Aperture and Depth of Field

Hey, how’s it going, I’m Andy Jones and this is episode 63 of the Photography Q&A Podcast.

This week I’m going to ramble on a bit about Aperture and Depth of Field. Next week I’ll cover Shutter Speed and ISO. I’ll go through a few different types of photography and explain how you choose the Aperture, Shutter Speed, and ISO.

So let's get started with Aperture and Depth of Field.

If you listened to the repeat episode of the Exposure Triangle you’ll know that the Aperture blades are in the lens. The aperture blades control how much light passes through the lens and onto the sensor.

The amount of light that passes through a lens is measured using the “focal stop scale”, abbreviated to f/ stop. The lower the number, the more light passes through.

I’m not going to go over how the stops work if you need to learn that you can listen to either of the two Exposure Triangle episodes. You can also sign up on the 50mmFramework.com website and download the Exposure Triangle PDF file.

One of the side effects of using low f/ numbers is a shallow Depth of Field. The wider the Aperture opening the shallower the depth of field is. 

Using aperture settings to control the depth of field is something you need to learn. Again, if you don’t understand it listen to the Exposure Triangle episode and download the PDF file.

If you have listened to some other episodes of this podcast you’ve heard me mention the PhotoPills app. It’s a great app, give it a try it’s free.

Ok, so let’s go through a few scenarios.

Most landscape photos have a very deep depth of field. That is everything from right in front of the camera to infinity is in focus. I’ve heard people say that you need to focus 1/3rd into the frame. This is a safe way to make sure that your depth of field is at its maximum, that is to infinity. 

If you want to be accurate you need to find out what the Hyperfocal distance is for your camera, lens focal length, and aperture combination. Using the PhotoPills Depth of field calculator app all you have to do is pick your camera body from the list, then set the focal length and aperture. The Hyperfocal Distance is at the top of the results.

Here is an example:

Nikon D3400, lens set to 24mm, and an aperture of f/11

The hyperfocal distance for this setup is 8.87 feet.

So if you focus on a rock 9 feet in front of the camera, everything behind the focus point will be in focus to infinity (∞). Anything less than 8.87 feet and the far reaches of the frame won’t be in focus.

Also, check out the DoF near limit, this is the distance from the camera to the subject that isn’t in focus. As a rule, wide-angle lenses like 24mm have a very small near limit. The longer the focal length the longer the out-of-focus near limit is. So if you are using a 50mm lens the DoF near limit expands to 30 feet. Which is bigger but if you are shooting mountains over a mile away that 30 feet won’t matter.

The focus 1/3rd into the scene is a foolproof method, but if you want to include something in the foreground it’s best to find out what the hyperfocal distance is for your setup. I’ll put a link in the show notes for the PhotoPills calculator. I would have the app on my phone so you can check stuff out in the field.

Next, let’s look at something a bit easier,

In portrait photography, the aim is to get the subject or subjects in focus, and in most cases, you want to get bokeh (blurry background). 

If you are taking a portrait of a family and they are staggered away from you, guestimate the distance from the front of the person nearest you to the person furthest away. Knowing this distance allows you to choose an aperture that will get everyone in focus.

Let’s do another PhotoPills example:

Canon T7i, 35mm Lens, and you are shooting from 12 feet away.

The distance from the front to the rear of your subjects is approximately 5 feet.

Set the aperture to f/4. Now, look at the Depth of Field behind measurement. It says 3.48 feet, but you need 5 feet, so change the aperture to f/5.6. Now the Depth of Field behind (the focus point) is 5.59 feet which will keep everyone in focus. 

Another example is a single-person head-shot portrait. The depth of field needed here is shallow because you only need to get the subject’s face in focus. You focus on one of the subject's eyes and might only need a depth of field of 4-5 inches. 

Canon T7i, 50mm lens, F/4, from 6 feet away gives you a Depth of Field behind of a little over 4 inches. It also gives you a Depth of Field in front of 3.69 inches which will keep the subject's nose in focus.

When I was shooting dog agility I needed a depth of field of 4 to 6 inches when I captured a dog coming straight at me over a jump. All I wanted in sharp focus were the dog's head and front paws, plus the rail they were jumping over. Depending on the size of the dog my aperture would be in the F/4 to f/8 range. 

So you need to think about how much depth you need and use PhotoPills to decide on the aperture. If you are shooting a fast-moving subject you need a fast shutter speed, but the first thing you should consider is Aperture. Yes, you might freeze the action perfectly but if the depth of field is only an inch the photo won’t work out.

So from now on, think about what you want to capture and how big the depth of field needs to be.

Ok, that’s it for another episode, I’ll be back next week with more waffle, bye for now.