n00b question on apertures

[DorsetDude]

Following on from the "Too narrow an aperture = bad?" thread has made me ponder something. I'll try and explain...

Say your camera sensor is a square of paper
Say you then have a pinhole 6" away to act as your aperture, the light source remains the same throughout.

The light rays passing through that pinhole will project a circular image on the paper.
If you widen the pinhole the circle on the paper would get larger wouldnt it?

So my question is, how come when you stop down your camera lens, the whole of the sensor still recieves a full coverage of light even thought the intensity of that light is less for smaller apertures?

I assume that it is something to do with the lens construction?

Or is my basic physics way off? Most likely!

Cheers for any enlightenment! If you can understand what Im asking!

 

[Donki]

As you stop the lens down you have to increase the shutterspeed or raise the ISO to get a good exposure.

Is that what your asking?

 

[Howi]

Following on from the "Too narrow an aperture = bad?" thread has made me ponder something. I'll try and explain...

Say your camera sensor is a square of paper
Say you then have a pinhole 6" away to act as your aperture, the light source remains the same throughout.

The light rays passing through that pinhole will project a circular image on the paper.
If you widen the pinhole the circle on the paper would get larger wouldnt it?

So my question is, how come when you stop down your camera lens, the whole of the sensor still recieves a full coverage of light even thought the intensity of that light is less for smaller apertures?

I assume that it is something to do with the lens construction?

Or is my basic physics way off? Most likely!

Cheers for any enlightenment! If you can understand what Im asking!

Logically that is what you would expect, unfortunately this is where the physics come in to play.
a pin hole, appears to act like a lens because of the way light works.
A pin hole only acts like a lens because it is a VERY small hole.
All that happens when you make the hole larger, is that you reduce the sharpness of the projected image(diffraction), but make the image brighter (because the larger hole lets in more light).
true lenses bend the light in a certain way, this is not affected by the aperture as such, only the amount of light let through, so the projected image (by the lens) will not increase in size if you vary the aperture.
This image is the field of view and is related to focal length not aperture.
This is why we have zoom lenses, we change the focal length to make something apear larger or smaller without moving our position.

Not sure whether I have done a particularly good job here, but don't want to get too technical.

 

[Suvv]

The aperture blades are inside the lens and do not affect the size of the view.

Each point in the scene reflects light and some of this reaches the lens, the thing to note is that the light from each point is spread across the whole of the lens. The lens then bends all these different rays so that they meet again at the other side of the lens - at the focus point.
If you close down the aperture you are not stopping all the light from particular points in the scene you are just stopping those that are being bent by the outer parts of the lens, rays which are nearer the centre of the lens still make it through and are focused.

This crude diag. may or may not help

Take a single point in the scene, S, and imagine all the light rays which make it to the lens from S. If you then close down the aperture some of the light rays are blocked, darkening the image, but other rays continue to the sensor / film

 

[DorsetDude]

As you stop the lens down you have to increase the shutterspeed or raise the ISO to get a good exposure.

Is that what your asking?

No. I follow all that reasonably ok.

Not sure whether I have done a particularly good job here, but don't want to get too technical.

No thats good, thanks very much.

, the thing to note is that the light from each point is spread across the whole of the lens.

Ah!

This crude diag. may or may not help

Yes, thanks very much!