Photography 101 - What Is ISO?

What the Heck is ISO?

I’m glad you asked! Welcome back for this week’s lesson on ISO (or ASA as some of you older folk might know it as) and what a blessing or a curse this can be for your photography. In simple terms, ISO refers to the Film Speed. No not how fast your film runs or the frames that it takes per second. This is Film Speed.

Film Speed = Light Sensitivity

We use ISO to measure the film’s sensitivity to light. The more sensitive the film is to light, the less time it will take to expose. There I go again, talking about those inverses! Being commonly termed slow film, relatively insensitive film requires much more time to expose.

Now I’m not going to really get into the mechanics and philosophy of where the terms came about because quite frankly, I know that most of you photographers don’t really care about that. Most of you are asking the question, “How does this help me take better photos?” Let’s play out a scenario.

You are at a wedding, in a church, where they do not under any circumstances allow flash photography. You take a gander at all of your options and factors: what are my natural light sources? How fast is my lens? What was that thing Josh said the other week about shutter speed? 1/60th was recommended for human motion? If you have ever been in a situation like this as a new photographer, your immediate focus is on aperture and shutter speed. Allow me to introduce the X factor.

ISO.

When you have your aperture wide open, and you cannot seem to get any more light in by slowing down your shutter speed because you will be putting in motion blur in places you don’t want it, ISO comes to the rescue to let in more light to the exposure.

If you are familiar with film photography, you have encountered this on the film you buy. 50 ASA. 200 ASA. 400 ASA. Needless to say the lower number provides insensitivity to light. A lower ISO is best used when you are in areas with a lot of light. Outdoor photos are a perfect example of a time when you would want to use a lower ISO. Higher numbers are best used when you are dealing in areas of low light. A higher ISO will allow more light to be captured – but at a price.

Ultimately what happens when you have a lower ISO, you are dealing with a bunch of small light gathering pieces of grain. The more you have, the less light each piece has to gather, and so you can have a lot of them giving you a clean and crisp photo. When you have high ISO, you have less of these pieces of grain that fit on your film, and each one must absorb more light. Because there is less, but they are bigger, they become more visible. You will always see this case in a “grainy” photograph.

I lose you yet?

I bet.

Tell me about it.

I completely agree with you.

In summary, ISO can be a great tool to employ in your low light scenarios. And with today’s editing software, grain removal is better than ever. As always, avoid using a high ISO if you can. But if you ever find yourself in that situation where you need to get that extra boost of light, ISO is the man for the job. So get out there and show all your clients and colleagues that you know EXACTLY what to do to satisfy their conditions!

-J