The 108-megapixel smartphone camera explained


A couple of weeks ago when Xiaomi announced its latest smartphone the Mi 10 which it is launching in India – its headline feature was its new 108-megapixel camera sensor. The company touted that this was the largest sensor that was ever installed in a smartphone and proudly pronounced that it had developed the technology in-tandem with Samsung’s imaging division. The component in question is the ISOCELL Bright HMX image sensor which has a 108-megapixel resolution. The 108-megapixel camera sensor first appeared late last year when the Beijing based company unveiled the Mi Mix Alpha phone, after that, it showed up in its Mi Note 10 Pro — both these phones were never launched in India. However, things got interesting in 2020 when Samsung announced its Galaxy S20 Ultra which also now had this camera system. After the announcement of the Mi 10 and Mi 10 Pro, even Motorola went ahead and announced the Moto Edge+ with the same sensor. So what’s really groundbreaking about this camera technology? And is it worth the hype, especially the way Xiaomi is talking it up? 

Xiaomi’s camera isn’t the largest sensor on a smartphone 

Firstly, it is very important to set the record straight — Xiaomi is lying quite blatantly if they call this 108-megapixel sensor the largest one on a smartphone. Currently, the phone with the largest sensor is the Huawei P40 Pro. This was pointed out first by Technical Guruji in a tweet last Friday. What’s more interesting is that even if one goes by history, this Samsung made 108-megapixel sensor isn’t the largest we have seen on a smartphone. Samsung and Xiaomi say that the sensor size is 1/1.33-inches which makes this smaller than the Huawei P40 Pro which has a 1/1.28-inch sensor. The plot thickens even further when we look at some older smartphones which high-level capabilities. In 2012, Nokia’s groundbreaking PureView 808 phone had a 1/1.2-inch sensor. In 2014, there was the Panasonic Lumix CM 1 which had a 1-inch sensor which remains the largest we have seen on a smartphone yet.

But why are brands posturing to get that fact across? That’s because of physics and marketing — the bigger the sensor, the thumb rule states that it would be able to absorb more light which usually means the better camera system. And, all of these phones are fighting for that slot of being the king camera phone which is why this posturing is all-important. 

That being said, the size of the sensor isn’t everything. For example, when Nokia updated the PureView 808 in 2013 with the Lumia 1020 – it actually used a smaller 41-megapixel sensor at 1/1.5-inches. It has evolved its oversampling technique which yielded better results at a 5-megapixel resolution. So there is no reason why a smaller sensor cannot reproduce better images than phones with larger sensors and higher resolutions. That’s also the reason, the iPhone 11 and Pixel 4 despite their pedestrian specifications have the best cameras for most people. 

So why the 108-megapixel resolution?

What are Samsung and Xiaomi trying to achieve with this insane 108-megapixel camera which is coupled with an abnormally large sensor for a smartphone?

The intent here is not to shoot 108-megapixel photos which even DSLRs don’t pursue. Instead, as most top-tier smartphones have proven that 12-megapixel is the optimal resolution for the photos and that’s what happens on the 108-megapixel trotting Galaxy S20 Ultra when it shoots in auto-mode. It achieves this resolution by a technique called oversampling introduced in the Nokia PureView 808 in 2012. It combines 9 pixels to form one superpixel which more light-sensitive.

That’s why the Nokia PureView 808 shot photos at 5-megapixels even though the main camera had a 41-megapixel sensor (38-megapixel effective resolution). This was done by combining 4 pixels into a superpixel with a subpixel size of 1.4um.

But why was there a need to downsize the resolution of the image and create superpixels? The problem was physics — despite the large sensor, the sensor size was still too small, constrained by the size of the smartphone. This also would result in a very low subpixel size on individual pixels as megapixels are divided by the size of the sensor. Hence, by combining the information for multiple pixels in one superpixel (like the one on the Nokia 808), these cameras compromise overall resolution for greater detail and clarity at a lower resolution and also allow for lossless zoom.

This works for most people as even a 5-megapixel image is great for printing photos and will look good on a 21-inch monitor. In the case of Samsung, you get 12-megapixel photos which are the same size as an iPhone photo.

But on the Mi 10, the sensor works slightly differently. In fact so much so that Xiaomi executives say that its sensor is a different module from what’s there in the S20 Ultra. This is also ratified by Samsung which calls the sensor on the S20 Ultra the ISOCELL Bright HM1. However, in practice, the Samsung Galaxy S20 Ultra is a minor update to the original ISOCELL Bright HMX as it interchanges the tetra cell pixel binning technology with a nona-cell pixel binning method in the chroma of the sensor.

This allows the phone to combine 9 pixels into one pixel instead of doing an oversampling of 4 pixels into 1. So, on the Xiaomi Mi 10 and even the Moto Edge+, the 108-megapixel camera is combining 4 pixels for a superpixel allowing for an effective higher resolution. On the Mi 10 users will be shooting 25-megapixel photos while on the Moto Edge+ the resolution is 27-megapixels. Xiaomi was able to reduce the resolution further as it believed that would yield better results.  

A sensor of such high resolution has benefits to overall image quality and even video — for instance, both the Galaxy S20 Ultra and the Mi 10 can shoot 8K video. It also allows for higher resolution photos with a higher subpixel size allowed by techniques like pixel binning and oversampling which creates superpixels by combining multiple pixels without sacrificing resolution.

A balancing act between subpixel size and overall resolution 

These superpixels now have a higher sub-pixel resolution which is the defining characteristic for image quality. For instance, on the Google Pixel 4, which has a standard 12-megapixel camera the sub-pixel size is 1.4um. This figure is the same for the iPhone 11 Pro and iPhone 11 primary sensor. On a phone like the Huawei P30 Pro which was one of the best camera phones of 2019, the sub-pixel count is higher than the Pixel 4 and iPhone 11 at 1.6um only when its 40-megapixel main camera shoots photos at 10-megapixels after oversampling. 

On a 108-megapixel camera sensor which has a size of 1/1.33-inches, the sub-pixel count goes down to 0.8um. This would be usable in day-light but quite poor in low-light. Using the nona-cell oversampling technique Samsung is able to make that subpixel size to go up to 2.4um which makes it very light-sensitive hence improving low-light performance at its 12-megapixel resolution. On the Mi 10, the camera shoots at 25-megapixels which is more than twice the resolution of what one can achieve on an iPhone 11 but you also get a higher sub-pixel resolution of 1.6um.

Back in 2013, HTC with the One series of smartphones introduced its UltraPixel cameras which had an effective resolution of 4-megapixels but the resolution of the pixels was at 2um. The reason that system failed was because 4-megapixels was deemed to be too low a resolution for a smartphone camera. It was also not great for marketing spin. Samsung has tripled the overall resolution while also improved the light sensitivity of the sub-pixels from that. 

Right now, it is too early to say which approach wins out, though the reality is neither phone is going to be shooting at 108-megapixels unless specifically asked to do so because the photos will be abnormally large in size and they wouldn’t be that great in low-light because of the low subpixel size. Both Samsung and Xiaomi are trying to provide a balance between resolution and low-light performance. 

A higher resolution means users can crop in more into the photo and can have theoretical lossless zooming properties. On the Galaxy S20 Ultra there is a dedicated 48-megapixel telephoto lens that does up to 5x optical zoom and 100x space zoom, while on the Mi 10, there is no zoom lens. It also boils down to the software capabilities of what each brand thinks it can do with the software. For instance, Samsung was late to adopt a night mode on its phones. It only came late last year with the Note 10+ while Xiaomi had been providing one for years even on its cheaper phones. It can be argued that Xiaomi has more evolved software which is why Samsung is using a more extreme nona-cell technique.

Camera optimisation is going to play a huge role in the end results of the camera 

Perhaps the biggest thing at play with most of these cameras is software optimisation. As Neil Shah, VP and partner of CounterPoint research said, “(hardware is) 40% of the contribution to the performance, software and optimisation is the rest 60%”. That’s why with seemingly inferior optical hardware phones like the Pixel and iPhone are able to rise beyond the optical capabilities of their camera systems. They piggyback off the capabilities of their software and silicon. Similarly, there will be a difference in the end result of the Samsung Galaxy S20 Ultra and Mi 10 108-megapixel camera approach and results. 

This difference is also further perpetuated by the difference in processing hardware which is married to different software and algorithms. On the S20 Ultra, the ISP comes from the Exynos 990 SoC while on the Mi 10, it will come from the Snapdragon 865 processor. Most people have concluded that the Snapdragon has the better ISP which could play in the hands of Xiaomi phone. 

Samsung’s phone has also been in the market for a while. There have been issues with related to minimum focusing distance, fringing and overall sharpness of a photo across the focus points. Xiaomi believes that has had more time to tune its software taking into account some of the problems faced by Samsung. It also claims that because that its sensor is different from the one Samsung uses, it will have fewer issues combined by the fact that it has used this camera system on two phones already which gives it the experience of tuning the software properly. 

The problem for Xiaomi could be physics. On phones like the Mi Note 10, its 108-megapixel camera did well but it had the focussing and fringing issues. Though it was also shooting at a higher resolution of 27-megapixels. Sure, it was hamstrung by a slower and inferior SoC but only so much can be done via software processing derived from a powerful chipset to overcome physics. 

The broader conversation around the viability of 108-megapixel cameras on phones will be decided by the performance of the Mi 10 in India considering the number of reviewers based out of the country.

It will also be first sub Rs 50,000 phone to use this technology which could democratise it if it works. In the case it doesn’t work than a more tempered approach would make more sense with sensors that are bigger but resolutions maxing out at 64-megapixels. This is argument is ratified by the fact that even today an 8-year-old Nokia PureView 808 is able to hold its own against the best camera phones out in the market — which is reflective of the conviction in Nokia’s approach and how advanced its camera technology was that it’s yet to be substantially surpassed.