We've made it to the final chapter of the Exposure Series, where I will explain ISO, and how it works in concert with Shutter Speed and Aperture. Make sure to ready the two previous installments, Exposure Part One: The Almighty Aperture and Exposure Part Two: All About Time AKA The Shutter (Speed), before continuing as this chapter focuses heavily on terms and concepts covered in those previous posts. Now with that out of the way, ISO is an interesting carry over from the days of film. With shutter speed and aperture, the mechanics are identical on a film camera and a digital camera. But since ISO is a quality of film, it's often overlooked when learning about digital photography. Let's remedy that!
Sitting Demon.
Simply put, ISO is the film's (or sensor's) sensitivity to light. To understand ISO from a pragmatic standpoint, we have to think about it in terms of its original application with film, as its usage in digital photography is indicated by its film use. Let's get the name out of the way first, ISO stands for International Organization of Standardization. We refer to ISO in film because there was a point where there were several film sensitivity standards, but ISO became the most widely accepted standard. Now that we know that, it's really not relevant to how film itself works, but it helps you understand a bit of the history of film. So, how does film work? Basically, you have a piece of celluloid with silver grains which are exposed to light, creating an image. If you have film with smaller grains of silver, you get a sharper, finer image. With larger grains, there's less contrast and sharpness, and you get a grainy look, almost as if there were grains of sand or dust on the photograph. Smaller grains take more light or a longer time to expose, and larger grains are able to be exposed with less time or less light.
Stallion.
Knowing the above, how to we apply that to digital photography? Well, the sensor in your digital camera has its own way of affecting exposure, which works differently but in parallel to film ISO. That is to say, the algorithms and machinations of digital sensors are too complicated to cover here, but they play by the same rules as film ISO. Lower ISO values, like 100, are less sensitive to light, but produce a less grainy image. And as you work your way up, the sensor becomes more sensitive, but you'll see noise on your photos, which is the sensor's equivalent to film grain. So, how does that fit into the exposure balance?
Station.
Think about it like this: shutter speed and aperture relate to each other in the same way they relate to ISO. For good exposure, you can change aperture, shutter speed, or ISO. Knowing when to change the right setting is key to creating great images. You may find yourself wanting to take a photo of a scene, and capture every single detail, so you set you camera to f22, but this makes your exposure darker. You have some elements in your scene that are moving, and would create motion blur at lower shutter speeds, so you need to be at least 1/150 shutter speed. At this point, you have the settings you want, but your exposure is still too dark. This is where ISO comes in, it gives you that extra exposure that helps you create the image you want. But don't forget that every time you raise your ISO number, you are increasing the potential noise.
Castle.
Sometimes, you don't want to have to set up your shot, your composition, your shutter speed, aperture, and then ISO if needed, so learning how all of the pillars of exposure interact is key to getting the shot you want quickly. Your camera's automatic mode does something similar by calculating one of the many options for perfect exposure, but it doesn't take in to account your artistic voice. In the last entry, I mentioned that when taking photos of still subjects, you should consider what aperture you should use first, then use shutter speed to compensate for over or under exposure. But now that you understand how ISO works, that is the first thing can consider when shooting, even before you leave your home! If you know that it's a sunny, bright day, you don't need to set your ISO too high. However, if you know that your primary subject will be deep, detailed landscapes, maybe you should set a higher ISO, which will give you more breathing room when it comes to your aperture and shutter speed.
Rainy Days.
Now that you know how all three pillars of exposure work together, and see the delicate balancing act, go out and shoot! Shoot on sunny days and see what settings you choose. Shoot on grey, dreary days; shoot indoors, outdoors! You'll start to visualize what your image's exposure will look like before even hitting the shutter button. As with all things, this takes a lot of practice, so go out there and shoot!