Sense and Sensor-bility

While I was looking things up for my post about how I ended up with the Canon Rebel T6 and my lens trifecta, I also got sucked into finding out about camera specifications, which led to camera sensors. Turns out it was a bit of a rabbit hole.

To start, digital camera sensors are the equivalent of film in analog cameras. They capture light and record color to reproduce images. Older digital cameras often used CCD (charge coupled device) sensors, but they’ve since been replaced by CMOS (complementary metal oxide semiconductor) sensors. Both types of sensors rely on photosensitive semiconductors that convert light into electric charge. The charge is then converted into voltage, and the correlation between light (brightness) and voltage can be used to recreate the photograph. CCD sensors are analog devices that send the charges from each semiconductor through an output node to be transformed into voltage while CMOS sensors convert charge to voltage at each individual pixel. While CCD sensors were originally the sensor of choice because of their low noise, CMOS sensors are cheaper and require less power, and technological advances have improved image quality to equal or better CCD sensors.

For color photography, a color filter array sits on top of the sensor. Commonly used in digital cameras today is the Bayer filter, which contains RBG color filters arranged in a checkerboard-like pattern on the pixel array of a sensor. The Bayer filter has 50% green filters, 25% red filters, and 25% blue filters, which reflects the physiology of the human eye. Because each pixel now only sees either red, blue, or green, the image needs to be processed with a demosaicing algorithm to give each pixel RBG values. For example, a green pixel has its green value measured directly, then can get its red and blue values by interpolating from its red and blue neighbors.

At the library
[Samsung Galaxy J3 Orbit, 3 mm, f/1.9, 1/40 s, ISO-125]

Finally, on to sensor size. In filmmaking and film photography, 35 mm film was the most commonly used film size, so when digital cameras became a thing, a sensor size that matched 35 mm film – 36mm x 24mm – became known as “full frame.” In general, bigger sensors/film require bigger lenses/cameras that are more expensive, so camera manufacturers also came out with so-called “crop sensor” or “APS-C” cameras with sensors typically sized at 22.3mm x 14.9mm (Canon) or 23.5mm x 15.6mm (every other brand). Then there’s the Micro Four Thirds System, 1-inch sensors, and once you get smaller than that (as in a lot of point and shoot/phone cameras), sensor size is usually expressed as something like 1/2.3”. This number ostensibly refers to the diagonal of the sensor, but it’s based on old video camera tubes, which were sized based on outer diameter, not usable inside area, so a 1/2.3” (0.435”) sensor actually has a diagonal of around 0.3” and dimensions of 0.24” x 0.18” (6.17mm x 4.55mm).

Argo Park on the Border to Border
[Fujifilm FinePix XP55, 7 mm, f/4.3, 1/58 s, ISO-100] 

Once you have your sensor, you then divide it into thousands of areas that each correspond to a pixel. The bigger a pixel is, the more light it individually captures, and the better it can capture whatever you’re photographing. For example, the Fujifilm FinePix XP55 has a 1/2.3” sensor and a resolution of 4320 x 3240 so has a pixel size of about 1.4 microns on a side (6.17 mm/4320 = 1.4 microns). My Rebel has the Canon APS-C sensor and a resolution of 5184 x 3456, giving a pixel size of ~4.3 microns. So even though the megapixels aren’t that different (4320 x 3240 = 14 million pixels, or 14 megapixels vs. 5184 x 3456 = 18 MP), the larger sensor and pixel sizes mean improved picture quality.

Tree at sunset
[Canon Rebel T6, 55 mm, f/8, 1/160 s, ISO-100]

Out of interest, I tabulated the specifications for a number of cameras I’ve owned below. The top of the table (click to enlarge) is digital cameras and the bottom is phone cameras. For fun I included Canon’s current most expensive professional mirrorless camera, the R5, which at $3899 for the body alone I don’t anticipate owning any time soon, if ever (I’ll be holding out for a mirrorless crop sensor R-mount camera for when I need to replace my T6; the current APS-C mirrorless line uses a different lens mount with few native lenses and like Canon’s entire range of DSLRs and lenses, seems to be a developmental dead end.). I also put in Apple’s latest and greatest offering, the iPhone 13 Pro, for comparison with my phone, which cost about the same as a pair of Airpods.

I was surprised by a few things, and unsurprised by others. First, I didn’t realize that my DSLR didn’t have many more megapixels than the FinePix XP55 or even the PowerShot SD1300. The sensor size, however, is over 11 times larger in area, and jumping up to a full frame sensor, though it doesn’t look massively larger, still more than doubles sensor area. On the flipside of things, I think I’m justified in my dislike for my phone camera, which at best has a sensor less than half the size of a 16-year-old digital camera. And even though one of the most advanced iPhones has surpassed my more recent point and shoots in sensor size, that sensor is still less than a sixth of the size of my entry level DSLR’s, which is one reason why people still pay hundreds and thousands of dollars for DSLRs and mirrorless cameras.

¹My phone camera (unsurprisingly?) not only has microscopic sensors, but also non-standard size sensors, so the inch equivalent is approximated from the sensor dimensions.

²I couldn’t find the aspect ratio for the iPhone’s sensor, but it’s stated to be a 1/1.65” sensor, so those are the approximate sensor dimensions and resolution based on a 4:3 aspect ratio and the given megapixels. Calculations are for the iPhone’s best (largest sensor/most megapixels) camera.