Depth of Field Charts: Does Sensor Size Matter?

In filmmaking, focus is a deliberate act that isn’t left to chance. We don’t autofocus, we actively choose what to focus on and how much of the scene around it to place within the focus range. All for effect. So let's look deeper at focus, why we have different focus charts for different sized sensors, how circles of confusion can be our friend, and some focusing tips.

Focus Charts
In our previous video on Hyperfocus I talked about focus range and how eventually you reach the hyperfocal distance where everything from your near focus mark to infinity is in focus. We talked about how to find that distance with charts and without.  But I was still curious why there were different charts for full frame and crop frame cameras.
If you look at the hyperfocal distance formula and don't worry it's not complicated, you’ll see three values: the focal length of your lens, like 50 or 35mm, the F stop you’re shooting at, like f/8 or f/5.6, and then a value called circle of confusion which specifies what acceptable sharpness is. It’s given in a diameter like 0.030mm. 

Basically the limit of focus where anything past that becomes blurry to the eye.
But I don't see the size of the camera sensor in that formula.
So why are there different focus range and hyperfocal charts for full and crop frame sensors?

Circles of Confusion
Well, when I was working on that video, I noticed something. At the bottom of the focus range chart I was using, DOFMaster gave a circle of confusion value of 0.019mm. This was for a crop framed camera, Canon 7D. I looked up the 5D, a full frame camera, and that value jumped up to 0.030mm. Why the change?
What I discovered? The size of the sensor does matter and it’s noted in the circle of confusion value.
When a point is in focus, it’s well, a point. When it’s not, it enlarges into a circle and you get a blur. In the world of optics, they define Circle of Confusion as the largest blur spot that will still be perceived by the human eye as a point. In other words, it’s still in focus. 

So the more out of focus an area of your subject is the bigger the circles become. That can actually be desirable at times when you want an out of focus effect in the background like our shots with James and Courtney. Even larger circles get their own term; Bokeh. It’s based on the Japanese word boke which means blur. 

Okay great. So why different Circle of Confusion values for full frame and crop frame sensors? It has to do with the area of the medium recording your image. In the print world, they had 35mm film but they also had larger formats which they used for very high-resolution images like full-page photos, posters, billboards, and the folded section of a magazine called a centerfold. 
In the film world, we had 35mm film and then 75mm with Vistavision and Imax. More area meant more resolution which meant you could blow up or project the image larger. Give the viewing public more detail.
The same with digital sensors. Yes, we get more and more megapixels and better quality sensors but you’re still focusing an image from the lens onto a smaller area. 
Today it’s a crop frame sensor but back in the day, it was 16mm film. So a more restrictive circle of confusion for their depth of field charts is needed. You’re technically blowing that smaller crop frame image up higher than a full frame sensor in order to view it at the same size.

Shoot Image Tests
Now don’t sweat this stuff too much. The purpose of these charts is to assist you in your work, to help you determine ahead of time what f/stop you might need to shoot at or what issues you may run into if have to shoot at with a wide f-stop and therefore a narrow depth of field. 
In the end, focus is subjective and you decide what works or doesn’t work in your shot. It helps guide the eye and create a mood or effect. The key is to know how to get what you want and when.
So our recommendation is to always check a test shot right before you shoot that setup. Check it on an average size LED monitor. This one’s a 23,” a size a viewer might watch our Youtube videos on. 

Check a test video shot.jpg

Another tip, when filming people, focus on the eyes. This way, your near and far focus range will be on either side of their face.

Professional assistant cameramen don’t use metal tape measures like we did in the last episode. You don’t want something metal near an actors eye.
They use a soft tape measure which hooks to a mount on the camera like the one we have on our C100.
But that works best with cine lenses with their more detailed focus rings.
Zooming into a point to focus gives us a good idea, as it gives you a shallower depth of field than when you’re zoomed out. 
But if you’re filming yourself, like I do? I place a grip stand where my eyes would be and focus on that. You can use your mic if you have it on a stand or any other object you can place there.
But you may notice at times when zoomed in, you still have a range where in your focus point is still in focus.

I find it best to set my focus in the middle of that range. But if I’m focusing down on a flat table top, for example, I may put it on the back end of that focus range if I need to see hands coming into the frame.
Note, when I say zoom in, I mean when using an actual zoom lens. The magnify function doesn’t zoom, it just simulates it by magnifying a section of your image. The depth of field stays the same. 
But it’s still useful just make sure you’re focusing in the middle of your focus range and check it on a larger LED screen.

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Courtney Shaffer

James Aaron Oh


DOF Master depth of field charts

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