What Is a Periscope Lens for Smartphone Cameras?

A diagram of a periscope lens on an Oppo smartphone.Oppo

The smartphone camera competition has always been a numbers game. Which company can boast of having the most megapixels, cameras, or (increasingly) zooms? When it comes to the laws of physics, however, optical zoom and thin phones don’t go hand in hand.

In July 2020, he was rumor that Apple could add a telephoto periscope to a future iPhone. Periscope lenses have been around for quite some time and they perfectly avoid the size issues encountered with traditional telephoto lenses.

Here’s how they work and what it means for the future of the smartphone industry.

In photography, size matters

The biggest limitations of photography have always been physical rather than technological. There are certain laws of optics that you simply cannot master. This is why the lenses of DSLR and mirrorless cameras are so big and heavy. To provide long focal lengths and wide openings, lenses themselves must be a certain size.

For example, a lens with a focal length of 200mm and a maximum aperture of f / 2.8 should have a front lens element more than 70mm (or 3 inches) wide. And that does not include any manufacturing considerations.

Smartphone cameras have the same limitations, but on a much smaller scale. Because they have smaller sensors, they get more magnification from shorter focal lengths. however, a lot of compromises come up with this arrangement.

The iPhone 11 Pro, for example, has a 52mm full-frame equivalent telephoto lens, which is actually just 6mm. It means if you want to take the same photo with professional dslr, you need 52mm lens. Since the iPhone’s telephoto camera sensor is 1 / 3.6 inches (about 5mm diagonally), you get equivalent magnification.

However, manufacturers are starting to run into problems. You can’t scale down much smaller camera sensors without making the tradeoffs unmanageable. Small sensors perform less well in low light and have more difficulty with higher resolutions.

If Apple wanted to get more zoom on the iPhone, it could (in theory) cut the sensor size in half. However, it would probably be expensive to produce and terrible to use.

The best option is to increase the size of the lens.

Avoid the problem

An illustration of the inside of the periscope lens on a Huawei P40 Pro + smartphone.Huawei

There are also problems with increasing the size of a lens. The iPhone 11 Pro is only 8.1mm thick. While a lens with a 6mm focal length doesn’t have to be exactly 6mm long, it should be close. Thus, it will always occupy a significant part of the space available on a smartphone. There just isn’t enough room to add a 12mm lens to a phone that’s only 8mm thick.

Unless you do it from the side.

A periscope lens works much like a periscope on a submarine. The light enters the front element and is then reflected at 90 degrees by a tilted mirror. It passes through all other lens elements before it hits the camera sensor and is then saved as a photo. By changing the direction in which the light travels, longer lenses should not be as deep as they can be wide.

For phone makers, that’s a big plus. Finding space for a longer telephoto horizontally is much more convenient than shrinking the sensor or making a thicker phone.

That way, manufacturers aren’t limited to 50mm equivalent lenses with 2x optical zoom (or, all of a sudden and with questionable marketing, 3x). It allows 100mm lenses (approx. 5x zoom) or even 200mm equivalent (approx. 10x zoom).

Of course, there are still trade-offs and the technology is new, but it perfectly skips the biggest limitation of adding optical zoom to a smartphone.

Digital zoom vs. optical zoom

Now if you think your iPhone already has 10x zoom, you are right, but also very wrong. There’s a reason we mostly referred to focal length, rather than zoom multipliers.

Indeed, there is an important distinction between optical and digital zoom (or enhanced, super-resolution, spatial or assisted by AI). With optical zoom, magnification is the result of the optical properties of a lens with a longer focal length. Distant objects look really closer, as if seen through a telescope, with no loss of image quality.

Example of a bad zoomed image of a dog on an iPhone. This 10x zoom on an iPhone is just a close crop of the 2x zoom.

Digital zoom, in its many forms, is just one fancy way of saying that a photo is cropped to look like an enlarged image. Certainly, digital zoom has come a long way. With high megapixel sensors, “binning” (several pixels treated as a single large pixel), and better scaling algorithms, manufacturers get better results.

Still, it’s really the same as just taking a photo and cropping it later. You are not getting the true magnification and there will always be a loss of image quality when you zoom in more.

Of course, you can’t build a marketing campaign around that little bit of truth.

Periscope lenses are available

Apple will not be the first to join the Periscope party. Chinese manufacturers (Oppo and Huawei in particular) have been playing with them for several years. The five-camera Huawei P40 Pro + has a 10x periscope telephoto lens that is equivalent to a 240mm lens on a full frame camera.

Wide shot of river in canyon. The wide-angle camera of the Huawei P40 Pro. Huawei

The more widely available Samsung Galaxy S20 Ultra has a 5x periscope telephoto lens that is roughly equivalent to a 100mm. However, Samsung’s exaggerated marketing tries its best to hide this information with some really ridiculous multipliers.

Zoom shot of man on rope climbing rocks beside river. The 5x optical zoom periscope. Check the image quality! Huawei

Like so many other features of the phone, even if Apple wasn’t the first, it will still make a splash when it hits the market. I think we can safely assume that between now and every time the eventual iPhone with a periscope lens launches, this will become a much more sought after feature.

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