It’s not you – it’s me. When I look at conventional explanations of the relationship between aperture, shutter speed and ISO, they are always shown as a triangle. That form of representation confuses my brain. I think they would be better shown as a slide rule. I don’t have the know how to do this as a properly sliding image so my featured image today is a simple Excel spreadsheet diagram.
The measure used for using the slide rule is the STOP. Each stop represents a halving or doubling of light. Over the past few days you should have seen that effect at work in photographs I’ve included.
If you use a faster shutter speed, you reduce the amount of light reaching the sensor because the shutter is open for a shorter time. That shorter time enables you to freeze motion or counter camera shake. If you open the shutter for longer you let more light in and you can blur motion.
If you use a larger aperture (the size of the opening of the shutter) the front to back sharpness or depth of field decreases, so you get a blurred background. Using a smaller aperture you get more of the depth of the photo in focus – as in a landscape shot. What confuses some people with aperture size is the unit used the f/stop. That’s because it’s shown as a ratio or a fraction. So, just as a half is larger than a quarter f/2 is a larger aperture than f/8. Every stop of aperture, even if the number below the f/ looks odd (mathematical reasons involving the square root of 2), the larger that number is eg f/22 the smaller the aperture is, while a smaller number eg f/11 is a smaller aperture (2 stops smaller)
If you use a smaller ISO (say 100) you get a clear image, but in low light conditions you may need to increase the ISO to 200, 400 or even 1600. However, the larger the ISO, the more likely it becomes that the graininess of the image will become noticeable. The ISO setting is used to increase brightness.
So what. Well, have a look at your subject and take a test shot. Make a note of the settings eg a landscape shot at f/8 at ISO 200 with a shutter speed of 1/500. Let’s say that you feel you’d like a greater depth of field . As a first step, try reducing the aperture by one stop to f/11. That lets in half as much light. Using Aperture priority as your mode, your shutter speed should decrease to 1/250 seconds to compensate and still produce a similar exposure – but sharper.
The slide rule gives you your guide. If you increase one aspect by two stops, to retain a similar exposure you will need to make a two steps correction between the other two elements.
I’ve mentioned camera modes in a paragraph above. Perhaps tomorrow I should explain something about the various modes available on most digital cameras and their uses.