Behind The Aperture: The Engineering Behind Mobile Phone Cameras

In previous weeks, we have guided you through the process of producing a digital image and have explained some of the interesting things that we can look forward to seeing in the future. We first talked about the human visual system and how it’s used for digital cameras to mimic the human eye. We then spoke about the mechanics of images sensors and the basic functions of an image signal processor.  Most of the aspects of imaging discussed up to now can be applied to any digital camera. So this week we complete our column with the challenges that face one of the fastest growing camera markets, the cell phone camera.  

In the past five years, smartphones have exploded in popularity and the resolution and quality of their cameras have increased exponentially.  The original iPhone, released in 2007, contained a fixed focus, 2 MP sensor that was noisy and lacked decent image quality. Just five years later, the latest iPhone contains an 8 MP, auto-focus camera that rivals digital still cameras in terms of overall image quality and rendition.

Cell phone cameras have to be small to fit within the confines of modern day mobile phones. Thus, shrinking the components you would typically find in a point and shoot into a package smaller than 8.5mm², generally called a ‘camera module’, is a feat of engineering. Making a small camera means decreasing the size of all of the components including the sensor, the auto-focus mechanism, and the lens. When any of these components are shrunk, the quality of images captured from these cameras tends to be worse. In the end, image quality is a balance between consumer wants, marketing needs, and engineering practicality.

As we discussed before, a smaller sensor means less area to collect light, which causes reduced low light performance.  A smaller lens means manufacturing yields become difficult to maintain as lenses have tight tolerances. Smaller auto-focus systems require a completely different focusing mechanism compared to digital still cameras and increase the overall size of the module.

The sensor is the most discussed component of a camera and there are a few things regarding the sensors that require trade offs as its surface area is reduced. The size of the sensor will significantly influence the size of the entire camera module. A larger sensor requires a larger spread of light over its area, which will require, not only a larger lens, but also greater distance between the back of the lens and the sensor. More distance between the lens and sensor means a thicker module which causes a device to be thicker.  In order to deal with the size constraints of the sensor, engineers have been working hard to improve the efficiency of the sensor as well as the lens in front of it.  Improvements include using a backside illuminated sensor which allows the light to hit the substrate with less circuitry in the way and design a lens system that can pass more light to a sensor at a wider angle referred to as ‘chief ray angle’.

As technology progress, the consumer market demands higher resolution cameras, in thinner phones with better image quality.  Marketing as well as market pressure has been pushing the camera resolution of mobile devices upwards because it is a quantifiable number they can slap on the box.  Therefore, it is difficult to push the envelope on the resolution of the sensor and still ensure that the image quality is up to the standard expected in the current market.

One of the major differences between a typical point and shoot camera and one in mobile devices is the way focus is performed.  Point and shoot cameras use mechanical gears to move the barrel of the lens back and forth in order to focus; however, due to the size of mobile devices, this is not a possibility.  There are three major ways camera phones focus. The first and simplest is to not focus at all.  Fixed Focus cameras are typically found on the front of phones and webcams and have a fixed focus point which is set a little bit closer than the typical use case of that camera.  The major drawback of this method is that anything closer than the focus point will be blurry and it is impossible to get a close up photo.  Extended Depth of Field (EDoF) is a relatively new technology that is in use on some current market devices. EDoF encompasses a proprietary software algorithm that focuses the different colour channels at different points and then uses software to recombine the three channels into an image that has a wider depth of field without any moving parts.  Next, the most popular method of AF is Voice Coil Motor (VCM) AF.  Much like how a speaker works, the lens barrel is wrapped in fine motor wire and surrounded by permanent magnets. Current applied through the coil of wire produces an electromagnet that positions itself based on the magnitude of current through the coil. The lens can move the focus point back and forth to provide a wide range of focus from macro to infinity.   The system must work reliably time after time in various operating temperatures and while taking various forms of physical abuse.  This is a major problem for designers and is a major consideration when creating a camera.

The last major consideration we will discuss regarding the challenges of mobile imaging devices is the optics used for these cameras.  Most consumer point and shoot and DSLR cameras use polished glass for their lenses because plastic is too variable with temperature at that size. However, when the lenses are as small as the ones required for these cameras, plastic becomes a cheaper and easier solution.  The major problem is that the smaller the lens the harder it is to make reliably and at that scale very small inconsistencies make a big differences in performance.  At the latest Apple announcement they advertised a five piece lens system which improved image quality over a traditional four piece lens.  This means that five lenses need to be perfectly created and aligned in order to create a pleasing image and stand up to the abuse of a typical cell phone user.

The next time you are shopping for your next cell phone remember the challenges associated with developing the camera inside and appreciate that a good cell phone camera is a major accomplishment.  It’s also important to remember that the more megapixels the camera has does not necessarily mean it’s any better.  There are a lot of different things that need to work together in order to produce a pleasing image and the megapixel count is very low on the list of major players.

This will be the last instalment of our column but if you have any suggestions or comments about current or future articles, please email The Iron Warrior and we will keep them in mind.  I hope you enjoyed learning a little bit about the imaging world from us and will allow you to understand why megapixels don’t say much about the quality of a camera.