Question about aperture

Beattie

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Sep 6, 2001
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I am new to dslr photography (christmas) and have been reading a lot today. I have a question that I didn't see answered anywhere.

If you have a subject at a given distance and you change the f-stop you are changing the size of the aperture. But if you make the aperture bigger aren't you letting light enter the camera and hit an area that is larger than the sensor? And vice versa, if you make the aperture smaller then you may be shading part of the sensor? I know there must be something wrong with this thought experiment because when I make the aperture small and look through the viewfinder or at pics I took they all seem to be full size. What am I missing?

Is the camera adjusting the focal length based on the aperture size so that it always lets the light fall on the sensor correctly?
 

xchangx

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Mar 23, 2000
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It has nothing to do with the size of the image circle, just how much light is getting there
 

Beattie

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It has nothing to do with the size of the image circle, just how much light is getting there

Yea, but light that comes through the aperture and doesn't hit the sensor is "wasted" and having the larger aperture wouldn't help you. Conversely, light that doesn't come through the aperture and thus doesn't hit the sensor is a shadow and would show up as black pixels in the viewfinder and in the image. The focus must be dependent on the aperture, right?
 

soydios

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Mar 12, 2006
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The size of the image circle is the size of the image circle and nothing more. A lens at f/5.6 transmits the same amount of light per unit area from the front to the image circle whether that image circle is in a tiny point-and-shoot or an 8"x10" film back.
 

troytime

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Jan 3, 2006
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i think because the aperture ring is so close to the sensor that there isn't really anything lost.
just like your pupil and your retina - bright light makes your pupils get smaller, but your field of view doesn't get smaller.
 

ElFenix

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Yea, but light that comes through the aperture and doesn't hit the sensor is "wasted" and having the larger aperture wouldn't help you. Conversely, light that doesn't come through the aperture and thus doesn't hit the sensor is a shadow and would show up as black pixels in the viewfinder and in the image. The focus must be dependent on the aperture, right?

focus shift does happen but it isn't a direct result of stopping down in the way that you're thinking. it's a result of spherical aberration.

light that goes through the aperture iris goes through the remainder of the lens and comes out the back in the image circle. the light intensity of any lens set to the same relative aperture is the same regardless of the size of the image circle.

light that goes through the front element but is blocked by the aperture blades should either be absorbed inside the lens or reflect back out the front. any stray light that goes through the back is a design flaw and could lead to halation. digital sensors are so bad at absorbing light that comes off-axis that it isn't as much a problem as it was in the film days (otherwise using film lenses with their large image circles would suck on digital bodies).
 
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corkyg

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Aperture does affect focus. In general simple terms, the depth of field increases as aperture gets smaller. Conversely, a wide open aperture has the shortest depth of field. And, in order to balanced the exposure, as aperture gets smaller, the shutter speed must get slower or the ISO value larger. One of the nice things about digicams is that you can play with these things and see comparative results very quickly. It is a learning process.
 

ElFenix

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Aperture does affect focus. In general simple terms, the depth of field increases as aperture gets smaller. Conversely, a wide open aperture has the shortest depth of field. And, in order to balanced the exposure, as aperture gets smaller, the shutter speed must get slower or the ISO value larger. One of the nice things about digicams is that you can play with these things and see comparative results very quickly. It is a learning process.

with an ideal lens there is only one plane of perfect focus. and dof is not what the OP is thinking of.
 

fuzzybabybunny

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Jan 2, 2006
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This is probably what you're confused about:

aperture.jpg


The red lines signify the rays of light that go through the lens and the aperture and end up on the sensor. When the aperture is wide open many rays of light from the tip of the arrow get through to the sensor, but when the aperture is stopped down less rays of light from the tip of the arrow make it to the sensor. This means:

1. the image gets darker as you stop down aperture
2. stopping down the aperture will not shade out part of the sensor, crop out part of the image, etc. As you can seen, even with the aperture stopped down raws of light from the tip of the arrow (and all parts of the arrow for that matter) still reach the sensor plane, resulting in a complete image.

Note that this image is a very simplistic attempt to show how aperture will not cut off parts of your picture, but it is not accurate for modern lenses beyond this. For example, you could incorrectly deduce from this example that the more you stop down, the more you use the outside edges of the lens. This is in fact not true because images tend to INCREASE in sharpness as you stop down (up to a point), but the outside edges of lenses are often the worst optically.
 
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corkyg

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with an ideal lens there is only one plane of perfect focus. and dof is not what the OP is thinking of.

You are correct, however, I'm not talking about an ideal lens or plane of perfect focus. OP asked if aperture affected focus. Yes it does - when talking about satisfactory focus - i.e., what most shooters work with.

The area in an image from front to back that is in focus. The smaller the aperture (the larger the f-stop number), the more objects are in focus both near and distant. The wider the aperture (the smaller the f-stop number), elements in front of and behind the object in focus appear soft or blurry.

Good diagram, Fuzzy.
 

Beattie

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Sep 6, 2001
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*bunch of good info*
This is exactly the answer to the question I was asking. Thanks.

And yea, as you stop down the sensor (which is in the middle) gets more (relative anyway) of the parallel rays through the middle of the lens which causes a larger depth of field.

It's all coming together.