The 1st AC Who Questioned the Default
A 1st AC is setting up a focus pull on an ARRI ALEXA Mini LF. The depth-of-field calculator on set asks for a circle of confusion value. The DP wants tack-sharp delivery for a 4K theatrical finish, but the default 0.030mm in the calculator looks suspicious. The sensor is larger than a standard Full Frame still sensor, and the delivery is 4K DCI, not a 12-inch print.
Pick a CoC too large and your depth-of-field math tells you focus is safe when it is not. The near and far limits shift by inches, and on a tight close-up at f/2.0, those inches are the difference between a usable take and a soft one. Pick a CoC too small and you stop down unnecessarily, lose a stop of light, and spend 20 minutes relighting a scene that was already lit.
This post explains how the cinema CoC standard works, what the sensor-width-divided-by-1500 rule actually means, and how to choose a CoC value for your camera and delivery format. The circle-of-confusion calculation is based on the same standard used by lens manufacturers and the ASC Manual: the blur spot must be smaller than the smallest resolvable detail in the final viewed image.
How the Cinema CoC Standard Actually Works
The circle of confusion is the largest blur spot that still appears sharp to a viewer at a defined viewing distance and display size. The standard cinema formula divides the sensor width by 1500 to get the CoC in millimeters. This comes from the assumption that a viewer sitting at a normal distance from a cinema screen cannot resolve detail smaller than 1/1500 of the image width.
For a Full Frame sensor at 36.0mm width: 36.0 / 1500 = 0.024mm. For a Super 35 sensor at 24.89mm width (the ARRI ALEXA 35 4K 16:9 mode): 24.89 / 1500 = 0.0166mm. For a Micro Four Thirds sensor at 17.3mm width: 17.3 / 1500 = 0.0115mm.
The Zeiss formula uses a stricter divisor of 1730 instead of 1500. Zeiss arrived at this number by targeting a smaller acceptable blur spot based on higher-resolution viewing conditions. The Zeiss formula for Full Frame: 36.0 / 1730 = 0.0208mm. The difference between 0.024mm and 0.0208mm is small in absolute terms but shifts your near and far focus limits by measurable amounts on tight shots.
Worked example: A 35mm lens at f/2.0 focused at 10 feet on a Super 35 sensor. Using a 0.030mm CoC (the still-photo default), the total DoF is approximately 2.1 feet. Using the cinema standard 0.0166mm CoC, the total DoF drops to approximately 1.2 feet. That 10.8-inch difference is the margin a 1st AC works with on every take. The 0.030mm CoC says the focus is safe when the cinema CoC says it is not.
Three Real-World Examples
Example 1: Theatrical Narrative Feature on ARRI ALEXA 35
Context: A narrative feature shooting on the ALEXA 35 in 4K 16:9 mode (24.89 x 14.00mm sensor area), finishing for 4K DCI theatrical delivery.
Inputs: Super 35 sensor, 35mm lens at f/2.0, subject at 10 feet, CoC = 0.0166mm (24.89 / 1500).
Output: Total DoF is approximately 1.2 feet, or 14 inches. The near limit is 9.4 feet and the far limit is 10.7 feet.
Decision: The 1st AC marks the focus at 10 feet and knows the actor has 7 inches of forward movement before going soft. Using 0.030mm would have suggested 2.1 feet of DoF, creating false confidence.
Example 2: Documentary on Sony FX6 in Super 35 Mode
Context: A run-and-gun documentary crew using the Sony FX6 in Super 35 crop mode, delivering 4K web-first.
Inputs: Super 35 sensor (approximately 23.6mm width), 24mm lens at f/2.8, subject at 6 feet, CoC = 0.0157mm (23.6 / 1500).
Output: Total DoF is approximately 2.8 feet. The near limit is 4.8 feet and the far limit is 7.8 feet.
Decision: The operator confirms the 24mm at f/2.8 gives enough DoF for a seated interview subject who shifts in their chair. The 0.0157mm CoC is slightly stricter than the 0.030mm default, which would have reported 5.4 feet of DoF and led to opening up to f/2.0 unnecessarily.
Example 3: Micro-Budget Short on BMPCC 4K
Context: A micro-budget short film shot on the Blackmagic Pocket Cinema Camera 4K (Micro Four Thirds, 17.3mm sensor width), finishing in 4K for festival submission.
Inputs: MFT sensor, 25mm lens at f/1.4, subject at 5 feet, CoC = 0.0115mm (17.3 / 1500).
Output: Total DoF is approximately 0.6 feet, or 7.2 inches. The near limit is 4.7 feet and the far limit is 5.4 feet.
Decision: The director decides to stop down to f/2.8 to bring DoF to 1.2 feet, giving the actor room to move. A 0.030mm CoC would have reported 1.6 feet of DoF at f/1.4, making the team think f/1.4 was safe for a moving subject on a 4K finish.
Circle of Confusion Values by Sensor and Use Case
The table below shows CoC values for four common sensor formats using both the cinema standard (width/1500) and the still-photo standard (0.030mm or width/1500 with a looser viewing assumption). The cinema CoC is tighter because cinema screens are larger and viewers sit closer relative to image size.
| Sensor | Width (mm) | Cinema CoC (width/1500) | Print CoC (0.030mm) | Best For |
|---|---|---|---|---|
| Full Frame | 36.0 | 0.024mm | 0.030mm | 4K theatrical, large screens |
| Super 35 | 24.89 | 0.0166mm | 0.030mm | Narrative cinema, digital projection |
| APS-C | 23.5 | 0.0157mm | 0.030mm | Indie features, web delivery |
| Micro Four Thirds | 17.3 | 0.0115mm | 0.030mm | Run-and-gun, high-resolution finish |
The gap between 0.0166mm and 0.030mm for Super 35 is nearly 2x. That is the difference between a focus pull that works and one that does not on a 4K theatrical finish.
How to Choose Your CoC in 5 Steps
- Identify your sensor width in millimeters. Use the actual active image area for your recording mode, not the marketing name. The ARRI ALEXA 35 4K 16:9 mode has a 24.89mm width, but the Open Gate mode is 27.99mm. Check the manufacturer spec sheet.
- Decide your delivery standard: 4K theatrical, 2K DCP, web, or broadcast. The delivery format determines how large the image will be displayed and how closely viewers will scrutinize focus.
- Use the cinema standard (width / 1500) for 4K+ finish or width / 1730 for pixel-peeped masters. For web-only delivery on small screens, the looser 0.030mm is acceptable but not ideal.
- Enter the CoC in the Circle of Confusion Calculator along with focal length, aperture, and subject distance to get your DoF numbers.
- Interpret the near and far limits as a safety zone, not an absolute guarantee. Lens breathing, focus ring play, and subject movement all eat into the calculated margin.
Pro Tips and Common Mistakes
Pro Tip: When a shot will be cropped in post or reframed, use a stricter CoC because the crop magnifies the original sensor image. A 1.3x post crop on a Super 35 frame effectively makes the blur spot 1.3x larger in the final image. Use 0.0128mm instead of 0.0166mm to account for the crop.
Pro Tip: Vintage lenses and anamorphic adapters often have softer edges than modern primes. A stricter CoC gives you margin for edge falloff that the formula does not model. On a 2x anamorphic shoot on Super 35, use the 6:5 sensor mode width (20.22mm) and divide by 1730 for a 0.0117mm CoC.
Pro Tip: The ARRI ALEXA 35 sensor is 27.99mm wide in Open Gate but 24.89mm in 4K 16:9. If you shoot Open Gate and crop to 16:9 in post, use the 24.89mm width for your CoC calculation, not the 27.99mm Open Gate width.
Common Mistake: Using the default 0.030mm CoC for a 4K finish. The numbers say you are in focus when the pixels say you are not. The fix: divide your sensor width by 1500 and enter that value.
Common Mistake: Confusing CoC with pixel size. Pixel pitch matters for sensor resolution, but CoC is about viewing conditions and final image size. A 6K sensor with 4K delivery does not need a CoC smaller than width/1500 unless the 6K master will be displayed at full resolution.
Frequently Asked Questions
Why is the cinema CoC smaller than the photography CoC?
Cinema screens are larger and viewers sit closer relative to the image size, so blur spots that look sharp in a 12-inch print become visible on a 40-foot screen. The tighter standard accounts for bigger display sizes and closer viewing distances in theatrical environments.
Does a 6K or 8K sensor need a smaller CoC?
Only if the final delivery is cropped or displayed large enough to resolve the extra pixels. If you finish in 4K from a 6K sensor, the 4K viewing standard still applies. The CoC is based on the delivered image, not the captured image.
What CoC should I use for anamorphic lenses?
Use the horizontal sensor width of the recording mode you are using. For 2x anamorphic on Super 35 in 6:5 mode, the sensor width is 20.22mm. The anamorphic squeeze does not change the CoC formula itself, but the desqueezed image is wider, so account for the final display width.
Is the Zeiss d/1730 formula better than d/1500?
It is more conservative. Use it when you need extra safety margin for large screens, aggressive reframing, or when the final master will be scrutinized at full resolution. The difference between 0.024mm and 0.0208mm for Full Frame is small but measurable on tight focus pulls.
Can I just trust the CoC in my camera app?
Most apps default to still-photo CoC values (0.030mm for Full Frame). For cinema, recalculate using the sensor width divided by 1500. The Circle of Confusion Calculator on this site lets you enter any sensor width and get the cinema CoC instantly.
Related Tools and Resources
The Circle of Confusion Calculator lets you calculate CoC for any sensor and viewing standard. Once you have your CoC, the Depth of Field Calculator runs the DoF numbers using that value. For maximum-depth-of-field work, the Hyperfocal Distance Calculator uses the same CoC. For a deeper dive into how CoC affects your focus math, see Depth of Field in Cinema: The Complete Practical Guide. For the difference between calculated DoF and what lens markings actually tell you, DoF Calculator vs. Lens Markings: Which Can You Trust? breaks down the gap.
The Right CoC Is About Delivery, Not Brand
The right circle of confusion is a function of sensor size and how the image will be viewed, not the camera brand or the marketing label on the sensor. A 0.030mm CoC is a still-photo standard that has no place in a 4K theatrical workflow. Divide your sensor width by 1500, enter that value, and your DoF math will match what the screen actually shows.
This post covers CoC for single-sensor spherical setups. Anamorphic CoC, large-format CoC for cameras like the ALEXA 35 in Open Gate, and post-crop scenarios each deserve their own treatment. The principles here apply to all of them, but the specific numbers change with the sensor mode and delivery format.
What CoC value do you use on set, and has it ever saved you from a soft-focus disaster? Share your workflow in the comments.
