I read about photography, a lot. I can read a book or magazine about photography or photo processing from cover to cover with no problem. With most any other genre of books I tend to get bored and put them down half way through, never to open them again. Books by Stephen King have been the notable exception. While I was leafing through a magazine yesterday, I came across an article about photography so I skimmed through it. It had some good information in it until I got to one line. I had to read that one line three times. You see this article was written by a well know author of photography books and was targeted at “new photographers.”
This part of the article listed three ways to help you hold your camera still in low light situations. The three tips listed are spot on. Here is the part I had to read three times: “If you have tried all three and you’re still getting blurry photos, raise your camera’s ISO setting, which increases your camera’s shutter speed, until your photos look nice and sharp.” Can you spot the reason I had to read it three times? On the surface it sounds like good information but it is not, it is misleading, especially for a new photographer.
Here’s the problem, ISO never has and never will “increase your camera’s shutter speed.” ISO does one thing and one thing only. ISO controls how sensitive the camera’s sensor it to light. That is all it does. This was true with film and it is also true with digital cameras. Changing the ISO on a digital camera or putting a different ISO film into a film camera will not change the camera’s shutter speed. Shutter speed and ISO are two completely different parts of the exposure triangle. ISO, Shutter Speed and Aperture make up the three parts of the exposure triangle. If you change one, you have to change one or both of the others if the lighting conditions stay the same and you want the overall exposure to stay the same.
Automatic modes on today’s cameras are great. I use aperture priority mode 95% of the time when I’m not working in a studio. In aperture priority mode, when I change the ISO does the shutter speed change? Yes it does. Did the ISO change the Shutter Speed? No it did not. There is a big difference! The shutter speed changes because the camera was forced to choose a new shutter speed based on the camera’s light meter and the fact that the camera is set to aperture priority and the lighting conditions did not change. If the camera is set to manual mode does changing the ISO change the shutter speed? No, you have to change the shutter speed based on what the camera’s light meter tells you if you want a similar exposure.
You might be thinking that I’m splitting hairs here and that’s fine. What is important here in this is very basic information all photographers should know. ISO, Shutter Speed and Aperture control three very different things. Each one is completely independent of the others and changing one does not change another. Changing one changes the overall exposure of the image. That is all it will do. I have been wondering why, when talking about ISO in the classes I teach, there is this confusion about the ISO changing the shutter speed. Now, I guess I know.