Aperture and Field of View

[Dalo]

This is a purely technical matter. I have consulted a few textbooks on Optics and Photography, but found no clear answer. I hope someone in this forum will be able to help.

As anybody who has used a camera will know, you can regulate the amount of light that falls through a lens in three different ways:

- the sensitivity of the film (ISO-norm) or the sensors (digital cameras);

- by a longer or shorter exposure (shutter speed);

- a wider or smaller aperture.

My problem concerns the last method.

Obviously, the larger the aperture the more light will fall on the film or sensors.

What I do not understand is how the field of view can remain unaffected. Thinking of the way light rays are manipulated by the lens, the only explanation that makes sense to me is that the diaphragm must be placed at, or very close to, the focal point. This way, however small the opening or aperture, the view remains unchanged. 

Does it make sense?

Once again, I am looking for a technical answer, that is, the way(s) it has been solved by manufacturers since cameras exist. It must be common knowledge not protected by patents because it has always existed. But no clear answers can be found in textbooks, only vague allusions about the general location of the diaphragm.

 

 

[Strange]

The image is formed by rays passing through all parts of the lens. From each point of the scene, rays will pass through each bit of the lens and arrive at the same point of the film (assuming the image is focussed!) So, if some of the lens is covered, the mage is still formed in the same way, but just with fewer rays (less light).

The field of view is determined solely by the focal length and the size of the film (or sensor, nowadays).

Here is a simple diagram.

Person being photographed on the left, lens in the middle, film on the right. Dotted lines are some example rays from the object to the film. To avoid confusing things, I have just shown how the rays from one point get from the person to the lens. There will, of course, be rays from every point to the corresponding position on the film.

Lets add a diaphragm:

You can see that some of the paths for the light are block but that doesn't change where light comes from. Just the amount.

 

I assume this a complete waste of time though. As you don't seem to understand the first thing about how light, lenses or sight works in the first place.

35 minutes ago, Dalo said:

Thinking of the way light rays are manipulated by the lens, the only explanation that makes sense to me is that the diaphragm must be placed at, or very close to, the focal point.

The diaphragm is normally placed as close to the lens as possible. In fact, because real lenses are complex compound lenses, the diaphragm is usually inside between the components of the lens.

[Dalo]

Your explanation certainly makes sense. Still, as I said, I consider my question purely as a technical matter, and I would like more than a theoretical answer. I would be very interested for instance in the design of an existing objective, however old. I am not interested in classified or proprietary information and I wouldn't expect anyone of providing such anyway.

[Strange]

11 minutes ago, Dalo said:

Still, as I said, I consider my question purely as a technical matter, and I would like more than a theoretical answer.

If my answer "make sense" what else are you looking for? You have a habit of not asking what your really want to know. Are you looking for the details of how a camera lens with multiple elements works: what each element is there for, etc? Like a more detailed version of this:

This one shows the position of the diaphragm

Edited November 27, 2017 by Strange

[Dalo]

3 minutes ago, Strange said:

Are you looking for the details of how a camera lens with multiple elements works: what each element is there for, etc?

Not really. In fact, the simpler the better. Something like the diagram in your first reply, the one with a diaphragm.

The EF 200-400mm is also very interesting. I have seen many diagrams like this in my search, and the more complex they are the less information they give me.

What I really want to know is the location of the (last) focal point relative to the diaphragm. That will answer my question in a practical and definitive manner of how aperture and field of view relate. I already know the theory, what I would like is to see how it is put in practice.

[Strange]

Just now, Dalo said:

What I really want to know is the location of the (last) focal point relative to the diaphragm. That will answer my question in a practical and definitive manner of how aperture and field of view relate.

Except the question doesn't make sense. What do you mean by "(last) focal point". For an object that is in focus, the focal point (more realistically, focal plane) is on the film.

2 minutes ago, Dalo said:

I already know the theory

Of course you do. Bless.

[studiot]

12 minutes ago, Strange said:

If my answer "make sense" what else are you looking for?

 

Yes indeed.

Dalo,

What do you mean by "field of view"?

 

Do you mean depth of field as Strange talks about?

Then you should google "Hyperfocal  distance and perhaps add the word charts or calculator

This determines how much of the scene is in focus.

Alternatively do you mean angle of view?

 

This is set by the focal length and determines how much of the scene the lens 'sees'.

 

BTW, irrelevant here,  the sensitivity of the film or electronic sensor does not affect the amount of light falling. Only the lens system through the shutter and aperture can do this.
The sensitivity affects the response of the sensor to a given quantity of light.

 

 

[Dalo]

1 minute ago, Strange said:

Except the question doesn't make sense. What do you mean by "(last) focal point". For an object that is in focus, the focal point (more realistically, focal plane) is on the film.

The focal point is defined as the point where all rays coming out of a lens cross each other and invert the image. It is usually denoted by f. The image, when it is real, is formed usually after the focal point. But then you already knew that.

[Strange]

18 minutes ago, studiot said:

Do you mean depth of field as Strange talks about?

Actually, I haven't mentioned depth of field. That is more complicated to understand.

18 minutes ago, Dalo said:

The focal point is defined as the point where all rays coming out of a lens cross each other and invert the image. It is usually denoted by f. The image, when it is real, is formed usually after the focal point. But then you already knew that

You are right. And that is also the position of the diaphragm.

 

[Dalo]

11 minutes ago, Strange said:

You are right. And that is also the position of the diaphragm.

Then I do not see why you find my question strange. The farther from the focal point, the more spread out the rays will be, which means that the position of the diaphragm is fundamental. As you said yourself, it is usually placed near the lens, but how near? Too near and many more rays will be blocked than when the diaphragm is placed a little further, when the rays have converged more, taking less space. Very close to the focal point would be more efficient. It would let more rays through even at small aperture. And so on.

But once again, I want to avoid a theoretical discussion, however interesting, and ask for practical information regarding the position of the diaphragm in an existing design.

edit: It would be in fact quite simple to emulate a simple setup. There are variable diaphragms in the market, and optical sets which would make it possible to do such experiments. The problem is their price which I cannot afford.

[Strange]

18 minutes ago, Dalo said:

But once again, I want to avoid a theoretical discussion, however interesting, and ask for practical information regarding the position of the diaphragm in an existing design.

Is this "too theoretical" because it is not for a specific lens:

 

How about this description of a specific Canon lens:

http://www.exclusivearchitecture.com/?p=853

This is the source of one of the images I posted earlier so I assume it is somehow "too theoretical" (whatever that means - I assume it is just an excuse to drag the thread out and ask increasingly stupid questions until either you or the moderators get bored).

18 minutes ago, Dalo said:

There are variable diaphragms in the market, and optical sets which would make it possible to do such experiments. The problem is their price which I cannot afford.

You might be able to buy broken lenses cheaply. Then you could take them apart to find out where the diaphragm is. Or just go to a camera shop and look at the lens. You can often see the diaphragm near the front of the lens:

 

Edited November 28, 2017 by Strange

[Dalo]

3 minutes ago, Strange said:

Is this "too theoretical" because it is not for a specific lens

I think you can answer the question yourself. Can you indicate where the image will fall after the diaphragm, and which part of it will be blocked? Also, where is the focal point after the diaphragm, relative to the image.

That is what I mean by practical information, and that is why my preference goes to simple designs, or even  an optical setup if someone happens to have the optical gear in his lab.

Still, I really appreciate your efforts.

[Strange]

Just now, Dalo said:

Can you indicate where the image will fall after the diaphragm

The diaphragm makes no difference to that.

Quote

 and which part of it will be blocked

None of the image will be blocked.

 

[Dalo]

1 minute ago, Strange said:

The diaphragm makes no difference to that.

None of the image will be blocked.

 

I really do not want to start a discussion which will only frustrate both of us. 

I prefer to wait for someone with, once again, practical information, or lab gear making it possible to answer my questions. Maybe then it will become apparent that I have asked the wrong thing and should approach the problem differently. But I will only know that in a practical situation.

I thank you once again and advise you not to waste your time.

[CharonY]

The OP has a number of misconceptions to start with. Obviously iso and shutter speed have of no influence on how much light passes through a lens. It only affects image exposure due to increased sensitivity of the detector (or film) or by collecting light for a longer time. 

What is not clear to me is what you mean specifically with field of view. If you mean the angular extent of an view imaged on a sensor. In photography, this is  depends on the focal length of the lens and the size of the sensor/film. If you think that aperture should influence that, I'd like a more precise explanation why you think so.

What the aperture does influence is the depth of focus, which is related to the circle of confusion. It would help if you could specify what you mean precisely and where you are getting confused.

[Dalo]

The simplest setup would look like this on an optic rail, from left to right:
1) a laser ray box (at least 3, preferably 5 beams or more)
2) a convex lens
3) a variable diaphragm
4) a screen

The first objective would be to position the screen, and/or the other elements, in such a way as to get a sharp image of all beams.

a) Determination of the position of the focal length (where all or most of the rays meet before they diverge again) relative to the diaphragm. (different from the focal plane on the screen)
b) Effects of different positions of the diaphragm on the image.
c) Effects of different apertures on the image.

Expected result: the number of beams reaching the screen will depend on the aperture and position of the diaphragm.

That might seem obvious until we compare it to what would happen if we used a single light source and a slide. In such a case we would expect the field of view to remain unchanged and only the brightness of the image to be affected.
 

edit: I am ignoring the depth of field in all this discussion.

edit 2: by putting gray filters in front of the beams we would decrease their intensity and make it possible for us to get a picture of the lamps themselves, as we would of the sun. My prediction is that the number of bright dots representing the lamps would remain unchanged, whatever the position of the diaphragm and its aperture.  In contrast with the number of beams that would still be able to reach the screen.

Edited November 28, 2017 by Dalo

[Strange]

33 minutes ago, Dalo said:

Expected result: the number of beams reaching the screen will depend on the aperture and position of the diaphragm.

That depends on the position at which they go through the lens. Those near the edge of the lens will obviously be blocked by a smaller aperture. This, equally, obviously, has no effect on the field of view. 

[Dalo]

4 minutes ago, Strange said:

This, equally, obviously, has no effect on the field of view. 

yes and no. It depends what you understand with field of view in different situations. I have already agreed  with the situation in which a slide is used, or only the images of the lamps are projected. You agree that some beams will be blocked. Which means that we will see, for instance, 3 instead of 5 beams. That is a kind of diminished field or angle of view, but I will not make a point of this.

More importantly, I hope someone will do the experiment and then we can discuss its interpretation.

[Strange]

3 minutes ago, Dalo said:

Which means that we will see, for instance, 3 instead of 5 beams.

You may or you may not. Depending where they go.

Quote

That is a kind of diminished field or angle of view, but I will not make a point of this.

It isn't. And I thought it was your entire point.

Quote

More importantly, I hope someone will do the experiment and then we can discuss its interpretation.

Why would they? Why not just take some photographs? Or draw a diagram? Or learn some basic optics?

[Dalo]

1 minute ago, Strange said:

Why would they? Why not just take some photographs? Or draw a diagram? Or learn some basic optics?

If nobody does it, I will just have to keep saving for the proper equipment and do it myself. It might take a while though.

[Strange]

7 minutes ago, Dalo said:

If nobody does it, I will just have to keep saving for the proper equipment and do it myself. It might take a while though.

You could buy a cheap laser pointer ($1), a cheap magnifying glass ($3) and a piece of cardboard ($0) to make different size apertures in. 

What do you hope to find? 

 

Edited November 28, 2017 by Strange

[swansont]

49 minutes ago, Dalo said:

yes and no. It depends what you understand with field of view in different situations. I have already agreed  with the situation in which a slide is used, or only the images of the lamps are projected. You agree that some beams will be blocked. Which means that we will see, for instance, 3 instead of 5 beams. That is a kind of diminished field or angle of view, but I will not make a point of this.

No, it's not. The number of rays going to a particular point on the image only affects the brightness.

I recall a talk from an AAPT conference I went to in grad school, on teaching methods in optics. Students had been asked what would happen to an image if part of the lens was blocked. The misconception that part of the image would be missing was common. All that does is make the image dimmer.  

Blocking the rays reduces the amount of light getting to the image. But light from any point on the object will will pass through any point you pick on the lens. The same concept applies to the aperture area — there will always be light going through the center of it, from all points on the object, to form the image.

Quote

More importantly, I hope someone will do the experiment and then we can discuss its interpretation.

Millions of photographers have done so. Changing the aperture does not change the field of view — that's a function of the focal length of the lens. It's why a wide-angle lens is always a wide-angle lens. It does not become a "regular" lens by changing the aperture, or vice-versa. There wouldn't be much of a point to having these different lenses if the aperture had this effect.

32 minutes ago, Strange said:

You could buy a cheap laser pointer ($1), a cheap magnifying glass ($3) and a piece of cardboard ($0) to make different size apertures in. 

Or borrow an SLR camera.

or use pictures that other have taken with different apertures

The image is the same size.

 

[Dalo]

1 minute ago, swansont said:

No, it's not. The number of rays going to a particular point on the image only affects the brightness.

I recall a talk from an AAPT conference I went to in grad school, on teaching methods in optics. Students had been asked what would happen to an image if part of the lens was blocked. The misconception that part of the image would be missing was common. All that does is make the image dimmer.  

Blocking the rays reduces the amount of light getting to the image. But light from any point on the object will will pass through any point you pick on the lens. The same concept applies to the aperture area — there will always be light going through the center of it, from all points on the object, to form the image.

Millions of photographers have done so. Changing the aperture does not change the field of view — that's a function of the focal length of the lens. It's why a wide-angle lens is always a wide-angle lens. It does not become a "regular" lens by changing the aperture, or vice-versa. There wouldn't be much of a point to having these different lenses if the aperture had this effect.

I completely agree with you as far as it concerns illuminated objects or scenes. The experiment I propose is to see whether the same principle is applicable to beams of light. Strange seems to agree that some beams would be blocked by the aperture.

Those are two different situations and it would be very interesting to see if the difference between them is real, and how we could interpret this difference. 

But first I would like to attest, in a non-amateurish way, whether there is indeed a difference.

[Strange]

4 minutes ago, Dalo said:

The experiment I propose is to see whether the same principle is applicable to beams of light.

Why wouldn't it be?

4 minutes ago, Dalo said:

Strange seems to agree that some beams would be blocked by the aperture.

That is why less light gets through.

Quote

Those are two different situations

In what way do you think they are different? Perhaps if we could understand where your confusion lies, it might be possible to explain where you are going wrong. (Although, based on past experience, there probably isn't much hope of that.)

[Dalo]

I will only react to posts concerning actual experiments, and not ideas about how or what they should look like.