Firstly I’d like to say a big thanks to all those who gave me such positive feedback on my first blog entry, sorry it's taken so long to publish another one! In my first blog I covered the relationship between Aperture and Shutter Speed, and in this second blog entry I’m going to discuss what the size of the aperture does to the focus range, or depth of field as it’s known.
So as I mentioned the bigger the aperture (smaller the f-number), the more light gets into the camera. Have you ever noticed that sports photographs are often blurred everywhere but the subject? This is because the photographer is using a wide aperture to get a fast shutter speed, but by doing this they are also displaying a key characteristic of a wide aperture: a shallow depth of field.
A shallow depth of field is produced when you are using a wide aperture, such as f2, and a large depth of field is produced when you are using a narrow aperture such as f18. The use of aperture and the control of depth of field is a fundamental skill in photography. When taking a photograph, you the photographer must always be in control of what you want your subject to be and how much of its environment you want the viewer to be aware of. The use of a wide aperture will isolate the subject from its environment, blurring the background and making the subject stand out. Shallow depths of field are essential in portrait photography.
A large depth of field is essential for landscape photography. As a landscape photographer you want to capture a whole scene, with front to back focus, where detail can be seen in the whole scene from the objects right in front of the camera all the way to the horizon. For this reason landscapes are taken with narrow apertures from f14 up to f22.
If you stick your camera on a tripod and focus on a static object with the aperture set to wide open and take a photo look at what’s in focus and what’s blurred. Now stop the aperture down and take the same photo and you’ll see what’s in focus has greatly increased. Simple yes? No, it isn’t. Depth of field is also controlled by focusing distance too. So if you have a wide aperture and you focus really close up to something the depth of field becomes very, very narrow, millimetres in fact. Using a wide aperture and focusing into the distance gives a greater depth of field. Likewise a narrow aperture up close again gives a shallower depth of field than if it were focused into the distance.
So to sum up so far:
Wide aperture (f2) – Shallow depth of field
Narrow aperture (f22) – Large depth of field
Focusing close – Shallow depth of field
Focusing far away – Large depth of field
However, that’s still not it! The focal length also affects depth of field. 10mm at f4 produces a much larger depth of field than say 200mm at f4. You can use this to your advantage, if you don’t have wide-apertured, expensive lenses then use your longest lens and set the aperture to its widest, keep your subject as close as possible and try and have the background far away and you’ll achieve the blur that you’re after.
Another thing to consider is how focusing works on your camera. It doesn't focus right in the middle of the depth of field, it gives you about a third in front of where you've focused and 2/3 behind. For this reason, don't focus on the horizon when you're taking a landscape, because you're wasting 2/3rds of your focusing field, which is a waste!