When it comes to the last side of the Exposure Triangle you need to be nice, because it’s sensitive. ISO is sensitive. REAL sensitive.
As a matter of fact it is the only thing in your camera that handles the sensor’s sensitivity to light. It works like this: Low ISO (50 or 100) is less sensitive to light and thus, needs more of it to make an exposure. As you move up the scale to 200, 400, 800, the sensor on your camera becomes more sensitive to light and needs less of it. You might think this is awesome. Just jack up the ISO to 32,000 and shoot in the dark! Wahoo!
Look, you’re camera is not a bunny and does not eat carrots (needed to get a bunny reference in) and there is a consequence for being more sensitive to light, it’s that the sensor is more sensitive to noise as well. Let’s take a look at how a sensor collects light. I’ll try to make this easy.
Pretty, isn’t it? That’s the sensor in your camera. It is made up of all these dots. MILLIONS of them (that’s the Mega in Megapixel). This image is looking straight at the sensor and there are various patterns to these dots, it’s ever evolving. Let’s turn it on its side.
This is looking at the side of the top row. Over each actual light sensor is a color filter to only let in red, green or blue. Above that is a microlens to focus the light. Look at your keyboard and pick four keys in a box shape, 2×2. Now imagine 10+ million of these sensors in that space. Amazing, simply amazing.
The sensor is energized with electricity to accept light and record when it is struck (when the shutter opens to let light hit it). All of those sensors are energized. And that’s where the limits come in. When set to a low sensitivity (ISO 100), there isn’t much current passing through the sensor area. This lack of sensitivity is overcome by letting in more light, either by a longer shutter speed or a fully open aperture. But when you adjust the ISO to around 400 or 800 something else starts to happen. That sensor area needs to become more energized to detect the lower level of light. As it becomes more and more energized, with electricity, every sensor gets so energized that little bits of electrical current get lose and starts to bug their neighbor. Not only are the sensors sensitive to light, they are are sensitive to electrical current (as that is how they measure and transfer the light information). Think of this neighbor pestering as akin to static on a radio. If you run a blender while the radio is going (humor me and pretend you have an actual radio that still gets an analog signal) that electrical current can cause static on the radio. And a mean margarita.
That ‘static’ causes noise when the camera computer gets all the info from every sensor and things aren’t 100% as they should be. There are a couple of types of noise and I’m not going to get into all of them. Noise (not grain, like in the film days) is ugly, should be shunned and possible fed to wolves. Noise is caused be the increased sensitivity required to capture lower levels of light. And it captures the electrical interference from neighbor sensor. Remember your keyboard? 10+ MILLION of these suckers in that space. Not a lot of elbow room.
Guess what we call the 50, 100, 200, 400, 800, 1600, 3200, 6400, etc… of the ISO scale? Stops. If you guessed it right you get a +50 bonus (oh yes, you get to keep score). 100 is twice as sensitive as 50. 200 is twice as sensitive as 100. Finally the math gets easy! This means 100 needs half as much light as 50 for the same exposure. Ha ha ha ha….you walked right into that one. I know, 50 to 100 is twice as much, but it needs half as much light? That’s the way it goes. Here’s the last piece of the puzzle.
At ISO 3200 the images will be brighter than at ISO 100 (if you leave shutter speed and aperture the same). High ISOs are great for speeding things up and capturing action. Lower ISOs are ideal for scenic shots as they will have less noise (and typically aren’t moving fast or at all).
How much noise is ok for you is something you need to play around with. For me and my Canon 7D I’m ok with 800 at times, but the lower the better. It also matters on the subject as dark solids can tend to show noise more readily than mixed light color areas. Try to use the lowest ISO you can for the most noise free images.
Let’s put them all together on one card.
You are welcome to download the image directly above as long as it is only used for personal information. You may not do commercial things with it, post it on the internet or do other things I do not like. You may print it to keep with you for reference sake. You may print an extra copy to send to your Mom. She called today and is wondering why you never write letters any more.
And that is the end of the Exposure Triangle. Shutter Speed affects blur. Aperture affects Depth Of Field. And ISO controls the overall sensor sensitivity to light. And they all need to be in balance to make a proper exposure.
Next Up: How To Hold A Camera
But not before a Pop Quiz!! How many stops of light difference is there between 1/60 f/8 ISO 400 and 1/15 f/8 ISO 100? And how many difference between 1/60 f/8 ISO 400 and 1/15 f/8 ISO 3200?
Questions? Pop ’em like Pez in the comments section below. And have a great weekend.
EDIT: Some examples of noise were requested. How silly of me to not show what the heck I was talking about! In these three examples, the noise may look grainy, but zooming in (click on image) you can see it’s slightly multicolored and cruddy. It takes away from the sharpness of the image as well.
Next Up: Metering Modes
31 Days To Better Photography is a series written by professional photographer Peter West Carey on The Carey Adventures.Com. The series is designed to unravel the mysteries of photography so you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.