Dumb question - how does a variable aperture reduce the amount of light without vignetting

[sirch]

I know it seems like common sense that a smaller hole lets less light through, and indeed if you partly draw the curtains in a room the room gets darker. BUT to avoid vignetting the aperture diaphragm is placed somewhere in the middle of the lens and from my naive reading of the diagrams this is the point where the rays of light converge to a point (or close to) so opening up the aperture has no effect because the light is focussed down anyway.

Obviously aperture diaphragms work, but how without significant vignetting? A diagram may help!

 

[Kodiak Qc]

A diagram may help!

Not really, I think you're well over that.

You know that the image coming in the lens
is reversed at a precise point. It is at this point
that the diaphragm is positioned; affecting the
amount of light coming in but not the image.

 

[sirch]

Not really, I think you're well over that.

You know that the image coming in the lens
is reversed at a precise point. It is at this point
that the diaphragm is positioned; affecting the
amount of light coming in but not the image.

But at that place in the optics all the light comes to a point so the aperture would need to be tiny to make a difference and would presumably turn every camera into a pin hole camera

 

[kendo1]

Err, no.

 

[sk66]

Vignetting is not caused by the aperture blades. Vignetting is caused by a wide aperture allowing restrictions at the edges (lens ring/barrel) to show in the image.

Basically, every point on the objective lens projects a complete image as seen from it's perspective (i.e. lens diameter does not limit FOV). And what a larger aperture setting does is "stack" more exposures/images taken from more locations on the objective lens. And if/when the edges of the lens/barrel are included, you get vignetting.

EDIT: This is why adding filters can cause vignetting... they extend the lens ring restricting the outer field of view for parts of the objective lens near the edges. The "fix" is to stop down to a smaller aperture so those portions of the objective lens are not included in the image.

The aperture is placed some distance from the objective element because that affects it's effective diameter (the size of the entrance pupil) which is what actually limits the amount of light. It is not to avoid vignetting.

 

[gunnar]

Illustrating what sk66 said.

Light from every point of the subject hits every point of the lens and gets focused in a single point of the sensor/film.

Stopped down aperture blocks some of it, thereby reducing the exposure

Light hitting the front element at more acute angles (or obtuse if measured from normal ) gets reflected by lens barrel and creates vignetting.
Hence stopping down reduces vignetting because those acute rays get cut off.

Longer lenses are less prone to vignetting, because light hits them near perpendicular

 

[gunnar]

deleted

 

[sirch]

Thanks for taking the time Alex, @gunnar that is really helpful