Camera basics: shutter-speed, aperture and ISO · by Simon Mackie
Aperture and depth of field
The other effect that aperture has, as well as letting more or less light onto the film or sensor, is to change the depth of field. The depth of field is the amount of the picture that will be in focus. Basically, the smaller the aperture setting, the larger the depth of field.
Take, for example, this landscape. As you can see, pretty much everything in the photo is in focus. This is because I used a fairly small aperture (f/11)
(It’s also because most things in the image are a little way away from the camera. Don’t worry too much about depth of field – I’ll cover it in more depth at another time.)
Why would we ever want to not have everything in focus? The really useful thing about using a large aperture (and thus have a small depth of field) is that you can use it to be creative and blur out the background. This really helps draw the viewers’ attention on the main subject of your photo, and is great, for example, in portraiture (although there’s plenty of other circumstances where you can be creative with shallow depth of field)
In this image, I used a large aperture (f /4, in this case) and as a result, the background of the image has blurred out, focusing our attention on our main subject.
So if you use a large aperture, you let more light in. This means to get a correct exposure you can use a shorter shutter speed. The tradeoff is that you lose some depth of field. Conversely, if you use a small aperture, you need to use a longer shutter speed to get the correct exposure, which brings with it the possibility of introducing camera shake.
If the day is bright this may not be a problem, but happens if there’s not enough light for you to get the picture you want? In circumstances like these, you need to either use a flash, or increase the ISO of your sensor or film.
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