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Photography What's That?

What is Depth of Field?

Short answer: Depth of field is the distance between the nearest ‘in focus’ object to the furthest within your frame.

Shallow Depth of Field

For example, check out the below image. This image has a very shallow, small or narrow depth of field. Only part of the button is in acceptable focus so this depth of field is just a few millimetres.

MEIKE MK S AF3B Sample Image 6
Macro photo displaying very shallow depth of field

Also notice that it’s a very small part of the button that is super sharp. Just the “O” in the centre and then above and below this is a small area of acceptable sharpness which counts as our depth of field and then focus drops off gradually. This is why we talk about acceptable sharpness, depth of field isn’t an exact portion of perfect focus it’s a section of acceptably ‘in focus’.

What Counts As In Focus?

In order to talk about what counts as in focus first we need to understand what a circle of confusion is. A circle of confusion is caused by light rays not perfectly converging on the camera sensor or film.

If you took a picture of a point of light and it was in perfect focus then the light rays would converge to form a point on the camera sensor. Anything outside of perfect focus would form a spot on the camera sensor and this spot is a circle of confusion.

What counts as in focus is an acceptable size for the circle of confusion. Essentially this is the size at which the circle of confusion still looks like a point to us; practically looks like a point and so is an acceptable size and acceptably sharp.

Resource: Wikipedia

Depth of Field Tables

So how do you know what the distances are that are acceptably sharp? That’s where you can use a depth of field table if you are interested. PhotoPills have a cool calculator where you can put your camera details in.

Here’s a snippet of one for the Sony A6000 with a focal length of 16mm:

Subject distance (m)f/4f/5.6f/8f/11
0.5m0.15m0.22m0.33m0.51m
1m0.68m1.06m1.94m6.48m
1.5m1.76m3.39m17.58m
2m3.98m14.39m
2.5m9.57m

So how do you read this? If I set my camera to f/8 and the distance to my subject is 1m then the depth of field is 1.94m. The subject is the sharpest part of the image and the 1.94m DOF is spread in front of and behind the subject.

What about a depth of field of infinity? In this case everything behind your subject will appear in focus and part of the distance to the subject is included in the depth of field.

Aperture and distance to subject are included in the table because they have such a large impact on the depth of field. Which we’ll explore below.

Aperture and Depth of Field

The size of the aperture has obvious impact on depth of field. In the table above you can see that the larger depth of field, f/4, has smaller depths of fields than the smaller apertures.

This is something to keep in mind when you are shooting especially if you want to take photos with background blur and bokeh. It’s also easy to play around with, switch your camera to aperture priority and set the f-stop to the lowest value you can and see what the effect is.

Why does aperture have such a large impact on depth of field? Aperture impacts depth of field because it impacts how shallow or wide the angle of light rays coming into the lens are. Rays coming in at a narrower angle stay within the acceptable circle of confusion from a greater distance in our scene.

Resource: Wikipedia

That’s a little tricky to get your head around and visualise so I recommend watching this video from Dylan Bennett on YouTube. He shows how the angle of the light rays coming in impacts the DoF.

The key point to bare in mind is, a lower aperture creates a shallower depth of field and so background blur is more likely, or more intense. If you want to take a portrait with background blur then you should switch to aperture priority and use your lenses’ largest aperture, lowest f-number.

If you are taking a photo and want more of the field in focus, for instance when taking a landscape, then set the f-stop higher, e.g. 11.

Distance to Subject

The distance to your subject also effects depth of field in a significant way. The closer you are to your subject the shallower the depth of, and the farther you are from the subject you wish to focus on the greater the depth of field.

You can see this in the depth of field table above. Even at 2.5m from your subject, with a 16mm lens, the depth of field is infinity for all apertures of f/5.6 or smaller.

Why is this? This is for the small reason as aperture impacting DOF. When you are closer to your subject, it fills more of your frame and the light entering the lens is coming from a greater area. As a result the light is coming in at a wider angle so the depth of field is shallower.

This is also demonstrated well in the above video from Dylan Bennett.

Focal Length and DOF

Focal length also has a significant impact on DOF. The longer the focal the length the shallower the depth of field, and the shorter the focal length, the greater the depth of field.

This is consistent with the points we spoke about for aperture and distance to subject. When the focal length is longer, then you are more zoomed in to your subject. The subject will take up more of the frame and so the light is coming in from a wider angle.

When the focal length is smaller, e.g. a wide angle lens like 16mm, then the subject takes up less of the frame and the light is coming in at a narrower angle causing the depth of field to be longer.

Here’s that depth of table again for Sony A6000 and a 16mm focal length:

Subject distance (m)f/4f/5.6f/8f/11
0.5m0.15m0.22m0.33m0.51m
1m0.68m1.06m1.94m6.48m
1.5m1.76m3.39m17.58m
2m3.98m14.39m
2.5m9.57m

Here it is again but with a 35mm focal length:

Subject distance (m)f/4f/5.6f/8f/11
0.5m0.03m0.04m0.06m0.09m
1m0.11m0.18m0.25m0.37m
1.5m0.26m0.41m0.59m0.87m
2m0.46m0.75m1.09m1.66m
2.5m0.73m1.2m1.79m2.84m

The depth of fields are greatly reduced with none of the apertures getting to infinity for the small section I’ve presented here.

Short Video Introduction

Check out this video from Kellan Reck for a quick introduction to depth of field and how you can effect it:

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