ISO {60/365}

Ooops!  I've been a bad boy.  Haven't been doing my promised Monday Lessons! Here to make up for it now . . . .

ISO stands for International Standards Organization.  Also the greek term, isos, meaning equal.


Ok, basically it means how sensitive your camera chip (or film) is to light.  The higher the sensitivity, the less light that is needed to make an exposure. If you shoot in bright daylight, a lower ISO is needed.  If you shoot in a darkened room lit only by candlelight, you need a much much higher ISO.

Knowing what your ISO is is VERY helpful.

Say, I’m at my sister-in-law's party, which is held inside at night with only a few household light fixtures to light the scene.  I point my camera at her as she's sitting there enjoying a rather large glass of Shiraz.  I take the picture.  Hmm.  Blurry.  My camera shot at f1.8 @8th of a second, but my ISO was 50.  If you remember from a previous lesson, to avoid the common blur, you must increase your shutter speed.  I would have to shoot at a 50th of a second to get a sharper image.  So, I increase my iso to 800 and I'm able to shoot at the 50th of a second as I increased my camera's sensor sensitivity.

So why not just up the ISO as high as I can to get the sharpest picture possible? This is because (dependant on how good the camera is) the HIGHER the ISO, the more ‘grainy’ the picture looks, grain is best avoided, especially if post-production is required/desired. You've seen them, images of your friends taken in low light conditions, but all yucky and muddy. Ideally you want to have the ISO set as low (low being the 50iso end) as possible to get the shot you need – but sometimes it is just not possible to get the shot you want at such a low ISO.

Look at the example below:

The image on the left is the original image; shot at 50iso.  The image above, right is a 100% crop of the center so you can see the details.  As you can see, no noise, very clear.  The image below, right, was shot at the fastest my camera can dish out, 25,600 iso.

As you can see the bottom image is very grainy.  This can cause many problems, from color distortion to fuzzy images and just plain yuckiness.

What causes it?  What's possibly happening is heat generation frees electrons from the image sensor itself, thus contaminating the "true" photoelectrons. These "thermal electrons" give rise to a form of noise called thermal noise or dark current.

Another type of noise is more akin to the 'grain' obtained by using a high ISO film. When we use a higher ISO, we are amplifying the signal we receive from the light photons. Unfortunately, as we amplify the signal, we also amplify the background electrical noise that is present in any electrical system.

In low light, there is not enough light for a proper exposure and the longer we allow the image sensor to collect the weak signal, the more background electrical noise it also collects. In this case the background electrical noise may be higher than the signal.

So when you're at your next party, taking shots of your sister-in-law enjoying a large bottle of Shiraz, think about what minimum shutter speed and aperture you need to get a proper exposure and shoot at the lowest possible ISO.

She'll thank you for it.