Trying to get into photography. How and why does aperture affect focus?
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As the aperture is set to a smaller diameter (higher F stop) rays of light from out-of-focus details in the image are channeled into smaller points on the sensor, which affects the apparent sharpness of these details. As the aperture diameter decreases, depth of field increases both in front of and behind the plane of focus.
With the aperture wide open (lower F stop), the only details that will be in sharp focus are those at the specific distance the lens is focused on. Everything closer to the camera and further away from the plane of focus becomes progressively softer the more distant they are from the point of focus.
There’s a lot of physics and math involved, but that’s the gist of it.
The aperture is a hole which lets light into the camera. The bigger it is, the more light can come in.
A smaller aperture means that a greater amount of things will be in focus (a larger depth of field). A pinhole camera has no lens, but a tiny pinhole sized aperture, so everything still is in focus, despite the lack of a lens.
That is because the smaller the aperture, the less different directions the light will be coming from.
However, you still want to let in enough light to get a photo! So if you make the aperture smaller, you will need to have the aperture open for a longer amount of time (a longer shutter speed).
Having a small aperture and a long shutter speed lets you have a greater focal length (the distance between the nearest and furthest away thing which will be in focus). But anything which moves will become blurry.
Having a large aperture and a short shutter speed will let you have a small focal length, so you can have effects like a crisp portrait with the background blurred. It will also allow you to freeze anything that is moving so that it isn’t blurred.
Aperture is the amount of light admitted into the camera body.
https://photographylife.com/what-is-aperture-in-photography
A really wide aperture allows a lot of light in, so its focal length is shorter (or you overexpose the image), so it has a shallow depth-of-field.
So what is our goal in making a camera? When we point a camera at something we want to gather the light from that object onto our camera sensor or a strip of film. Sounds easy enough right? It’s not that simple though, we want each point on the object to have its light fall on one point and only one point on the sensor. If it falls on other points on the sensor which have other light coming onto it as well it ends up melding those two together and your result is a blurry image. Each point of an object radiates light in every direction, so if we just hold up a strip of film to the object it’s not gonna work, everything is going to get blurry.
To solve this, the earliest cameras were what we call pinhole cameras, you just take a box, put a tiny hole in it, and it works. No lens, no anything, something like [this](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Pinhole-camera.svg/800px-Pinhole-camera.svg.png).
This works because two points form a line and only one line. For each point on an object you are trying to photograph, there is exactly one direction the ray of light can go to go into the pinhole and onto the film or whatever on the other side.
The only downside is that the only ray of light that we can use is the one that goes in that direction. It’s not a lot of light, our sensors aren’t that good at picking them up nor is our film. We can maybe run things for long exposure or use flash but it doesn’t help much.
So we then to lenses. Lenses bend light, we take the light that’s radiates out and bend it back onto a point on our sensor. This way we can use a lot more light, and we can actually open up the hole to be wider to allow more light in.
The only problem with this is that based on the distance from the object, the light comes in at different angles, [here’s not an amazing diagram but it shows it](https://inst.eecs.berkeley.edu/~ee198-4/fa07/images/week12_dofdistance.gif). Our goal on this image is to make the two rays from each distance line up on the green line which is the sensor (there are a lot more rays in reality but this uses 2 to reduce clutter). As you see, the light from different points comes in at different angles and the lens is only adjusted to really correct for one angle. The result is things in the foreground and background are blurry, only the stuff along where it is focused is clear.
This image also shows another fact, by moving the lens forward and back we can adjust the distance from the lens to the sensor to focus in on different distances, which is what cameras do.
The blur is going to be worse based on aperture. Our pinhole camera is so focused already due to a small aperture it needs no lens, but the more we widen the hole the less the camera approximates this pinhole camera ideal, more light is let in at different angles into the camera, a lot of which doesn’t go where it’s supposed to.
At least if we don’t want it to go there. I find bokeh pretty, it’s a limitation we have to deal with but makes things a bit more fun in my mind.
imagine you’re looking through a tiny window. If the window is really big (like a wide aperture), you can see a lot of things clearly, but everything else around it is blurry. But if the window is very small (like a narrow aperture), you can see a smaller area very clearly, and more things around it are also clear.
In photography, the aperture is like the size of the window that lets light into the camera. A big aperture lets in a lot of light, which can make things look sharp in the front but blurry in the back. A small aperture lets in less light, which can make more things look clear from front to back.
So, choosing the right aperture helps the photographer decide what they want to be in focus and what they want to be blurry in the picture.
It doesn’t. Changing the aperture affects how much appears in focus but there is only ever one plane parallel to the sensor/film exactly in focus no matter what aperture is used.
> 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.
[This diagram shows it pretty nicely.](https://www.vision-doctor.com/images/stories/optik/grundlagen/Fundamentals_depth_of_field.png)
When a point of an object is in focus, it means that all the light rays emanating from that point converge to a single point on the sensor. But once they converge, those light rays will continue to go in a straight line and end up diverging. If you move your sensor back (bring things out of focus), you will capture those diverging rays and end up with a blurry “bokeh” circle instead of a point. The further back you move your sensor, the larger that circle becomes, because the light rays just keep diverging further apart from each other.
If you make your aperture smaller, you block the outermost light rays, which means that the light rays coming in at the steepest angle will get blocked. Only the ones coming in straight will get through. The straight ones simply don’t diverge as much. So with a small aperture, the only light rays that make it through are coming in quite parallel to each other, which means they can’t diverge as much, which in turn means your bokeh circles will be smaller.
Imagine you have a spray can of paint. You press the nozzle and make a large spot on the wall. But if you hold a piece of paper with a hole in it in front of the nozzle, the spot on the wall will be smaller, because the particles of paint that go more sideways simply get stopped by the paper.
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