Friday, June 11, 2010

Photography Tips- What is ISO?

What does ISO stand for?

What ISO really is:
the term ISO stands for International Standards Organization. Every time you buy film it has a number like 200 or 400 or 800 on it and the Kodak boxes will usually let you know if it’s good for indoor or outdoor lighting. This description is a direct result of the film’s ISO. Now digital cameras have ISO settings because ISO also affects the exposure of your images. I will touch on this bit in a minute.

So what is the ISO number?
The ISO number of film indicates how sensitive that film is to light. Film is produced differently so that some is more sensitive than others to light. This is why some film is good for outdoors while some films have ISO’s that make it better for indoor or night photography. Since ISO stands for the International Standards Organization, all this really means is they determine what sensitivity gets what number indication, so that when you purchase a roll of film that says 400 on it, it will react the same to light as the last roll of film you bought, no matter what brand you bought from.

ISO numbers:
So which ISO numbers are used for which types of light? The lower the ISO number, the LEAST sensitive it is to light. So an ISO of 100 or lower is going to be ideal for outdoors on a sunny day because the light is very strong. The higher the ISO the MORE sensitive it is to light, which means it takes very little light to create a good exposure. So an ISO of 800 or more would be ideal at night, so that if you’re recording moving subjects in dim lighting, you can get a decent exposure while stopping the motion of those subjects.

Why ISO is important:
ISO is very important for manual users. If you ever use your camera on manual then you probably know the relationship between shutter speed and aperture to make a good exposure. If you don’t know this, don’t worry, I have an article that explains it in more detail. Shutter speed is used to stop or blur motion, while aperture is used mostly to determine how much of a photograph will be in focus. But when you change either the shutter speed or the aperture you directly affect the exposure and must compensate to get a good one. But let’s say I want a small aperture opening so that I can get everything in focus, well this opening lets in a small amount of light, so we compensate by leaving the shutter open longer. Well what if I’m taking a photograph of my dog who is running around in my dimly lit apartment, but I want everything in focus. I would use a small aperture and I’d use a slower shutter speed. But then my dog would be blurred and I wouldn’t get a good photograph. By changing my ISO from something like 400 to 1000 it allows me to use a faster shutter speed, because the film or digital sensor is now more sensitive to light, and doesn’t need as much light let in to make a good exposure. Now I can stop the motion of my dog running, have a smaller aperture, and still make a good exposure.

Why not use 800+ ISO all the time?
Although this seems like a good strategy for all lighting situations, the higher the ISO is the grainier the image is. If you are working digitally then this translates to how much noise will be in your photograph. To get a really nice clean photo, you want to use a lower ISO. It can be an artistic choice, however, to give an image noise or grain and in that case you may choose a higher ISO. But for most people, we want to use the lowest number possible for our subject matter.
So a quick recap:

Low ISO (ex 100) is good for outdoor, strong lights, and still subjects. It also produces a very crisp clean image.

High ISO (ex 800 or 1600) is good for low light or night photography and moving subjects. However it also produces more noise and grain in the image.

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