Friday, 4 February 2011

Exercise – Focus at Different Apertures.

The aim of this exercise is to show how altering the aperture will alter the depth of field in the picture, that is, how much of the picture is in sharp focus. A wide aperture gives a small depth of field so only a small part of the picture will be sharply focused while a small aperture gives a large depth of field and therefore more (or all) of the picture is in focus. This is important to consider when setting up the camera as there are times when you may want the whole picture in focus, for instance in landscapes, and other times when it gives a more interesting effect to have some out of focus areas such as the background in a portrait.
I made several attempts to show this, initially trying outside using a row of houses, a pathway lined by trees and a fence and steps. The variation of focus could be seen, however did not make a very obvious change in any of these situations. This was probably because the light was not very bright and I had to give a fairly long exposure time at f/22 and therefore the picture was not pin sharp. I was also taking distance photographs and the effect of varying the depth of field is less in this case.
Example 1.
                      f/2.5                                        f/6.3                                        f/16
f 2_edited-1f 5_edited-1
f 16_edited-1

I then repeated the exercise with a still life set up using a tripod with the jugs lit from the side with a spotlight. I used a pancake lens with a focal length of 20mm (equivalent to 40mm) and was close to the jugs. This shows a much more obvious effect of the changing aperture. My preference her is the 2nd picture with a slight softening of focus to the rear as the gaze concentrates on the central jug without being too distracted by the out of focus areas that occur in picture 3.
Example 2.
jugs f 16                                                               aperture f/16 – the whole picture is in focus
jugs f8                                                 aperture f/8 – mostly in focus but the back is slightly soft
jugs f1_edited-1                              aperture f/1.7 – the only area completely sharp is the central jug that is at the focal point

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