Depth of field(DOF)

Ironlung · Admin Joined: 24 May 2008 Posts: 118

What Is Depth of Field (DOF)?

Depth of field (DOF) is the distance wherein objects are in focus.

This is the pragmatic explanation.

To be technically correct, DOF is the zone of acceptable sharpness, the area in front of, and behind, a focused subject that appears in focus.

Circle of Confusion (COF)

Technically, only the subject in focus -- and all other objects at the same distance -- are in focus; everything else in front and behind are out of focus. How much out of focus depends on a term called the "Circle of Confusion (COF, or COC)."

Let's see if we can make that term clear, but if you don't understand it, don't worry. Many photographers take superb pictures using depth of field to their advantage never having heard of COF. I never did until I started researching this topic. And, I find that it just confuses the hell out of most people, so if you want to skip to the next paragraph, here is your chance to do so.

OK, back to the COF.

Imagine we are photographing three (3) dots. They are the tiniest dots the human eye can clearly make out, and of course, we are assuming perfect 20-20 vision and ideal light condition.

So, here we have these 3 dots, arranged one behind the other (with the closest dot to the right of the middle dot, and the farthest dot to the left of the middle dot, so a camera can take a picture of all 3 dots).

Now, we focus our lens on the middle dot, which comes out in perfect focus. The two other dots also appear in focus, but peering closely at the resultant photograph, we notice, however, that the dot in front of and behind the middle dot appears as circles instead of perfect dots. I.e., technically, they are out of focus, but to our naked eyes (at a "normal" distance), they "appear" in focus.

It is this circle that we call the "Circle of Confusion." So the COF is the diameter of a dot such that when we view it with the naked eye, it appears in focus. If this circle gets past this diameter, our eyes tell us it's out of focus.

Lens manufacturers have to decide what that diameter is going to be and design their lenses accordingly to be able to resolve a dot within that COF so that it appears sharp to us. For a more technically detailed explanation of how COF is determined by lens manufacturers, read Michael Reichmann's excellent tutorial: Understanding Depth of Field.

That is about all I am going to say on COF. Do you need to understand COF to use DOF? Personally, I don't think so. But if you are one of the people who absolutely must understand COF, its history, how lens manufacturers calculate DOF based on the COF, etc. etc., just do a search on Google. There's enough material there to keep a technical mind happy for many hours.

What Affects DOF

What we really want to know as photographers is what affects DOF so we can control DOF in our pictures. For a long, long time, photographers have gone with the following three criteria:

lens aperture
distance from subject
focal length

While the first two are technically correct, the third one has raised somewhat of a storm of controversy among certain circles. Why exactly, we will make clear later. Let's look at each of the three criteria in more detail.

Lens Aperture

The aperture is simply the size of the opening that allows light to go through the lens. It is expressed in f/stops (also referred to as f/value or aperture value), and a typical aperture range is f/2.8 - f/8, giving the range from maximum (large at f/2.8) to minimum (small at f/8) aperture.

A small f/value (e.g. f/2.8) indicates a large aperture.

A large f/value (e.g. f/8) indicates a small aperture.

So, f/2.8 is a larger aperture than f/8.

Generally, a large aperture gives a shallow DOF, and a small aperture gives great DOF.

Putting Aperture into practice:

If you want only the subject the lens focuses on to be sharp, and everything else to be out of focus -- such as a portrait with the background nicely blurred -- then you would "open up the aperture," i.e. use a large aperture.

If you need most of your picture to be in sharp focus -- such as a landscape scene -- then you would "stop down the aperture," i.e. use a small aperture.

Shallow DOF

Note how the use of a large aperture throws the flowers in the background
out of focus.Focus has to be precise

Great DOF

Using a small aperture
extends the DOF from the foreground all the way to the background.

In most consumer digital cameras, you may not be able to directly control the aperture. Many, however, provide a Portrait scene mode and a Landscape scene mode that basically do what we are after, i.e. use a large and small aperture, respectively.

As most of you have found out by now, it is quite difficult to obtain a shallow depth of field with most consumer digital cameras even with the aperture opened up wide. Why? See the section on focal length below.

Distance From Subject

When you focus on a subject close to the camera, the DOF is less than when you focus on the subject farther away from the camera.

Putting Distance From Subject into practice:

Step away from your subject to obtain greater DOF, or move in closer to decrease DOF.

Focal Length

Set your subject in front (say, about 3m or more) of a bush (or a tree with lots of leaves, or some other kind of busy background). Use wide-angle and take a picture. Both your subject and the bush would most probably be in sharp focus.

The background may distract from your main subject -- unless you're aiming for a Product Shot where you want to situate your subject in his or her environment, and do want both subject and background to be in sharp focus.

Now zoom in and fill in the screen with your subject's face and shoulders. Look at the resulting image and you'll probably notice that, though your subject is still in focus, the bush now appears out of focus, giving a nice blurred background that does not steal attention from your subject.

Photographers use this technique very effectively to "affect" DOF.

So, a wide-angle lens has greater DOF than a telephoto lens. Most consumer digital cameras have very short focal lengths and that is why it is so difficult to obtain shallow DOF, even with the aperture opened up wide.

I promised to come back to the storm of controversy brewing in some circles that disputes that focal length has any effect on DOF.

The contention is that, for a fixed image size in your screen, the DOF is unchanged irrespective of focal length used.

However, photography is part technics and part art. The technics part may well tell us that DOF is the same at all focal lengths for a fixed image size on screen. What this is saying is that we achieve the same DOF whether we fill the screen with the face by walking to the subject, or backing off and zooming in.

The art side, however, tells us that a long lens (i.e. long focal length) reduces the distance from the subject and thus provides a shallow DOF. Yes, we can achieve the same narrow DOF by walking close to the subject. However, the results look very different! Why? Because a long lens also provides a "flattened" perspective which makes the narrow depth of field much more prominent than walking closer to the subject using a lens with normal focal length does.

Putting Focal Length into practice:

Use long focal lengths to achieve the effect of a shallow DOF.

Summary

Even though the subject you focus on is the only thing technically in focus, some objects in front and behind your focused subject also appear to our eyes to be acceptably in focus (thanks to the COF). This zone of acceptable sharpness is the DOF.

We can increase the DOF (or the appearance of it) by using a small aperture, moving away from our subject, or using a wide-angle lens.

We can decrease the DOF (or the appearance of it) by using a large aperture, moving closer to our subject, or using a long focal length.

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