Trying to get into photography. How and why does aperture affect focus?
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> How and why does aperture affect focus?
Aperture doesn’t affect focus, but affects depth of field. In that respect it’s just geometry.
A point of your subject emits light towards the lens, which concentrates it at a point on the sensor (or film). This means that said light forms a *cone*, with base (roughly) the diaphragm and apex on the sensor.
When the diaphragm is wide open (corresponding to *small* numbers on your lens’ settings, like 1.4, 2, 2.8…) that cone is rather fat/broad; and conversely when the diaphragm is closed (big numbers: 8, 11, 16…) the light cone is slender/pointy.
Now suppose you move the sensor towards the lens. The intercept of the cone and the sensor (a plane) will be a disk and not a point anymore: the image will be blurred. And with a broad cone that disk will be rather big, while rather small with a pointy cone. In other words, the same displacement of the sensor will create a lot of blur at wide aperture and much less when stopped down.
In practice we of course don’t move the sensor but the lens (focusing), but the effect remains the same.
Now to depth of field (at last!). Suppose you’ve focused on a point A in the field of view. Its light cone, having the apex on the sensor, will give a sharp/small point. A point B of the subject, nearer to the camera, will generate a cone with an apex behind the sensor: its image is a disk and will look blurry. Similarly a point C, farther away than A, will have its cone of light rays converging in front of the sensor, not on it, and then spreading out to form a disk there too.
The blurriness (size of the disks) will increase with:
* the difference of the distances of B or C to the “correct” one, where you set the focus, that of A;
* the aperture (which defines the geometry of the cones).
But real life photography doesn’t deal with perfect geometrical points: even with a very good lens everything is a bit blurry, the question is just “Do we notice it?” That’s the meaning of depth of field: how far can we deviate from the ideal focus distance (that of A) before we consider the subject (B or C) to be out of focus?
As said, with a great aperture that deviation is small, the slice of the subject which seems in focus is slim, while a small aperture ensures sharpness over a much larger interval of distances.
Most lenses have engravings which allow to gauge that. I’ll refer to this [picture](https://i.imgur.com/XeYw75u.jpeg): the focus is set at 6 m (red triangle) and the marks left and right of it show that, stopped down to f/16 (red point at top) everything will seem sharp from 2.5 m to infinity.
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