Understanding Depth Of Field

In Uncategorized by tfwm

A quick primer on the concept from a creative and partially technical perspective

What is depth of field?

1. Technical Definition-
A term that describes the focus distance between objects within the viewing area of the camera lens, how much foreground and background are in focus compared to the main subject.

2. Creative Definition-
Depth of field is a method to center or focus the viewers’ attention to a specific visual zone. Shorter depth of field focuses attention, where wider opens up the area to a wider zone.

Drivers for depth of field

The two technical factors that drive depth of field are:

1. How zoomed-in (or how wide) your lens is set: the wider the lens the more depth of field, and…
2. How much light you have: The more light you have, the more closed your lens is (f-stop number larger), the more depth-of-field. The less light you have, the more your lens is open (f-stop number smaller), the less depth-of-field.

In some cases, you can control the depth of field. Example: You take out your camcorder, shoot video and control the environment with lighting, camera position and use depth of field to focus the viewer’s attention on the parts of the content you want them focused on. When understood and harnessed properly, depth of field can be a very valuable creative tool.

Challenges of Depth of Field in the Worship Space
When doing live IMAG or broadcast work in the worship space you often times lose some of this creative freedom. In this case, the depth of field capabilities can present more of a technical challenge than a creative capability.

Challenge #1 • Camera Placement
In the worship space, we want to have cameras located in position so that we can achieve nice angles and shots, specifically close-up shots of the pastor during his or her message. We also want to reduce the number of seats that we occupy with the technology, and the number of seats we “burn” behind the camera location. That forces us to want to put the cameras further away from the platform. Now we have to use longer focal length lenses to achieve close-up shots because we’re zooming in further. And the question becomes, do we have enough lens to achieve the close-up that we want to?

Challenge #2 • Stage Setup & Configuration
If we are doing IMAG during a service, it is important that we identify the items on stage that we want to capture and their spatial relationship to other stage elements.

Example: If we want to capture content with depth of field onstage, are there objects that will take away from the creative element in that shot selection or do they build the shot up?

Challenge # 3 • Camera Shot Selection
Is the depth of field appropriate for the shots we’re going to accomplish? In other words, is the shot we want to take going to give the look/feel and commensurate response that we would like it to?

Example: When the pastor looks directly at the lens in a close up and says “YOU WILL BE SAVED”, it would certainly be best that other items in front of and behind the pastor stay out of focus to intensify the emotion of the pastor’s expression. So, we want shallow depth of field. If we have a camera that’s a hundred feet away from the platform, it’s going to be with a telephoto lens, and probably an awfully long one. Plus, chances are, we’re going to be in a low light situation, so we’re going to have very shallow depth of field (long lens and small f-stop).

Challenges With Using A Lens Extender
One of the things that some people will want to do is say “Well, we’ll just buy a lens with a 2X extender on it, so now our 7.6mm – 130mm lens (17×7.6) goes to 15.2mm – 260mm, to get twice the close up effect. But when we put that extender on the lens, we reduce the light going through it by half. We get even less depth of field and we need more light on the stage to achieve a good, high-quality image. So the 2X extender is rarely effective indoors because of the lighting required.

Depth of Field as A Creative Tool Within Videography
Let’s say you’ve got three salt shakers; one that is two feet, one that is two and a-half feet, and one that is three feet away. If you want the audience’s attention to be focused on the middle salt shaker, then you want a shallow depth of field. In a shallow depth of field, the closest salt shaker and the one that is furthest away will be out of focus. Our eye is naturally driven to watch the item that we’ve selected to be in focus.

As a creative tool, you can shift the depth of field, say during a conversation. Let’s say we’ve got three people at a table, and we’ve got the depth of field set to be very shallow, so that the person in the middle is in focus, and everybody else is out of focus. During that shot, the person in the middle finishes their line and we get a verbal or non-verbal response from the person who is closer. If we shift the depth of field by shifting the focus point to the person who is closer, we’ll be able to see very clearly and concisely the response from that person. It creates a dynamic effect and therefore becomes a useful creative tool.

To reiterate, there are two technical things that drive the ability to use depth of field: the lens (how wide or telephoto the lens is), and the iris – (how open or how closed the iris is). Try to experiment using different subjects, live or not. Of course, I would suggest experimenting in a live scenario if you’re not sure what you’re going for- that’s what rehearsals are for…