion is directly proportional to the aperture size, which is why stopping down increases the depth of field.

I managed to follow the explanation in the link from Ted, but it does assume that you are comfrtable with the algebra.

John

[Manfred M]

Interesting, but looks quite wide of the mark. Let me give you a highly simplified view of what is happening.

A light source emits photons. These photons have different energy levels, which is linked to the wavelength of light associated with them. In the visible spectrum, he lowest energy level photons are the ones we associate with red light and the ones the highest energy levels are the ones associated with violet light.

These travel and hit your subject. Depending on the property of the material they hit they can either go through it (transmission) like what happens when it hits a glass element in your lens or be reflected by it if the object is opaque. Of course, in the real world, real properties are not quite this perfect and we have all kinds of in between states (translucence).

When light hits an object we consider to be black, the light is absorbed and not reflected; a white object reflects all light. We get some objects that absorb certain wavelengths of light and reflect others. These are those photons that come back towards our camera lens. Photons are neither in nor out of focus. They really have two main properties that interest a photographer; the energy level with the associated wavelength and direction the photon is traveling in. There is no such thing as an out of focus photon (it doesn't have this particular property).

When the photon hits the lens,depending on the angle it hits it (angle of incidence) and it travels through it, the photon slows down a bit, with higher energy photon less so that a lower energy one. A photon hitting near the outer edge of the front element of the lens bends more than one hitting the dead centre of the lens.

A lens is technically only focused at the focal plane selected when you focus the lens. Anything other than this plane (which by definition is infinitely thin) is out of focus. This is where the concept of "circle of confusion" comes in. Depending on a number of factors; the level of magnification of the object, the viewing distance of the image, the size of the image, the amount of sharpness is "good enough".

This is where it gets a bit muddled and hard to put into words. A ray tracing diagram is a lot easier to understand, so please feel free to improve on my explanation language. So, for a given level of magnification, viewing distance and image size, when we shoot at a large aperture, the photons entering the lens are bent from the entirety of the front element of the lens; the outside, middle and centre. All this bending means that the photons hit the sensors having been brought in from all directions and this shows in the final image as being less distinct (more distributed?). Another artifact from shooting wide open is vignetting. It is also more apparent as light from the edges of the lens have to travel through more glass, and some of this light is absorbed.

When the lens is stopped down to a very small aperture (ignoring diffraction), comes in through the very centre of the lens only, with far less bending and far more "accurate" projection on the sensor.

Clear as mud... But depth of field has nothing to do with photons hitting a digital sensor per se. Project the lens onto film or onto an opaque surface, the differences in depth of field (if we can make them out) would be identical.

[rachel]

We physicists use the term 'rays' which is, as stated above, essentially a 'stream' of photons.

And to a physicist, focus is about whether different rays - travelling along different paths but from the same point on the object - meet at the sensor or not (ie whether they meet at the sensor or meet before or after it).

So this bit

Background out of focus streams:
These photons do reach their focal point however, this focal point is in front of the sensor.
Passing the focal point, these photons spread out in an ever widening cone shape toward the sensor.
Photons in these out of focus streams, are also parallel/several photons wide.
As these photons impact the sensor, it’s in the shape of a cross section of a cone.
This stream of photons impact more than one pixel as well.
This causes a blurring of that photon [ light ] source as it is represented in the picture taken.
These streams of photons, build the out of focus portions of the exposure.

is pretty much it, but we physicists like to keep things as simple as possible. Consider a spherical cow.

[george013]

We physicists use the term 'rays' which is, as stated above, essentially a 'stream' of photons.

And to a physicist, focus is about whether different rays - travelling along different paths but from the same point on the object - meet at the sensor or not (ie whether they meet at the sensor or meet before or after it).

So this bit


is pretty much it, but we physicists like to keep things as simple as possible. Consider a spherical cow.

Just some thoughts of me.

Physical and optical oproaches are different.

A foton is a symbol/quantum of energie. You can't use it for a focus statement. Only for exposure.

A light ray can't be in focus or out of focus. It's just there. A point can be in focus when light rays travelling from a point through a lens are coming together again in that image point. It's only in focus for that point.

There is no connection between aperture diameter and difraction. Difraction depends on the top angle of the light cone made by aperture diamater and image distance.



George

[xpatUSA]

There is no connection between aperture diameter and diffraction.

George

Astounding, George, unless I misunderstand the meaning?

No connection here, then?:

https://en.wikipedia.org/wiki/Diffra...cular_aperture

Or here?:

https://en.wikipedia.org/wiki/Airy_disk#Size
.

[george013]

Astounding, George, unless I misunderstand the meaning?

No connection here, then:

https://en.wikipedia.org/wiki/Diffra...cular_aperture

Or here:

https://en.wikipedia.org/wiki/Airy_disk#Size
.

It would be nice if you quote right.

There is no connection between aperture diameter and difraction. Difraction depends on the top angle of the light cone made by aperture diamater and image distance.

To make it easy for you, I marked the second sentence.
And read your own links. It's all in there.

George

[DeF02003]

If it isn't, it ought to be

HI Jim:

Thanks for responding. I'm that much more confident in my camera education.

D

[DeF02003]

Hi All:

My above post to Jim was when I thought to respond to y'all individually. But, lucky me, there were many respondents so a group thanks to you all!!!

Before writing to this forum, I DID spend hours on the web researching. But, after picking up shreds of information here and there plus misstatements, etc. I 'connected the dots' as best I could and only then wrote to you.
Your responses have given me more confidence in my understanding BUT, they have created more questions as well.
Ain't it the way ?
So, back to researching points and articles you have sent to me.

Y'all have been gracious in sharing your hard won knowledge.

Thank you.

D

[xpatUSA]

It would be nice if you quote right.


To make it easy for you, I marked the second sentence.
And read your own links. It's all in there.

George

I did not understand the second sentence, sorry. My comment was specifically about the first sentence.

So, George, are you claiming that your first sentence "There is no connection between aperture diameter and difraction" is correct?

If you are, I can assure you that absolutely nobody here will agree with you.

Please try very, very hard to: a) just answer my question, and b) answer it with a simple "yes" or "no". That way, there will no further misunderstandings.