Bottom of the camera bag: Kelly Wheel

I’ve collected a few camera tools over the years. One that I’ve found relevant a few times recently is the depth-of-field calculator, better known as a Kelly Wheel.

The Kelly Wheel is a handy device that helps determine focus depth. Information especially helpful on set to a first assistant camera operator. The 1st AC is (among many other things) in charge of ensuring a scene is properly in focus, and will pull or roll focus to keep a subject sharp throughout a shot. So for a job like this, knowing how much focus depth you have to work with is crucial.

It looks a little complex at first but to find your focus depth start by locating your focal length, in my case 60mm. Next, rotate the Kelly Wheel so the arrow above the focal length is on your subject’s distance from camera (remember this is measured from the focal plane, not the end of the lens). In this case 9’. At this point I’m sure you’ll notice the red numbers that extend out to the left and right of the focal length. This your aperture setting, in my case I’m shooting at f/3.5 so we’ll use the tick mark between 2 and 4.

Looking at those tick marks and referencing the line above we can now see that with a 60mm lens at f/3.5 with the subject at a distance of 9’ the in focus area will be from just under 8’6” to just past 9’6”.

This sample image was taken from roughly 9” as opposed to 9’ but it helps visualize focus depth. The Kelly Wheel isn’t something I use on a daily basis but it is nice to reference once in a while.

Hopefully, reading this will help you notice other tools for determining focus area. Like the reference marks on this Voigtlander 40mm.

And if Kelly Wheels aren’t your jam, well don’t worry. There’s plenty more things to nerd out on in the bottom of my camera bag.