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What is ISO? Photography tutorial for DSLR beginners

What is ISO? An explanation of what it is and how it relates to light, exposure, stops, shutter speed and aperture for DSLR photography beginners.

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In this video, we’re going to ask the question: What is ISO?

If you are old enough to remember the days before digital, you will recall going into a chemists or a photo lab and seeing stacks of film with numbers like 100, 200 and 400 on the boxes. These numbers still apply in the digital world, although as you will notice twiddling the dial on your camera, there are many more numbers now available to you.

Let’s take a look…
There’s 100, 125, 160, 200, 250, 320, 400 and on and on.

For our purposes, the ones to remember are:

100, 200, 400, 800, 1600, 3200

These numbers refer to the sensitivity of the sensor (or film) to the light. 3,200 is very sensitive, which means that you can take photos in very low light often without the need for flash. The downside is that the picture will be very ‘noisy’ (or grainy on film).

In bright sunlight, 3,200, a ‘high’ ISO, would be far too sensitive. The image would be completely burnt or whited out. 100, a ‘low’ ISO, has a low sensitivity to light and is therefore perfect for very bright conditions – like a sunny day.

So as a general guide we might use ISO 200 on a cloudy day, 400 outdoors but in the shade, 1600 indoors in a well-lit room. I very rarely use 3,200 or higher ISOs because they are too ‘noisy’. That means the picture isn’t sharp and has an ugly pixelly look.

I try to use the lowest ISO I can given the lighting conditions in order to reduce noise as much as possible.

Let’s look again at those numbers. There’s an obvious pattern. 200 is double 100, 400 is twice 200 and so on. 400 is half 800, which in turn is half 1600.

100 to 200, or 400 to 800, is known as a ‘stop’. Stops work in doubles and halves.

To make sense of this, let’s look at an example. You are taking photos on a sunny day, so you have the ISO set to 100. Suddenly the sun goes behind a cloud. Now there is roughly half the amount of light hitting your subject and being reflected into your camera to form an image. If there’s half the light we need to DOUBLE the ISO to achieve the same exposure (in other words for the image not to be too dark). The sun comes back out and the amount of light doubles, therefore we need to halve the ISO – back down to 100.

Each time we double or half the ISO we are adjusting it by a ‘stop’. Stops are measures of light and they represent the most fundamental principle of photography.

Stops – in doubles and halves – also apply to shutter speed and aperture.

Assuming the light remains the same, any change in ISO must be compensated by a change to either shutter speed or aperture in order to achieve the same exposure (brightness or darkness).

Setting the ISO is generally the first thing I do when starting a shoot before I even think about shutter speed and aperture.

Compared with shutter speed and aperture, ISO isn’t very creative but it is at least easy to get to grips with.

I hope you found that useful. Thanks for watching and see you next time!

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