Mastering Depth of Field - Part 1

Every shooter wants to be able to direct the viewers' attention to the important aspects of a shot, and manipulating your depth of field is a great way to do it. In this two part series we talk about controlling your depth of field with aperture, focal length, and distance. In part two we talk about perceived depth of field, how sensor size and angle of view might affect your ability to get the depth of field you're striving for, and rack focusing. Knowing the different ways to achieve the depth of field you want for your scene will ultimately help you to craft more effective scenes.

Mastering Depth-of-Field Part 2

Video Transcript

Whether you're producing short films, shooting weddings, or creating a family video, every shooter wants to be able to direct the viewers' attention to the important aspects of a shot, and manipulating your depth of field is a great way to do it.
In this two part series we talk about controlling your depth of field with aperture, focal length, and distance. Then we'll talk about how sensor size, and angle of view might affect your ability to get the depth of field you're striving for.
Knowing the different ways to achieve the depth of field you want for your scene will ultimately help you to craft scenes to be more effective.

Scientifically speaking, only one plane of perfect focus is truly possible, but in practice, a given area in front and behind the plane appear to be in focus to the human eye.  This perceived area of focus is know as the Depth of Field. There are three things you can control to manipulate depth of field.  Aperture, distance, and focal length. Let's start with your aperture.
Aperture refers to the hole that allows light to pass through your lens to your sensor.  The iris is the mechanism that closes or opens to increase or decrease the size of the aperture. The F number or f-stop is a numerical representation that tells you relatively how much light is being passed through to your cameras' sensor.  F numbers typically range from f1.2, which lets in a ton of light to f22 which lets in a very small amount of light. Simply put, the lower the f-stop, Or the more light you let in, the shallower your depth of field will be.
Now, it sounds simple, but you'll need to be able to control the light in your scene to shoot at the f-stop you want.
There are basically four tools to help you.  Lights, Neutral Density Filters, Shutter Speed,  and ISO or GAIN. Let's look at an example.
We want to shoot this scene at f1.2, but as you can see, it's totally overexposed because there's too much light coming in.  If we were indoors, we could simply reduce the intensity of the lights in our scene to give us proper exposure. But since we can't turn down the sun, another option is to use a neutral density filter on our lens. Some video cameras have these filters built in, but DSLR users will have to purchase one separately. You can see that we can now shoot at 1.2, with a very shallow depth of field.  If you don't have the proper neutral density filter, you can choose a faster shutter speed, which will expose each frame for less time, and therefore let less light in.  Of course, the downside to this method is that it will create a strobe effect on moving subjects in your scene, which will be more pronounced as you increase your shutter speed.
Now let's take a look at the opposite situation.  Where we want deep depth of field.  We can use lighting, and ISO or gain to accomplish this.
We want deeper depth of field in this shot, but there isn't enough light coming in to shoot at a higher f stop, so we've got two options.  We can either increase the overall light level in the scene, or boost the ISO or Gain.  Of course, the downside to using ISO or Gain is that you will increase the amount of noise in your image.  We're at f1.2 and ISO 100, with fairly low lighting.  First let's close down the iris.  You can see that as we do, we're getting less light in, and our scene darkens.  Let's try boosting the ISO to make up the difference.  In this case, we have to boost it to around 500 to get a relatively decent exposure.  Now let's go back to ISO 100, and rather than boosting the ISO, let's increase the overall light level in order to keep our iso at 100.  You can see that we've now got proper exposure without boosting ISO.  Of course, now that our overall light level is higher, we should be able to stop down even more and boost the ISO.  We're going to go from f3.5 all the way to f10. Now let's boost the ISO from 100 up to 800.  You can see that combining a higher light level with a reasonable boost in ISO has allowed us a far deeper depth of field  than the f1.2 aperture at iso 100.
A second way you can manipulate your depth of field is by changing your focal distance.
Focal distance, or focal length, is the measurement from the optical center of your lens to the camera sensor, and lenses are typically referred to by their focal length.  The important thing to know here is that the shorter the focal length of a lens, the deeper the depth of field it will produce. 
Let's take a look at an example.  Here's our setup: we're shooting with a 24mm lens at f4 and we're focused on the gear shift knob.  Now here's the shot from the same location at f4 with a 105mm lens, still focused on the gear shift. If we look at the shot in split screen, we can see that the 105mm shot has a narrower depth of field, due to the longer focal length.
So if you want shallow depth of field, why not just shoot with a 300mm lens all the time? Well, there are a few factors to consider.  First is the amount of space you have to shoot in.  In many cases, you won't be able to back up far enough to frame the shot you want with such a long lens. You'll also need to take a lens'es maximum aperture into consideration.
The maximum aperture of a lens refers to the lowest f-stop the lens can shoot at. Lenses with lower f-stops are referred to as fast lenses, while lenses with higher f-stops are referred to as slow lenses.  In general, shorter lenses are available with lower maximum apertures, while longer lenses tend to have higher ones.  Especially the affordable ones.  This is important because shooting with a fast lens allows you more control over the aperture, which we know can give you shallower depth of field.
It's also important to understand that on many video cameras with zoom lenses, the maximum aperture changes as you increase your focal length.  This increase in f stop will decrease your depth of field.
If we take this zoom lens and push in on our scene, you can see that the f-stop increases as our lens gets longer, and therefore lets less light in.
A final way to manipulate your depth of field is by changing the distance between you camera, your subject, and your background.  The basic rule here is that the closer the camera is to the subject, and the further away the subject is from the background, the more shallow the depth of field will appear.
In this example we start with the camera, and background 10 feet apart, and the subject directly in the middle.  You can see  the amount of depth we're able to achieve.  First, let's move our subject closer to the camera, which creates tighter framing.  You can see that even though our actual depth of field remains the same, we're focused closer to the subject, which throws the background  further out of focus.  Now, if we keep the same distance from camera to the subject, but move further away from the background, we can see that the back wall is now more out of focus than before.
So there you have it.  Aperture, focal length, and distance, the three primary factors that influence your depth of field.  In part two of this series, we talk about perceived depth of field, how sensor size and angle of view can affect your ability to control your depth of field, and rack focusing.  Thanks for watching.