Question about camera lens taking a picture

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    Camera Lens Picture
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Discussion Overview

The discussion revolves around the question of how a camera can take a picture even when half of its lens is covered. Participants explore the principles of light rays, refraction, and the effects of lens obstruction on image formation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant explains that light rays from an object, even when partially blocked, can still enter the lens and form an image, as long as the rays originate from a single point on the object.
  • Another participant inquires whether the light rays bend due to refraction, to which a subsequent reply confirms that they do.
  • Some participants note that covering the lens symmetrically affects brightness but does not fundamentally change the image formation process.
  • There is a discussion about the implications of non-symmetrical lens obstruction, which could lead to different optical processing techniques.
  • One participant expresses uncertainty about whether the original poster considered a symmetrical or asymmetrical obstruction when asking the question.

Areas of Agreement / Disagreement

Participants generally agree on the basic principle that light rays can still form an image even if part of the lens is covered. However, there is uncertainty regarding the specific nature of the obstruction (symmetrical vs. asymmetrical) and its implications for image quality and optical techniques.

Contextual Notes

Some assumptions about the nature of the lens covering and its symmetry are not explicitly clarified, which may affect the interpretation of the discussion.

Dixanadu
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Hey guys,

Can someone give a basic explanation of why a camera can take a picture even if half of its lens is covered up?

Thanks guys...
 
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In the image below, the red lines represent light rays from an object directly in front of the lens and located far away so that the light is very nearly parallel when it enters the lens. If you block off the outer rays, perhaps by closing the iris diaphragm, you still have rays entering the camera so you still have an image. Perhaps the key thing to understand is that all of these rays originated from a single point on the object. So if you took a picture of your friend, where the center of the image is at his chest, then these rays all came from a single point on his chest located directly in front of the center of the lens. The rays that came from his nose would be tilted at an angle when entering the lens (imagine tilting all the red rays so that they are traveling downwards slightly). The lens would then focus all of those rays onto a spot at the focal plane further down from the center.

Does that make sense?

optics-kids-lenses-fig1.gif
 
Okay, thank you I think I get it. Also, do the light rays bend due to refraction?
 
Dixanadu said:
Okay, thank you I think I get it. Also, do the light rays bend due to refraction?

Yes they do.
 
Aside: "Covering" the lens is effectively what the iris in the eye or aperture in a camera does. It's primary effect is to alter the brightness of the image.
 
It's worth pointing out that this thread assumes that the lens is covered symmetrically- stopping down or conversely, introducing a central obscuration (catadiopteric lenses). Obscuring the lens in a non-axisymmetric fashion leads to various optical processing techniques, such as:

Foucault knife-edge test
Schlieren imaging
Aperture synthesis
 
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Andy Resnick said:
It's worth pointing out that this thread assumes that the lens is covered symmetrically- stopping down or conversely, introducing a central obscuration (catadiopteric lenses). Obscuring the lens in a non-axisymmetric fashion leads to various optical processing techniques, such as:

Foucault knife-edge test
Schlieren imaging
Aperture synthesis
agreed, Andy

when I first read the OP stating the lens was 1/2 covered I was considering a blockage coming up say from the bottom so only the top 1/2 was passing light

optics-kids-lenses-fig2.GIF


Drakikith assumed a symmetrical aperture closing arrangement ... not sure if that was what the OP had in mind ?
 
davenn said:
Drakikith assumed a symmetrical aperture closing arrangement ... not sure if that was what the OP had in mind ?

That's a possibility. Still, the basic explanation doesn't change. Rays of light are still getting through even with half the lens covered so there's still an image formed.
 
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OK :) I was trying to visualise that
 

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