What is Depth of Focus in Film?

If you’re new to cinematography, there’s a chance that you haven’t been fully informed yet. As to the various differences in things like depth of focus and depth of field. In fact, you may find yourself hearing about these various terms and wondering, “What is depth of focus in film?” Often referred to as the amount of lens to film tolerance. Depth of focus represents the distance that exists between the camera lens and the camera sensor.

What is Depth of Focus?

As we look at answering the common question, “What is depth of focus in film?” One must be aware of how the camera lens works in relation to the image plane. Which may otherwise be referred to as the film plane or represented by the camera sensor.

The focus viewable area will adjust when the camera sensor is moved. In order to come closer, or further from the camera lens. This is depth of focus.

Depth of focus isn’t always adjusted just by the movement of the camera though. Sometimes depth of focus is increased when various objects are placed behind the lens. In order to add things like filers or gels. That adjust the color or balance of the light coming into the lens.

As such, understanding how to calculate the depth of focus. Especially when the distance between the film plane and the camera lens is adjusted can help the cinematographer. To better understand the range within which they should be adjusting to ensure sharp focus.

Calculating Depth of Focus in Film

Now that we’ve answered the question, “What is depth of focus in film,” we’ll take a look at how depth of focus can be calculated. The calculation requires understanding of the relationship between the camera lens and the camera sensor.

It’s also important to understand that calculating depth of focus can occur with a simple calculation or one that is rather complex. 

The Equation

The equation is as follows: t=2Nc(v/f) But what does all of this mean? The equation states that t is equal to the total depth of focus where N equals the lens f-number, c equals the circle of confusion, and v equals the image distance while f is representative of the lens focal length.

That’s the complex version of calculating depth of focus in film. Most of the time, as long as there is minimal magnification then the depth of focus can be calculated in a much simpler manner where t=2Nc.

In this calculation, the total depth of focus is approximated to be equal to 2 times the lens f-number and the circle of confusion. Now that you know what depth of focus is in film and how to calculate depth of focus.

You’ll better be able to determine when it is best to use filters and gels that are placed in between your camera lens and the image sensor. And when it is better to choose another option to achieve the desired results.