Understanding Apertures

February 7, 2024

If you are new to photography you probably came across the exposure triangle. This triangle is made up of ISO, Shutter Speed and Aperture. I have already covered the first two in previous blog posts, but today I am going to cover the latter. We are going to walk through the basics of aperture and how it effects your photography.

What is Aperture?

Aperture refers to your structures within your lens that change the amount of light coming into your camera. They are leaf like structures that expand and contract within your lens allowing for more or less light to enter. Wikipedia has really good in-depth articles on it and images of it here. Below is the image from Wikipedia.

Aperture is measured in what we call f-stops (f/5.6 for example). Now, one thing to note is that as the number gets smaller, you can see that the opening gets wider. On face value, as the number gets smaller more light can pass through the lens into your camera. This does a couple things. First, it requires less light to make an image at lower aperture numbers because more light is entering the camera. Second it takes less time to collect sufficient light in order to make an image at lower aperture numbers. This is why lenses that have f2.8 or lower are considered very fast because they don't need a bunch of time in order to get the right amount of light to make an image.

How does Aperture Effect Images?

At face value, lower f-stop number means you have brighter images and it requires less time take a properly exposed image. But your aperture effects other aspects of your image too, in particular it effects depth of field (how much of your image is in focus) and how sharp (or clear) your image can be.

What is Depth of Field?

Below is a graphic that demonstrates depth of field. In the center of the image is a square that represents what might be called the plain of focus. The plain of focus is the spot within your image that is perfectly clear or in photography vernacular, "sharp." As you move away from this center plain of focus the image becomes fuzzier and less clear. With this note in mind, if you are trying to photograph someone, you want that plain of focus right on their eye so we can see their face clearly, but have the background be out of focus.

I want to add one more term to our understanding of depth of field. I will call this the acceptable field of sharpness. Every image has a plain of focus that is the maximum level of sharpness that the lens can produce. But on either side of that razor sharp plain of focus is an area that is still clear (see graphic below). In fact it is so clear our eyes cannot detect that it is not sharp or even if it is detectable, it is not significant enough to be of concern. As we move to the edges of that acceptable field of sharpness the image becomes less clear and begins to look fuzzy or soft.

We now have two terms to work with, plain of focus and acceptable field of sharpness. Let's apply aperture to that and see what happens.

Aperture sits on a sliding scale from most frequent f1.8 to something like f22 or even f32. As stated above if our aperture number is small we have a large opening within our lens, and if our aperture is large, we have a small opening within our lens (see wiki image above). One other thing to note, our plain of focus is always the same on your lens. No matter what you do or where you focus, your plain of focus is always the same. That spot is as sharp as it will ever be for that lens.

When our aperture is small our plain of focus is the same, but the area of acceptable field of sharpness reduces in size (see image below). This fundamentally means that your image has a reduced area that is acceptably clear. If you are taking an image of a person with a low aperture (lets say f1.8) you have a very narrow band of acceptably clear image with the rest of it being out of focus.

To see this in action let me show you a small crop of an image I took of my daughter. I photographed this at f4, so a relatively small aperture. You can see her face is in focus but the background is out of focus. In-between the background and her eyes, you can see that her hair line becomes fuzzy showing that acceptable field of sharpness concept in action within this image.

On the reverse end of this acceptable field of sharpness idea is the high aperture and how it increases the acceptable field of sharpness. Most often when you are photographing people you want the background to fade away so that the person becomes the main subject in which you are trying to highlight. We don't want to see the messy background, even if it is pretty. As a landscape photographer though, we often want to see the background and the foreground and everything in-between.

Using Larger Apertures

I would say most landscape photographers f-stops sits between f11 and f16 for their images. The main reason why is that this gives you the maximum depth of field while having the clearest sharpest image (more on that in a bit). As you increase your aperture your acceptable field of sharpness increases in size (see graphic below). Now this does not mean you can have infinite size acceptable field of sharpness. There is no such thing. As you notice, if you look at the diagram below the AFOS does not go to the very end of the diagram. There is always a fall off point, but whether that fall off point is within view of your image is another question. But in all areas of life, there are trade offs to what you do when you increase or decrease your aperture, the great quest of photography is using those trade offs to your advantage.

Increasing F-Stop Beyond f16

There is a curious little conundrum that occurs when your f-stop number begins to climb higher than f16. As your aperture increases your images have a tendency to begin to look softer and softer. Infact, if you are shooting at your highest f-stop possible on your lens, you may find your images are so fuzzy they begin to look really bad. Why is this the case?

The culprit is called diffraction.

Diffraction is light that is being bent as it passes through an opening. Using a graphic from Wikipedia you can see how the light as it enters through the opening is actually warping rather than coming through perfectly. The smaller the opening the greater the warp. One thing to note as you look at the graphic below, you will see the center of the image will have the greatest clarity and the edges will have the least. This too applies to your photos you create.

Distance From subject

One thing you realize as you fiddle with your aperture is that the further away you are from your subject the easier it is to get all parts of your image in focus. The main reason (I think) for this is because you camera sees the world in 2D space, flattening all aspects of it into a single plane. The further you get from the camera, the harder it is to depict the difference between objects. Vis versa, the closer you get to your subject the harder it is to get all of it in focus.

This can be used to your advantage as a photographer. If you are taking an image of a grand landscape on the edge of a cliff, it allows for lower f-stop numbers to obtain an overall clearer images. If you are closer to your subject you have to use higher f-stop numbers to achieve maximum levels of sharpness. What f-stop is perfect for your image? It depends. What do you want to accomplish? If you want to have a clear image from front to back, you can either increase your f-stop and risk diffraction or you can decrease your f-stop and take multiple images focusing on different spots to achieve maximum sharpness throughout the image. The latter requires more technical know how but it will results in an overall better image.

Putting It All Together

The aperture you choose should serve the photograph. That is the moral of the story. There is not fancy lesson or rule that says at this distance you need to be at this f-stop. What does the photo need for your vision. At first this might be hard when trying to figure out how to achieve the best images, but in the end, that is the price you have to pay to get good at photography.

The Great White Throne in Zion National Park with fall colors and the virgin river flowing through the seen.
Rode to the Throne

Every once in a while I re-visit views in Zion with better gear, more experience and better conditions. In 2023 I had it a goal to go back and photograph the Great White Throne of Zion near Big Bend. The fall colors were not in full glory, but were doing there thing in a decent manner. In addition the clouds really came to life creating some dynamic conditions.

Posted in Education.