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I've been teaching myself the various settings in cameras and now have a much fuller understanding of how to use my camera effectively, but one thing is still bugging me.

When the aperture size is changed, are any other lenses moving to refocus the ray bundles to the full frame size?

What I mean is, to me, the aperture would seem like it would "crop" the image to a smaller and smaller circle (or n-sided shape, rather). While I understand that this obviously means less light, is there something else going on that refocuses the light bundle back to fit over the entire sensor when it is adjusted?

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I know your question leads towards focusing but all the aperture does is limit the light and does not crop. Think of squinting your eyes. Your field of view doesn't change but less light is let in. –  BBking Jan 2 '13 at 2:33
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4 Answers 4

up vote 17 down vote accepted

Lenses don't shine light directly through, they focus it, if you forgive the lousy ray-tracing: alt text

The green lines represent the cone of light striking the lens with a narrow aperture, the red is the cone of light allowed by a wide aperture. In this case you can imagine the aperture immediately in front or immediately behind the lens (there's not much difference with a simple lens) Regardless of the aperture, the light is still focused to the same point.

That's a simple lens, but the same is true of complex ones. That's where your intuition about cropping (typically called vignetting in this context) the image comes into play: in a complex lens, the aperture has to be at an appropriate location. Obstructions in other parts of the lens will cause vignetting.

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I understand how lenses work. To clarify, this was what my intuition was scratching noggins at: img838.imageshack.us/img838/2347/52606135.png (ahh, good ol' mspaint) –  Nick Bedford Aug 17 '10 at 1:06
    
@Nick - I think where your intuition is misleading you is with respect to the point of focus. If you have a point source, it should focus to a point on the sensor. The proper point of focus in your diagram is where the lines cross; in which case, our diagrams are equivalent. Basically: the aperture restricts the cone of light that passes through the lens, but light from the same point will always focus to the same place (imagining the ideal lens, that is). –  ex-ms Aug 17 '10 at 4:26
    
@Nick - also worth pointing out is that your diagram does indicate why wider apertures lead to blurrier backgrounds! The light from the out-of-focus point is spread wider across the sensor. –  ex-ms Aug 17 '10 at 4:53
    
Matt, sorry about my own confusion. I now understand. It would be better to say that the focal point of each point source falls on the image plane. This is what I was confusing. –  Nick Bedford Aug 17 '10 at 5:24
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I was recently trying to figure this out myself, and found this question. I didn't feel the accepted answer was quite complete, so here's my shot (no pun intended!):

The first thing to understand is that the light that reflects off any one point on a surface isn't one beam of light, but many, coming in at many different angles and reflected off at many different angles. Most of these beams will never hit the lens on the camera; however, some do, and will be focused onto a single point on the image-sensor (assuming that point is in focus).

Light from an in-focus point going through a lens
Light from an in-focus point going through a lens

So what happens if we put an aperture behind (or in front of) the lens?

Aperture behind lens
Aperture behind lens

Light from the point still hits the image-sensor, so it will still show up in the image. However, we now simply have less light from that point striking the sensor. This is why using a smaller aperture (or a smaller lens) requires longer exposure time; the sensor requires a longer amount of time to absorb the same amount of light.


So, if making the aperture smaller forces you to use longer exposure times, what's the point of having an aperture at all? Reducing the exposed light can sometimes be useful (for example, that is the purpose of an eyeball's pupil, which is exactly analogous to an aperture), but the primary reason for having an aperture in a camera actually has to do with points that are out of focus.

Out of focus point - too far away
Out of focus point - too far away

Out of focus point - too close
Out of focus point - too close

Notice that, in both cases, the light beams all come from a single point, but they don't all hit the image-sensor at a single point. Rather, they are spread out in a circle. This is what causes out-of-focus points to appear blurred in a photo.

(This circle is sometimes called the Circle of Confusion. Incidentally, this also explains why out-of-focus points that are brighter than the surrounding points appear as circular discs)

So, what happens when we put an aperture behind (or in front of) the lens in this case?

Out of focus point with aperture
Out of focus point with aperture

We see once again that less light hits the sensor, meaning we'll again need a longer exposure. However, something else has happened: the circle of light (from our point) striking the sensor has become smaller. This will cause the point to appear more in-focus in the final image! Hence, a smaller aperture will increase the depth-range at which objects appear in focus, ie. it increases the depth of field.

Thus, the larger the aperture (or lens), the less exposure time you'll need (due to there being more light), but the shallower your depth-of-field will be (due to the light from out-of-focus points striking a larger area). Inversely, the smaller your aperture (or lens), the larger your depth-of-field will be, but the more exposure time you'll need.

If we could get an infinitesimally-small aperture†, we could get everything in focus in one shot... but we'd need an extremely long exposure time, or an extremely sensitive sensor! This is essentially how a pinhole camera works.

Well, the opening would still need to be larger than the light's wavelength, but that's a whole other topic...


I generated the above images using this awesome tool.

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No, there is no refocusing needed when you change the aperture, and the aperture doesn't crop the image.

As you might know, the image is mirrored and upside down when it hits the sensor. The image is focused into a single point inside the lens, and comes out flipped on the other side. This point where all light rays meet is the aperture, that's why the image can pass through such a small hole without being cropped.

The lens elements in a lens is actually not needed to produce the image, only the aperture is needed. A pinhole camera doesn't have any lenses at all, it only has a small pinhole that works as aperture and projects the image onto the film.

The effect is named camera obscura and is where the camera gets its name from.

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point where all light rays meet is the aperture... If that were true, changing the size of the aperture wouldn't have any impact on the amount of light reaching the sensor: all the light would pass through that single point. –  Caleb Jan 1 '13 at 2:21
    
@Caleb: The point is not a theoretical point with the size zero, it's a point the size of the aperture. –  Guffa Jan 1 '13 at 7:33
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I assume you're talking about a point along the optical axis, but this makes your second paragraph confusing: a location where all rays meet is a point of the zero-size variety. Your third paragraph also seems misleading -- it gives the incorrect impression that any aperture, not just a pinhole, can produce an image. In practice, of course, any aperture larger than a pinhole requires a lens to focus light into a usable image. –  Caleb Jan 1 '13 at 8:19
    
@Caleb: What is it exactly that you don't understand? –  Guffa Jan 1 '13 at 8:23
    
Okay. This is 4 years later and unbelievably picky, but in the real world you do sometimes have to adjust focus after stopping down. That's not fundamental optical theory, it's real-world lenses in the real world. Focus isn't exact; it's about getting as many of the rays to converge at the same point as practically possible, giving the highest degree of contrast. Since lenses are never perfect, there can be a shift in where the "consensus" lies as you reduce the number of "votes" coming from the perimeter of a lens with a smidge of spherical aberration; it gets worse the closer you focus. –  user28116 Jul 18 at 8:08
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It might help to think about your eye. Your eye is essentially a camera, and the iris in your eye works performs the same function as the iris in your camera's lens. When you walk outside on a bright day, your irises constrict to reduce the amount of light that hits your retina, but you still have the same field of view -- the image isn't cropped. The same thing obviously happens with your camera's lens.

The reason that the image isn't cropped is that rays from the entire image fall on the entire surface of the lens (whether your camera or your eye). You can imagine a cone of rays leaving each point in the field of view, where the tip of the cone is at the point and the base of the cone is the lens. The iris reduces the diameter of the base of that cone, so there's less light, but part of the cone nevertheless passes through the iris and is focused by the lens to a single point on the sensor (or retina).

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