One of the features Apple (AAPL 1.66%) seems most proud of with each new iPhone release is the rear-facing camera. Each year, Apple makes substantial advances in sensor technology, optics, and processing algorithms to improve -- sometimes dramatically -- the quality of the photos and videos its devices can deliver.

Though Apple makes, by all accounts, excellent mobile cameras, there's one area where Apple has consistently fallen behind some its flagship smartphone competitors: low-light performance. For example, according to testing at Tom's Guide, the iPhone X delivers worse low-light performance than the Pixel 2 XL smartphone.  

A person holding and looking at an iPhone X to unlock it.

Image source: Apple.

Here's why I think Apple has consistently lagged its competitors in this area and why I think it will finally close that competitive gap with next year's iPhones.

What causes Apple to fall behind?

There are two important technology factors that affect a mobile camera's low-light performance: the size of the image sensor and the maximum aperture of the lens. A bigger sensor tends to produce better low-light photos, because the size of each pixel is larger, which means each pixel can let in more light. In situations where light is scarce, the ability to let in more light is key to producing decent photos. And as website Expert Photography explains, "If you want to take a well-exposed photo in low light, you need a lens with a wide enough aperture to let more light in."

The iPhone X, Apple's best smartphone with its best camera, has two image sensors. According to TechInsights, the first sensor, which handles the main camera, has a die size of 32.8 millimeters squared -- and so does the second sensor. By contrast, the Samsung (NASDAQOTH: SSNLF) Galaxy S8 has a rear-facing camera sensor that measures in at 38.1 square millimeters. The number of pixels on the Galaxy S8's rear-facing camera sensor is the same as the one on the iPhone, but because the sensor is larger, each pixel can be significantly larger, helping low-light performance.

Apple also seems to be slightly behind Samsung's best in terms of lens aperture. The Galaxy S8's rear-facing camera has a maximum aperture of f/1.7 -- the lower the number, the wider the aperture. The maximum aperture on the primary cameras on the iPhone 8, 8 Plus, and X is f/1.8.

The solution? Bigger sensor, more aggressive optics

The sense that I get is that Apple has been limited by a few factors with respect to its iPhone cameras over the past few years. First, it had to squeeze its latest camera sensors and optics into a phone with the same basic shape as the iPhone 6 for four straight generations -- the iPhone 6, 6s, 7, and 8/X.

The larger-bodied Plus models, as well as this year's iPhone X, were endowed with technologies that couldn't fit into the standard iPhone casings, such as dual-lens cameras, but fundamentally, all three phones had to use the same optics and sensor technologies for the main rear-facing camera.

Apple's iPhone X.

Image source: Apple.

Next year, Apple is expected to introduce three new iPhones: an upgraded version of this year's iPhone X, a lower-cost iPhone with a 6.1-inch liquid crystal display, and a larger version of this year's iPhone X with, of course, upgraded internals. These fundamentally larger and redesigned casings should allow Apple to fit in much larger rear-facing camera sensors to dramatically boost image quality with an emphasis on low-light performance.

On top of that, I think Apple has been fundamentally hamstrung by the cost structures of its devices. Apple had to develop optics and sensor technology that would fit comfortably in a device that sells for $699 and is expected to generate substantial profit per device. With Apple experiencing a fundamental surge in the average selling prices of its new iPhones as users have shifted toward Apple's Plus models as well as its ultra-premium iPhone X, Apple can now afford to build technologies targeted at pricier devices. That means more headroom for Apple to develop and use pricier components in its devices. 

So, next year, I expect all of Apple's new iPhones to ship with dramatically upgraded camera sensors and for at least some of the new models -- probably the higher-end iPhone X successors at least -- to deliver dramatic improvements in optics as well.