Wednesday, 18 August 2010

Exercise : Focus at different apertures

Hooray ! It's finally stopped raining........

This exercise involves taking three photographs of a subject that will show the effect of the change in aperture on the area of focus. Throughout the exercise exposure and the point of focus have to remain the same. i.e if the aperture is decreased by 1 stops from say f/16 to f/22 then an increase of a corresponding amount has to be made to the shutter speed to maintain a constant exposure. The smaller the lens aperture the longer the aperture has to stay open to allow in the same amount of light.

When the three photographs were processed the effect the change in aperture has on the depth of field can be clearly seen. The examples below are marked to show the area of sharp focus.

Fig.1 f/4.5                                                                                         Fig. 2 f/22
For the purpose of this exercise these images were scanned onto the computer and are at a lower resolution than the original photographs therefore they do not show the areas of focus clearly.

Fig 1.taken with the aperture wide open at f/4.5. A relatively small area around the point of focus is sharp. Fig 2. taken with a small aperture of f/22 shows sharp detail throughout the image from front to back.

As the aperture that is used to take a picture has a direct effect on the area in focus it is a valuable creative tool. You can use depth of field to emphasise a focal point or remove distractions from an image e.g. in a portrait with the aperture wide open you can concentrate the viewers focus on the eyes of the subject and throw the background out of focus.  However in order to maximise depth of field in an image use the smallest possible aperture and focus approx 1/3 of the distance into the frame.

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