Thread: Motorsports Photography Part 2 - To Blur or not to Blur

Depth of Field

If you start poking around the photography world you're bound to run into this term sooner or later. What is it and why is it important? Better yet, how do you make it work? Let's find out.

Quite simply, Depth of Field (DoF) is the amount of distance between the nearest and farthest objects in a photo that are in sharp focus. A photo can usually be defined as having 'great' (meaning deep) or 'shallow' DoF.

In the first photo, both the lead riders and the following riders are in focus. This is an example 'great' DoF. In the second photo we still have four riders in almost the same location, covering the same distance, but only the first rider is in focus. This is 'shallow' DoF.

Easy enough, right? So what's the big deal? The deal is not so much of a technical nature as it is an artistic one. Let's call it a matter of personal preference that is heavily influenced by the type of photography you are doing.

First, a couple of quick technical details: aperture, or f/stop as it is commonly called, is a ratio between the focal length of a lens and the diameter of the opening (aperture).

Focal length is the distance from the lens to the film or digital sensor. A longer focal length results in more magnification or a larger image.

The most common f/stops are:

1.4 / 2.0 / 2.8 / 4 / 5.6 / 8 / 11 / 16 / 22

Here's the tricky part; f/stops are inversely related to size, meaning that the smaller numbers (f/1.4) represent larger diameter openings in the lens, thereby letting in more light. An aperture of f/22 would then be much smaller, letting in much less light. Got it?

Now that your head is hurting, let's get back to the fun stuff.

When shooting sports or action photography I will almost always be using a large aperture setting such as f/2.8 or f/4. This will isolate the action and force the viewer to focus on your intended subject. I really don't want the spectators cheering in the background to be in focus; I want you to be looking at the action.

Whether a bike on the track, or a kid kicking a soccer ball - it doesn't matter. The viewer will focus on the parts of an image that are nice and sharp and ignore the rest. Since we're the ones telling the story, we get decide what's in focus.

How about the other other end of the scale? f/22 must get pretty lonely, right? Not really. Ever hear of Ansel Adams? More than a few of his landscape photographs were shot at f/64. He wanted us to be able to see everything in sharp detail. This will apply more to photos of your bike in front of a scenic overlook where you'd like the bike and the scenery to be nice and crisp.

These concepts are readily apparent in pretty much any magazine you care to look through. Pick up a Sports Illustrated and you'll see endless examples of shallow DoF with the heart of the action clearly in focus while the background is not. Feel like a little National Geographic? They use a lot of smaller apertures for greater DoF in order to show you all of those cool landscapes. Playboy uses a little of both...

There is one very important aspect of DoF that you need to know about. When shooting wide open at f/2.8, your DoF will be very, very shallow so you need to make sure you are carefully focused on the most important part of the image. It is not uncommon in portrait photography to have part of a person's face in focus while the rest is out of focus if you mess up. We're talking about a DoF that is inches deep.

On the other side of things, when shooting with smaller a DoF say f/22, you want to focus on the nearest object you want to be in focus. Everything else behind that focus point will fall into focus.

This can be demonstrated very simply with cans on a floor. The cans are placed at approximately one foot intervals with the camera about one foot away from the first can.



In the first image my aperture is set at f/1.8, the largest aperture I can get with the equipment that I have, and I have focused on the first can. Notice how very little of the image behind the can is in focus? This is shallow DoF.
The second image is also at f/1.8 but I have focused on the middle can. This is a great example of how DoF works both in front of and behind your subject. The first two and last two cans are out of focus.
In the third image I have changed the aperture to F/22 and focused again on the first can. Now compare it to the first example at f/1.8. Starting to make sense now?
The fourth image is still at f/22 but is now focused on the middle can and the last image is focused on the last can.

That wasn't so bad now, was it? Now we know how to get things in focus and fuzzy all at the same time. We're good to go. Well, hold on to your breeches, Grasshoppa. We're only just getting started. By changing the size of our aperture we're changing how much light we're letting into the camera and that's going to directly impact what our shutter speed needs to be set at. We'll leave that until next next time, though. For now, go take some photos of soda cans and have fun.

Thanks, Taz!