Shallow portrait of Kuiper. Only his eyes are in focus.

Depth of Field, Part 1

Kuiper gets out of his depth (of field)

Have you ever wondered how some pictures appear to have only the subject in focus, but others appear to have everything in focus? Depth of field is due to an effect called the CIRCLE of CONFUSION (yes, really!!) To avoid creating our own CIRCLE of CONFUSION, let’s first look at the basics of how cameras work.

A camera is fundamentally just a hole that souls…er..light rays travel into so we can catch them. In a DSLR, the light hits a mirror, then bounces to a rectangular electronic image sensor (or film if your camera is retro.) In mirrorless cameras and phones the light hits the sensor directly.

(The bigger the sensor is physically, the more light the camera can gather, which is a lot of why “real” cameras can still generally take better pictures than phones, particularly indoors. My mirrorless sensor has nearly 15x the surface area of my phone camera’s sensor. Outside it doesn’t matter quite as much with today’s phones, but inside it makes a huge difference.)

Ok, so we have a hole for light and a little electronic thingy in the camera to collect it, but our camera won’t be very useful unless that light is actually hitting our sensor (otherwise known as “focusing.”) Lenses to the rescue! Because physics, we can use a roundish piece of glass or plastic to condense light into a “cone,” converging into a single point on our sensor.

In this picture, I was aiming to to get the light reflected by Kuiper’s eyes into focus (directly onto my sensor.) Since my sensor is two dimensional (flat), the “plane” of focus will also be flat (think of holding up a piece of paper and focusing on it with your eyes while you move it closer and further away from you.) The further away from that “slice,” the further out of focus other parts of the picture (e.g. his nose, the tree in the background) will be.

With me so far, or do you need some diagrams? Turn on post notifications so you don’t miss part 2: Aperture Science!

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