If you read Nancy’s post you will find the factors involved in limiting depth of field explained. The f-stop description is as Nancy says – counter intuitive. The key to understanding is that f-stops are a ratio with smaller numbers indicating a wider opening to let light in. One of the best ways of understanding this is to view the number as being the number of holes of that size that would fit on your film so 2.8 is, by definition, a much bigger hole than the hole that would fit onto your film 16 times. f-stops have been around a long time and the direct correlation to film / sensor size no longer exists. Nancy mentioned that focal length also plays a part in the depth of field – it’s worth adding that cropped sensors affect your lens focal length compared to full frame sensors and therefore also have an effect on the depth of field in your final image. I hope my additional thoughts help.
Limiting depth of field is a technique used in sports photography to isolate the action from the background. Thus, you will find many sports photographers shooting using aperture priority to keep the depth of field small whilst adjusting the ISO setting to maintain a suitable shutter speed. Some photographers will shoot with the lens wide open – I tend to try and use somewhere between f4 and f5 because sometimes I find that f2.8 is a bit too tight for my subject matter. Of course when it gets dark and I’m photographing under the poor quality floodlights at a non-league football match then I have to resort to shutter priority and adjust the ISO to get reasonable exposures at the widest aperture – such is the life of a football photographer 😉 Here’s a couple of shots from a recent match at Leatherhead – one of action and one of a couple of stewards…
