What is an Aperture?
Aperture affects Depth Of Field. Do you still hate math? You might hate it more after this. Check it out.
Getting back to yesterday’s diagram:
Before light leaves the lens, it passes through the Aperture. Most Apertures are made up of a number of blades which effectively make a smaller and smaller hole for the light to pass through. Why? Because it will increase Depth Of Field as it does so. It looks, more or less, like this:
From left to right, the aperture is open, closing, closed as far as it will go.
Most smartphones and drones have a fixed Aperture, meaning you can’t change the value or close the Aperture. Although things are changing with those cameras. The same is true with action cameras, but most mirrorless cameras have an adjustable Aperture.
Let’s Diagram How Aperture Works
Now, it doesn’t actually block the field of view as it might seem, it’s blocking light that isn’t coming into focus at a given plane. Let me try a diagram.
Light comes from a source, let’s say the sun, hits the bunny and bounces into the camera lens. In this simple diagram, one element in the lens is then moved front to back to focus (just like a magnifying glass). That’s the Focusing Lens in the diagram. Focus means the light from the bunny hits the plane of the sensor evenly (front to back), in a two dimensional way.
Another horrible way to think of focus with a lens is frying ants with a magnifying glass. I know, it’s gross, but if the glass was too far away, the light would not be properly focused and no ants would be harmed.
In this diagram, the aperture is open all the way. This focus is only good for the bunny, for how far away the bunny is from the camera. The Bunny Plane™ if you will.
If the bunny moves or the camera moves or the focus is changed, this happens.
It’s a fact that you can only focus on one plane at a given distance with your lens. Maybe that bunny is 15 feet away. In the top diagram the focus was set to 15 feet. In the bottom one, maybe it was set to 20 feet and thus, the focal plane (Bunny Plane™ = focal plane) was off of the sensor.
IMPORTANT NOTE: ALL of the bunny light is still hitting the sensor. It is just hitting it while being focused on a plane past the sensor. If the light (in these diagrams) is not focused on the plane of the sensor, it is blurred.
Different Distances and Focal Planes
“Well then,” you may be asking, “how the heck do I get things in focus that are different distances away from the camera?”
Aperture! First let me show you what it looks like having different objects at different distances as the light comes into your camera.
In this case then, the tree and the Yeti, being further back and having the aperture open all the way, do not have their light focused on the correct plane of the sensor. They come out blurred. Now let’s use the aperture and close it a bit. I’m going to remove the bunny for a moment. Don’t freak out, the Yeti doesn’t get her.
With the aperture partially closed, as represented by the increased darkness around the edge of the aperture, that ‘extra’ light is cut off and only the light that will come to a high focus is let in. From this point forward, the tree will continue to be in focus. This can be a hard concept to illustrate in two dimensions.
When you close down the Aperture, you cut out the extra light from an object that was not going to hit the sensor in sharp focus and are attempting to only let in the light that will be in focus.
Aperture settings are often called f-stops. Remember when I mentioned stops yesterday? These function the same way, letting in half as much light as the f-stop goes from one setting to another and doubling the amount of light hitting the sensor when changed in the opposite direction.
In the case of the Aperture, the sequence looks like f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45 and so on. The f-stop number is a function of the lens focal length and the size of the hole the Aperture makes (which is the same concept as how your pupil works, by the way). The smaller the Aperture/pupil size, the less light. Because we are working with fractions again, and the equation looks like it does at right (thanks Wikipedia): the higher the number, the less light.
The higher the f-stop number, the less light and the greater the depth of field.
Depth of Field
Depth Of Field (DoF) simply means the the number of planes (or things, if that makes it easier), front to back, that can be in focus.
Written another way DoF = amount of stuff, front to back, in focus.
If we focus on the bunny and use f/1.4 for instance, the Depth of Field might only be a foot. Meaning from the point you are focused, maybe the bunny’s nose, the only objects that will be in focus will be .33 feet in front of that point and .67 feet behind that point = 1 foot total. (Side Note: This 1/3 in front and 2/3 behind ratio is true of your eye as well, but that’s a lot more boring physics than we need here)
Now change the aperture to the next stop, f/2 (which, as it is another stop, lets in half as much light) and you might have increased the Depth Of Field to 2 feet. .66 feet in front of the nose and 1.34 feet behind it. Can you see how more and more things will come into focus as the f-stop is increased?
Here’s another piece of the puzzle:
The person at top is to represent portrait work, where a wide open aperture and its shallow depth of field is often desired to help the person stand out from the background. The mountain scene at the bottom represents a time when you might want many things, front to back, in focus and thus a smaller aperture is desired.
Rarely do you shoot at the extremes, so in reality that portrait might be at f/4, for instance and the mountain scene will be at f/11 maybe.
While changing to a higher f-stop number does give a greater Depth Of Field, it lets in less light. What does that do to the Exposure Triangle? It means either Shutter Speed needs to change to compensate for a good exposure, or ISO does.
Knowing what you do about Shutter Speed and how it works with stops, can you now see how if you change the Aperture by one stop darker, you need to change the Shutter Speed by one stop more light? (We’ll get to ISO tomorrow, for now we’ll keep it simple with Shutter Speed and Aperture.)
It’s those Teeter Totters Of Doom in action! If Aperture goes one way to make the Depth of Field deeper, then the Shutter Speed needs to go the other way to compensate.
What does this look like in the real world? Here’s a series shot of things I found already on the floor in my house. Each object is about 6″ away from the one in front of it. Focus is locked on the golf ball in all shots. Click on a picture for a larger image.
Can you see how you more and more of the objects (and dirt on my floor) come into focus as the Aperture gets smaller and smaller (remembering that as the aperture itself gets smaller, the f/ number gets bigger)? Do you also notice what happens to the Shutter Speed? It gets slower and slower to let in more light to compensate for the aperture.
Whew! I know that is a lot to take in and it is one of the harder topics for people to grasp. To make it very basic: Aperture affects Depth Of Field.
And with all that reading, it’s time to put it into practice!! Here is an assignment for you to try at home to help reinforce the ideas of Aperture and Depth Of Field:
Questions? Pop ’em like Pez in the comments section below. or email me at firstname.lastname@example.org.
Photography Basics – A 43 Day Adventure, and its companion 40 Photography Experiments, are series written by professional photographer Peter West Carey. The series are designed to unravel the mysteries of photography, helping you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.
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