Back to basics Part 3- What is ISO and how to put it all together?

The last piece of the puzzle that makes up the exposure triangle is ISO. Quite simply, ISO is a measure of how sensitive the sensor is to light.

At a low ISO, the sensor has a low sensitivity to light, giving you a darker image. As you increase the ISO, the sensor becomes more sensitive to light and you image becomes brighter. The trade off is that as you increase the ISO, more grain or ‘noise’ is introduced into your images.

Each camera has it’s own level where grain becomes more noticeable. Whilst you may not notice the grain immediately, if you zoom into an image, the grain is more evident. This is the primary reason why ISO should be left as low as possible when you are shooting food photography. On a Canon Mark III OR Mark IV, grain becomes apparent at around an ISO of 500. When shooting for a food photography client, keep the ISO to 100, especially if you are shooting on a tripod. You can adjust the aperture and shutter speed to let more light into the camera.

Grain can also be removed in post production in most editing software.

So there you have it, the three variables that make up the exposure triangle- aperture, shutter speed and ISO. All three elements work in harmony to control the amount of light entering the camera. Changing one of these elements has an effect how bright or dark our image is and so, you may find that you need to change something else in order to compensate.

Your aim is to have the perfect exposure. Not too dark, not too light and definitely not blurred. When you have the perfect exposure, all three elements of the triangle are working in perfect harmony.

If you suddenly change any one of these elements, the exposure would either get underexposed or overexposed. In order to bring the harmony back in the triangle, you would need to make an adjustment to one of the other two elements to get that perfect exposure again.

To conclude, here are the main points covered:

• ISO is how sensitive the camera’s sensor is to light
• As you increase the ISO number, your image becomes brighter
• As you increase ISO, you introduce grain into your images which can be corrected in post production to a certain extent
• Exposure triangle is when aperture, shutter speed and ISO work in balance to give a perfect exposure
• Change one element of the exposure triangle and ‘triangle’ is no longer balanced
• To create balance again and a perfect exposure, one of the other two elements needs to be changed to compensate