Today we’re going to talk about the aperture and depth of field relationship.
Here’s how it works.
The size of the aperture opening in your lens helps you control the depth of filed in your image.
The larger the opening is, the shallower the depth of field will be, and the smaller the opening is, the greater the depth of field will be.
The reason for this has to do with how light spreads as it travels, and how the lens manipulates the light as it’s passing through to it.
When you’re photographing a subject, light reflects off of the subject and then travels through the lens to be captured by your image sensor.
As that light travels, it spreads, and when it enters the lens, the glass elements in the lens are working to gather that light back up so that it strikes the image sensor as a perfect point.
The depth of field in your image is actually defined by how far the light can spread out and still be rendered back into a point by the lens.
Which brings us back to the aperture.
Let’s say you’re taking a portrait and you’ve focused on the eye of the subject, which is 1 foot away from the camera.
As the light bounces off of the eye and travels towards the lens, it’s spreading out, and as it goes through the lens elements it’s gathered back up into a point right as it hits the sensor.
And then you have another point of light bouncing off of the ear, which is one foot and three inches away from the camera, and by the time that light gets to the point where the nose is, it’s already spread out into a three inch beam
Now these numbers are completely made up, but for the sake of this example, let’s say that the limit for how far the light can spread and still be focused by the camera is two inches.
When the light from the ear gets to the lens, the lens tries to bring it back together, but because it’s already spread out too far, instead of bringing it to a point at the image sensor, it comes to a point behind the image sensor.
Since the light can’t travel past the image sensor, when it strikes the sensor, it’s a big bokeh ball of light instead of a small, focused point.
Now, let’s look at this again, but with a smaller aperture opening in the lens.
We’ve got the light from the eye, spreading out as normal, and then getting to the lens.
Once inside the lens we have this smaller aperture, so some of that light is cut off from entering the camera, which means that effectively, the light is not as spread out as it was with the larger aperture opening!
Because we’re still focused on the eye, that light comes to a perfect point on the sensor as expected.
And again, we have the point of light from the ear, which spreads out as normal, and then hits that aperture, and gets cut off.
Now if we cut off all the excess light and look at the spread of just the light that is able to pass through the aperture, we have light from the ear that’s not spread out nearly as far as before, for the sake of this example we’ll say it’s only spread out to one inch, which is within the range that the lens can focus!
What that means is that this point of light is brought together right on the image sensor, rendering it in focus!
What this means is when the aperture opening is smaller, it restricts the light passing through the lens into narrower beams, which keeps it from spreading out too far, meaning that light coming from distances in front of or behind the point of focus are more likely to be rendered in focus in the image.
And when the aperture opening in the lens is larger, the light that enters the lens is able to spread out more, making it more likely that light coming from distances in front of or behind the point of focus are more likely to be rendered out of focus in the image.
So that’s how the relationship works, but now let’s take a moment to talk about the relationship between the aperture number and the depth of field.
I used to be scared of my flash, and I understand how intimidating flash photography can be, but a flash is just a tool. Once you understand how that tool works you’ll be using your flash to create photos that used to seem impossible.
If you want to master your flash and take those amazing photos, check out my Understanding Flash Photography Video Course.
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