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
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
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?
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
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
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,