My name is Rob, and math is a part of my job. As promised, I'm going to school you on the exposure triangle. This is the first of four posts in a three part series on the exposure triangle. I'll talk about the three elements that determine the exposure of an image - ISO, Aperture, and Shutter Speed.
But before we jump into the first element we need to know a thing or two about "stops"
A stop is a term used in photography to describe an amount of light - the exposure value. Stops are a difficult thing to wrap your head around until you've been tinkering with a camera for a while. Think of them as units of exposure for an image, and they work on a logarithmic scale. That is to say, if you increase the exposure of an image by one stop, you've added double the amount of light - increase it by two stops, you get quadruple the amount of light. Subsequently, if you decrease the exposure by one stop, you get half the amount of light, and so on.
That sounds like a huge jump, right? It's not as large as you'd think, as light intensity doesn't work on a linear scale either, but that's another post. That being said, an entire stop is still a considerable jump. Stops are generally broken down into 1/3, 1/2, and full stops when making adjustments.
So how do you make adjustments? That's where the triangle comes in.
It's not really a triangle. There are just three elements that, when all taken into account, determine the exposure for an image. Three elements, three sides to a triangle. See? It's really a crappy metaphor. When trying to illustrate changes in one aspect of exposure, the shape of the triangle changes, but it doesn't really convey what happens to the exposure.
...or maybe I suck at drawing. I'm just going to use this keyboard to explain the whole thing.
First up - ISO.
ISO is a carry-over from the film days. It stands for International Organization for Standardization(It should really be IOS, right? Whatever, that's what it stands for). ISO 5-1:2009 primarily provides a system for describing methods of measuring or specifying the transmission and reflection properties of photographic and graphic arts materials(taken from iso.org). In other words, it describes the films sensitivity to light. Before the take-over of digital photography, films were commonly rated at ISO 100, 200, 400, 800, or 1600.
See how each number is double the last? There are stops in action for you.
With the advent of digital photographic sensors, ISO refers to the sensor's sensitivity to light. Nowadays, digital cameras have gone just a little bit bonkers. The latest full-frame camera from Nikon goes up to ISO 409,600 - a whopping 8 stops above 1600.
The thing about ISOs that high, is that the resulting image is basically unusable due to noise.
On a modern sensor, there are little buckets that collect light, and produce an electrical signal. These buckets can't change size, so the sensor makes up for this by amplifying the signal produced. When you jack up the voltage, digital noise is introduced into the image. Also, it pretty closely resembles the grain you got from film.
So, ideally, you want to use the lowest ISO value possible to achieve the exposure you want. Though with modern digital cameras, you don't have to be parked on ISO 100 all the time to get stellar image quality. At most concerts I shoot, I have zero qualms with shooting at ISO 6400, because the noise level is more than acceptable for a properly exposed image.
Notice I say "properly exposed" up there. When you have to adjust the exposure in post-processing, you'll introduce noise as well. Cameras have a hard time retaining detail in shadows and highlights when adjusting exposure.
And, that, boys and girls, concludes the first of four parts in this three part series. Stay tuned for more information than you ever wanted about the word aperture.