Looking to get off the “auto” setting on your digital SLR? That was me but lots of the information I found was way too confusing. I’m hoping to provide a simple, straightforward way to learn the three basics you need to understand to start being more creative: aperture, shutter speed, and ISO.

This is part 3 – if you haven’t read part one or part two, you’ll want to read Part 1: Aperture (Fuzzy Factor) then Part 2: Shutter Speed (Capturing Movement)

Understanding ISO

ISO stands for something really boring (don’t even ask) and difficult to explain in detail. But the semantics and technical details are NOT what we’re here for.

When you take a picture with a digital camera, obviously it’s not hitting film – instead, it’s hitting a sensor that captures the picture. That sensor can be set to be much more or less sensitive to light. This is ISO.

Crank up the ISO and the camera will give you a much brighter image. Crank down the ISO and it will be much darker.

Let’s go back to the image of our car racing down the ramp from part 2:


If we were to crank the shutter speed to freeze the car, we’d be in trouble because the image would get really dark.

Unless we were to crank up the ISO to have the sensor to be more sensitive to light. Look what we have here:


Not perfect, but the car is virtually frozen in time with enough light to see what’s going on.

On the opposite end, if we were to want to blur the car even more than the original photo we started with, we’d be in trouble because leaving the shutter open for longer would mean that the image would get way too bright.

Unless we crank down the ISO to make the sensor less sensitive to light but stay open longer to blur the image more:


Now it’s really, really blurry, but the light is about the same as the other two images.

This is the magic of ISO. It simply allows you to adjust how much light you capture on your photos.

So why wouldn’t I just leave my ISO super-high all the time so I could crank up the shutter speed in order to not worry about my shaky hands or blurry objects?

The problem with ISO

The catch here is that if you crank up the ISO too high, your image starts to look really grainy and lifeless because the sensor is having to artificially adjust for that missing light.

Click on one the images below to compare bigger. One was taken at ISO 6400 (very high) and one was taken at ISO 200 (very low):

It should be very obvious that one looks a lot better than the other – even at such a small size.

Imagine if you tried to blow up the one of the left to a poster? It would be awful.

How to use ISO

So ISO is the balancing game between what you’re trying to accomplish. In general, you never want it to get too high as your pictures will degrade, but in some cases, it’s better to have a photo that’s a bit grainy as opposed to one that’s way too dark or super blurry.

So when it’s dark, crank it up so a slow shutter speed mixed with shaky hands or moving subjects doesn’t lead to a blurry mess.

And when it’s really bright out or you need to leave the shutter open for a long time, crank it way down to keep your pictures from being way too bright (overexposed).

Next Up: Read Part 4 – Putting it all Together