How variable aperture works on phones

A phone's main camera is a stack of parts about as tall as a sugar cube. Light comes in through the cover glass and the aperture, the lens focuses it, a filter takes out infrared light, and the sensor turns it into a photo. In most phones the aperture is just a fixed hole in the front of the lens. A variable aperture replaces it with a ring of six tiny blades.

The whole iris is driven by one part, the drive ring. Each blade turns on a fixed pin, and a second pin on the blade rides in a slanted slot in the ring. Turn the ring and the slots push all six pins at once, so the blades swing in or out together and the hole stays round. A magnet and a coil turn the ring, and a sensor tells the phone where it is, so it can stop at any f-number in its range.

The lens is focused on the yellow arrow, so its light meets in a sharp point on the sensor. Light from the far arrow meets just in front of the sensor and lands as a small disc instead, which is what blur is. A smaller hole sends a narrower cone of light, so that disc shrinks and more of the scene looks sharp. It also lets in less light: f/4 passes an eighth of what f/1.4 does, so the phone uses a slower shutter or a higher ISO to keep the photo bright.

Light reaches the sensor as a cone. At f/1.4 the cone is wide and comes to a spot about one 1.6 micron pixel across. At f/4 the cone is narrow, and light squeezing through a small hole spreads out a little, so the spot grows to over three pixels and the finest detail softens. Add the cost of the motor and the space it takes, and the fact that small phone sensors already keep most of a scene sharp, and most makers simply leave the aperture wide open.

Nokia tried it in 2009, Samsung's Galaxy S9+ switched between two settings, and Huawei brought real blades back in 2022. Today it turns up mostly on phones with big 1-inch-type sensors, where a wide-open lens can blur too much of a close-up.