Cameras and Lenses

Summary

December 7, 2020 Pictures have always been a meaningful part of the human experience. From the first cave drawings, to sketches and paintings, to modern photography, we’ve mastered the art of recording what we see. Cameras and the lenses inside them may seem a little mystifying. In this blog post I’d like to explain not only how they work, but also how adjusting a few tunable parameters can produce fairly different results: Over the course of this article we’ll build a simple camera from first principles. Our first steps will be very modest – we’ll simply try to take any picture. To do that we need to have a sensor capable of detecting and measuring light that shines onto it. Recording Light Before the dawn of the digital era, photographs were taken on a piece of film covered in crystals of silver halide. Those compounds are light-sensitive and when exposed to light they form a speck of metallic silver that can later be developed with further chemical processes. For better or for worse, I’m not going to discuss analog devices – these days most cameras are digital. Before we continue the discussion relating to light we’ll use the classic trick of turning the illumination off. Don’t worry though, we’re not going to stay in darkness for too long. The image sensor of a digital camera consists of a grid of photodetectors. A photodetector converts photons into electric current that can be measured – the more photons hitting the detector the higher the signal. In the demonstration below you can observe how photons fall onto the arrangement of detectors represented by small squares. After some processing, the value read by each detector is converted to the brightness of the resulting image pixels which you can see on the right side. I’m also symbolically showing which photosite was hit with a short highlight. The slider below controls the flow of time: The longer the time of collection of photons the more of them are hitting the detectors and the brighter the resulting pixe...