In week 3 we learned about ISO and how it is the third setting responsible for exposure in the camera (in addition to aperture and shutter speed.)

ISO stands for International Standardisation Organisation and determines the sensitivity of the sensor to the light.

When shooting in Aperture priority with the aperture wide open (a low f number) the only other way that you can let more light into the camera is by increasing the ISO. However the more you increase it, the grainier your photographs will be.  The sensor records more pixels when using a lower ISO. A higher ISO means that there will be no fine detail.

Also, when shooting in Aperture Priority mode,  your shutter speed will be worked out by the camera, and with the aperture wide open, the shutter speed will be faster (higher number.)

If you want to increase the aperture to get more of the picture in focus, your shutter speed will be slower (low number)  meaning that the exposure time will be longer as you’re closing the size of the hole and allowing less light to reach the sensor. You therefore need to ensure that you are holding the camera steady to avoid camera shake, e.g. on a tripod or resting on something. And again, to let more light into the camera so that your shutter speed remains fast, you will need to increase the ISO. The ISO doesn’t change the brightness of the photo; only the shutter speed. The higher the ISO, the faster the shutter speed.

e.g. The following example shows how the shutter speed gets faster the higher your ISO

ISO Shutter speed
100 1/3
400 1/13
800 1/30
1600 1/60
3200 1/100
6400 1/200

When looking at slow shutter speeds, you will see the ” symbol. This means ‘seconds’

  • 4″ = 4 seconds
  • 1/8 = One eighth of a second

Using Exposure compensation changes the brightness of a photo. It will also give you more control over the shutter speed, however, making the photo brighter increases the ISO, resulting in a lower quality / grainier picture.

Exposure compensation: Changing the brightness of the photo

In the following examples, one of the photos uses Exposure Compensation to demonstrate how the settings change:

Left photo: ISO 100 Shutter speed 1/80 Aperture 5.6
Right photo: ISO 125 Shutter speed 1/60 Aperture 5.6 Exposure Compensation +0.7

When trying to capture the following two photographs, I found it difficult as the subject, a butterfly, was in front of a window with lots of light behind it, and therefore the butterfly was dark. I increased the Exposure Compensation +1.7 steps in order to brighten in. This has also brightened the background and the flowers and they could be in danger of looking overexposed, but I quite like the effect:

Top photo: ISO 100 Shutter speed 1/125 Aperture 5.6 Exposure Compensation +1.7
Bottom photo: ISO 100 Shutter speed 1/100 Aperture 5.6 Exposure Compensation +1.7


The photographs of butterflies were taken in the very hot and busy Glasshouse at RHS Wisley. I had to prepare my lenses before going inside by keeping them as warm as possible so that they wouldn’t steam up so much in the humid conditions and I took a special cloth with me to keep them clean once I was in there. I also had to be aware of other people as it was very busy. I tried not to get in the way by taking photographs in narrow areas and I also tried not to spend too much time photographing one particular butterfly so that other people had a chance to see it.

When to use a low ISO

  • Portrait photos
  • landscape photos

…otherwise your photograph will be of a lower quality.

Landscape photographers don’t use more than 100 ISO and at least f16 aperture.

When to use a high ISO

  • In low lighting situations
  • If you would like to record movement that requires a higher shutter speed to freeze the subject

Setting the camera for shooting in Aperture Priority:

  • Check the ISO setting and put it to the lowest number
  • Choose the aperture, taking into consideration depth of field and movement
  • If the shutter speed is not fast enough, increase the ISO

Selecting the lowest aperture mean that you get the best shutter speed you can possibly get from your camera. If you need a faster shutter speed, all you can do is increase the ISO

Fast lenses

The advantage of a fast lens (e.g. those that go down as far as an aperture of 1.4 or 2.8) is that the lower apertures will give faster shutter speeds.


Criteria covered in this blog:

Unit 01: 1.5, 1.6, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 2.10