Understanding Depth of Field (DOF) - A field guide.

If you do a search for Depth of Field on the internet you will get some fairly high tech explanations on what it is and how it works. However, for the average photographer, it doesn't need to be so complex. I'll attempt to explain it in a simplified way so you can use it in your photography out in the field.

So, what is DOF?

First of all let's talk about focus and for this, take out your SLR camera (any lens will do). Now the easiest way to demonstrate focus is for you to aim your camera along a textured surface. The carpet is a good choice. Get down on your belly and aim the camera along the surface of the carpet. Now as you turn the focus ring on the lens notice how the area of focus moves back and forth on the carpet, getting closer or further away depending on which way you turn the focus ring. Think of this area of focus as a vertical plane of glass (focal plane) that is parallel to your camera and moves back and forth as you change focus.

Now that we're moving our focal plane back and forth along the carpet notice how much of the carpet is in focus at any one time. It will be very narrow close to the camera and as you focus further away it gets progressively (if only in small increments) wider. This little band of carpet that is "in focus" is called depth of field and it extends in front of and behind the actual exact point of focus. We can describe DOF as the area considered in focus between the nearest point of focus and the furtherest point of focus that is either side of the focal plane. Of the area in focus, one third of it is in front of the point of actual focus and two thirds is behind.

But this little band of focus (DOF) is so small, how do I make it bigger?

Excellent question! We change the DOF by changing the aperture, also known as the F-stop or AV. For a quick introduction to aperture have a look at the "What is aperture" page found in this section of my site.

The smaller the F-stop number, i.e. f2 (wide aperture), the narrower the DOF. The larger the F-stop, i.e. f16 (narrow aperture), the wider the DOF.

Narrow aperture = Wide DOF
Wide aperture = Narrow DOF

You can see this effect by changing the AV (aperture value) on your camera. You may need to use your DOF preview button (if your camera has one) to see the results of the change. Why do I need to use a DOF preview? Because when you look through the lens of a recent model SLR camera (last 30 years or so) it will, by default, show you what your scene looks like on the widest aperture. But why? Because the wider the aperture the more light comes through the lens. You need lots of light so you can compose your photograph. When you press the shutter button, your camera will automatically close (stop down) the aperture to the value you have dialed in for it to use. Trust me.. this is how it works.

If your camera doesn't have a DOF preview button (check your camera manual) you will need to take a couple of images with different AV values to see the difference.

Ok so now I know what DOF is and how it works, how do I use it?

Another excellent question!

This is best answered by taking some photographs. The first is a landscape. When you take a landscape image you generally want to capture the entire scene all in focus, yes? Ok, so how do you ensure you get the entire scene in focus, from the nearest point of interest to infinity (which is around 30 metres or greater). Well, to get all this in focus, the first thing you'll need to do is set your aperture to the narrowest you can. This will depend on your lens and is usually around f16 or f22. Got it set? Good.

Now, focus your lens at infinity. If your lens barrel doesn't have an infinity mark (a horizontal 8) you can focus on an object that is either greater than 30 metres away or on that mountain range you see on the horizon. Once you have this done, press your DOF preview button while looking through the lens. Look for the closest point of focus, the closest object that looks in good focus to your eye. Once you have identified this object (it could be that small bunch of wild flowers about 5 metres in front of you) take your finger off the DOF preview button (notice how the bunch of flowers falls out of focus again) and refocus your lens so that those flowers are now in focus. Don't worry that the mountain range has now fallen out of focus. Take the photo.

So what did this achieve?

Think back to the carpet exercise. As we move the plane of focus back and forth we change what part of the scene is considered in focus. Remember also, that DOF has a near point and a far point of focus... focus has depth. As we "stop down" our DOF gets wider either side of the exact point of focus. So... when we focus on infinity we are actually wasting the two thirds of available focus that lies on the far side of infinity and we only can see the one third that lies on the near side. I'll give you a moment to digest that last sentence. You may need to read it a couple of times.

Ready to move on? ...good.

So, when we move the focal plane to the bunch of flowers in front of us, we bring back into play the area of focus that was extended past infinity. And, at the same time bring the nearest point of focus closer to us to capture more of the landscape scene. In other words, when your camera stops down the aperture to take the photograph, the area of focus will extend from in front of those flowers, all the way back to the mountain range, thus rendering your landscape nice and sharp all the way through the scene.

As an aside, if you wanted to, you can calculate how much of a scene will be in focus given any focal distance and any aperture. For example, how much of a scene would be in focus if you focus at 1 metre with an aperture of f8? I can answer that with a DOF table that I have created for my own use. I will link to it here so you can download and use it. I also have the formula so if you wish to play around with it send me a message and I'll forward it to you.

Moving on.

Now that we know how to make all the scene sharp by creating maximum DOF, what about limiting DOF? Why would we do this? Well, we limit DOF in a scene so that distracting elements such as a background, fall out of focus. We can use this in portraiture. If we're taking a photo of a person, generally, we want attention on the person and not on that green hedge behind them. So to make the hedge fall out of focus we make sure it falls out side our DOF. F5.6 to F8 is considered a good aperture for taking photographs of people. These AV's provide enough DOF to render the person sharp from nose to tail but not much else. You will need to experiment because it will depend on how far the person is from your camera and what aperture you've chosen.

So now that we know what DOF is and how to use it, go back and read the pages on Aperture and Shutter speed, and then go out and take photographs. Experiment with DOF in different situations so you know and can see how creative a tool it can be.


Till next time,
Ben.