We say a lens "covers" a sensor when its image circle produces an image over the full sensor size. So how do we determine if a lens’ image projection will "cover" a specific sensor? Just overlay the two. Look at the graphic of the image circle of a Zeiss CP.2 prime lens (right), which produces an image circle of 43mm (diagonally), completely covering every sensor size from full-frame 35mm down to 1/3-inch sensors.
From the diagram, you can see that a Zeiss CP.2 will work with a huge variety of sensors. But what about a Super 16mm lens with a 15mm image circle?
In the graphic below, the Super 16mm lens doesn’t cover anything larger than the "Super16"-sensor type. That makes sense, of course, but notice what happens to all the larger sensors; they vignette. This is exactly what you would see if you put a Super 16mm lens on a camera like the ARRI ALEXA or Sony F55, which both have sensors around the size of Super 35mm film.
Okay, so lens coverage isn’t all that complicated when you compare sensor sizes to a lens image circle. But many modern cameras offer a variety of aspect ratios and resolution modes, which allow the camera to record different areas of its sensor. Cameras that have this option are effectively altering their sensor size. The RED DRAGON is a great example of a camera that has these modes.
In this graphic (below, right), I’ve overlaid a standard Super 35mm lens image circle over a DRAGON sensor. The lines over the sensor indicate different recording areas and aspect ratios. As you can see, the area outside the 6K image isn’t covered by the lens, but the inside areas are all covered just fine. So this lens doesn’t cover the 6K resolution area on the RED DRAGON.
This brings up another question: What does resolution have to do with lens coverage, anyway? The answer: Nothing. The 6K resolution area of the DRAGON is physically 30.7×15.8mm (34.5 diagonal) in dimension, meaning that each photosite on the sensor is around 4.9um (0.0049mm) in size. A different sensor type could have larger or smaller photosites in the same area, producing lower or higher resolutions.
Let’s go back to our original questions: "Does my 16mm lens cover the Blackmagic Cinema Camera sensor?" and "Does my lens cover 6K on the RED DRAGON camera?" Super 16mm lenses don’t cover the Blackmagic Cinema Camera sensor, as the image circle of Super16 glass is 15mm, and the BMCC sensor has a diagonal area of 18.4mm. As for the RED DRAGON at 6K, the answer depends on the lens image circle. Having a 6K resolution has nothing to do with lens coverage, but it does describe a large area of the sensor. Lenses like the Zeiss CP.2 primes and CPZ.2 zooms are designed to cover the larger full-frame-sensor cameras, so we know they will work. Other Super 35mm lenses generally won’t cover that image area, but the only way to know for sure is to test them. AbelCine’s technical blog and other online resources are available that list various image circles, so visit our site to learn more.
Andy Shipsides is a N.Y.-based Camera Technology Specialist and Manager of AbelCine’s Training Department. To learn more about AbelCine’s Understanding HD Series, you can visit their website at training.abelcine.com.