A Word About Camera Resolution
September 16, 2019
Huawei has been shipping now two generations of smartphones, the P20 Pro and P30 Pro, with a 40 megapixel, 1/1.7″ sensor camera.
Figure 1: Camera Sensor Configurations
This class of larger sensor of 40 or 48 megapixels uses a trick with the color filters. Most digital cameras use a Bayer color filter, taking in one color per photodiode and interpolating from the neighbors to deliver R, G, and B color samples for each pixel. These new sensors use a modified “quad Bayer” filter pattern. This allows a high resolution image to be shot per photodiode, or groups of four to be ganged together to function as a lower resolution camera. The lower resolution mode will capture more light, greatly improving the quality of an image in low light. The higher resolution image may look better in bright light, and can also be cropped-to-zoom while still staying within the 10–12 megapixel normal for phone cameras. On Sony’s 48-megapixel sensor, featured in some new phones like the Nokia X71 and the Xiaomi Mi 9, the individual pixels are really, really small. The pixels are about 800nm in diameter. But the lowest frequency of light being records, at the far red end of the visual spectrum, is going to be at around 700nm. As a result, shrinking the pixels to less than 700nm, creates a sensor with inconsistent color response, or requires growing the sensor to about twice the area. There have been phones, like the Nokia N808 and Nokia Lumia 1020, with larger sensors. To support a larger sensor, requires a larger lens and a longer focal length — a longer distance between sensor and lens, unpopular with most consumers.
There is also a diffraction problem in resolving 48 megapixels, where the lens would have to be f/1.4 or faster. A f/1.8 lens, provides an effective resolution of about 33 megapixels. The Huawei phones use hardware, including a lens that matches the sensor, and they’re adding a bunch of software on top of it, under the guidance of Leica Camera Corp. Even without the color issue, doubling the pixel count isn’t going benefit the image resolution without going to a far more exotic, large, and expensive lens. The fastest phone lenses recently as the f/1.5 main camera lenses on various Samsung phones, which thought the image was too soft at f/1.5, so they offer a stopped-down f/2.4 mode as well. Not an option anyway if you want that much resolution with tiny, tiny pixels.
Samsung recently released a higher-still resolution sensor, which was pretty brazen given that even with a higher end professional camera, 100 megapixels is not needed. But in fierce competition, “because we can” often wins out over “because we should.”