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I have some conceptual difficulties with understanding field of view and aperture. Let us say, I have a wide-angle lens, for example 20mm. Field of view directly correlates to the focal length of my camera. If I have a small aperture which is pretty much the opening of how much light comes on, how come I am not limiting the field of view? If I am decreasing the aperture am I not limiting the amount of rays hitting the image sensor, therefore decreasing the field of view?

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Answer with an image : the guy on the left represents your (vertical) field of view and its image on the sensor is inverted on the right (in other terms, the guy fills your entire image).

Aperture vs FoV

As you can see in this illustration, the rays of light going through the center of your lens aren't concerned by your aperture settings, they are still getting in, no matter how small it is.

Of course, when your aperture is too small, diffraction comes but it is another story (What is a "diffraction limit"?).

Have a look at What is "angle of view" in photography?

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    Olivier - thank you for the illustration. You updated it compared to yesterday's illustration which was confusing. Please correct me if I am wrong: So that point that represents the top of the guy's head would still hit the image plane even if the aperture is small? When the aperture is wider, more rays for that point will be hitting the image plane. – oneiros Sep 21 '16 at 14:42
  • Yes Oneiros, that is basically it : the aperture controls the amount of rays coming from point "P" that will hit it's image P' in the image plane... if the point "P" is in focus. – Olivier Sep 21 '16 at 17:03
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This is a good question but most have difficultly visualizing the answer.

First the definition: The angle of view sustained by a lens is the angle between the light rays that go to opposite corners of the frame and image with sufficiently good definition over entire frame.

What is hard to grasp is: All the light rays from the vista pass through a point called the “rear nodal” on their way to film or digital sensor. To measure the angle of view, we draw lines from the far opposite corners of the frame to the rear nodal. The angle of view measured will differ with subject distance. This is because, to obtain sharp focus on nearby objects we shift the lens forward away from film or digital sensor. As a result, the distance from film or digital sensor to the rear nodal is elongated. Now the angle traced as above changes; it is dependent on the back focus distance. Stated differently, the angle of view is at maximum when the camera is focused at infinity.

Because all image forming rays pass through the rear nodal, the angle of view does not change as we open up or close down the iris diaphragm.

A caveat: Every lens projects a circular image area. Only the central portion possesses sufficient definition to be photographically useful. As we stop down, the twin demons, diffraction and interference begin to degrade the image. As these faults intensify, the size of the circle of good definition will contract. At some point, you might judge that the angle of view has been abridged.

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    "All the light rays from the vista pass through a point called the rear nodal” No they don't, as you explain later in the very same paragraph. Rays from a closer object will converge at a point farther back than rays from a more distant object. – David Richerby Sep 21 '16 at 9:05
  • Two significant points that define an optical system are the front and rear nodal points. The front nodal point is a position on the optical axis where entering ray cross the optical axis. The second or rear nodal point is a position on the optical axis where the departing rays cross the optical axis. These points can fall inside the lens barrel or in air ahead or behind depending on lens design. The position of these nodal points can even be inverted. – Alan Marcus Sep 21 '16 at 15:01
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    But all light rays do not pass through the nodal points. Any light ray passing through one of the nodal points will be refracted through the lens such that it will appear to have come from the other nodal point. But a paraxial light ray that is not not on the optical axis by definition will not pass through the nodal points. – scottbb Sep 21 '16 at 16:52
  • Alan, your comment does nothing to address the point that I and @scottbb raised. – David Richerby Sep 22 '16 at 9:07

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