Definition
A camera lens design for thin devices like smartphones. It bends light at a right angle and lays the lens down along the inside of the body, so you can zoom in clearly on faraway subjects without adding thickness. Like a submarine's periscope, it uses a prism to get a long focal length.
Why did smartphone telephoto cameras stick out so much?
Think of a telescope. The tube is really long. To see a faraway object big and sharp without losing quality, the physical distance between the lens and the image sensor (the plate that receives light and turns it into a digital signal) has to be long.
But the smartphone we use every day is less than 1 centimeter thick. If you stuck a high-magnification telephoto lens straight through the thickness of a thin phone, the camera would have to stick out about a finger joint, causing a serious camera bump.
Of course, there is also digital zoom, which just enlarges the photo on screen. But digital zoom crops and stretches the image by force, so the quality breaks down and it turns blocky like a mosaic. We needed a clever trick that keeps real image quality without spoiling the thin design.
Bringing the submarine's eye inside the smartphone
The clever solution to this puzzle is the submarine's periscope. A submarine hides just below the surface, and uses a prism mirror inside a periscope that is extended upward to bend light at a right angle and look at the scenery above the water.
The periscope lens carries this same principle into the smartphone. Light that enters through the lens opening on the outside of the phone hits a prism tilted at 45 degrees, and is bent 90 degrees into the body of the phone.
Because the light is bent sideways, several lens elements and the image sensor can be laid out like a train along the wide inside of the phone. The phone stays thin, yet it gets the long light path that telephoto shooting needs.
To be a little more precise
A phone with a periscope lens can zoom in on a subject 5x or 10x farther away, and the picture stays sharp instead of turning mushy. That is because it is true optical zoom: it gathers and magnifies real light, instead of stretching the image pixel by pixel.
To be a little more precise, the periscope design has technical difficulties too. Because the lens is laid down inside a narrow body, it is hard to make the lens wide, so the amount of light coming in tends to be small. In a dark room or at night, when light is short, photos can easily come out blurry from shaking.
To get past this limit, recent smartphones use optical image stabilization in the telephoto camera, which moves the prism, lens, or image sensor in tiny amounts to cancel out the shake. Thanks to this complex optical technology, we can capture the surface of the Moon or a stage in a far-off concert hall with a thin phone in our pocket.
Common misconceptions
A periscope lens is a feature that enlarges photos with AI or software.
It is not digital zoom that adds pixels with software. It is a pure optical zoom technology that physically arranges a real prism and lenses to gather light.
Where you meet it
When you zoom in on the craters on the surface of the Moon with a smartphone camera, the periscope lens is at work.
When you zoom in on a singer's face on stage from the second-floor seats of a big concert hall, it is used as high-magnification optical zoom.
In one sentence
By bending light 90 degrees and laying the lens down inside the phone, this technology creates clean, high-magnification optical zoom in a thin body.