Exercise 3: Focus at different apertures – cameras with a
3 photographs – prints needed to note results
For this exercise, find a similar subject to the previous exercise. One that will show the results very clearly is a row of things seen from an angle: railings, parked cars, terraced houses, for example. Stand at an angle to this row and, if you have a tripod, set the camera on it so that each photograph will be framed identically. Focus on an obvious point somewhere near the middle, and make a note in your field notebook of this focus point.
Take the first picture with the lens at its widest aperture. Take the second with the lens stopped down to the mid-point of its scale of numbers. Take the third with the lens at its smallest aperture. You will, naturally, need to adjust the shutter speed so that the exposure stays the same, or have the camera set so that it automatically adjusts the shutter speed for a constant exposure (see its instruction manual). Remember that there is a reciprocal relationship between shutter speed and aperture. Just as shutter speeds are graduated in steps that double or halve the time (and so the exposure), the main steps in the aperture control are separated by the same amount. Each stop down from the widest aperture halves the area of the circular opening. This means that, in practice, one stop down to a slower speed accompanied by one stop down to a smaller aperture makes no change to the exposure.
Have a print made from each of the photographs; number them and compare them. There should be an obvious difference between at least the first and the third. With a pencil or marker, draw on each print what you see as the limits of sharpness. This will give you a more precise picture of the depth of field at different apertures. You can see that each pencil band is arranged around the point on which you focused. Keep these and other prints in, or with, your learning log for future reference.
Four images taken at a fixed focal length with the focal plane adjusted to the middle soldier
TOP LEFT: f/5.6 1/30sec Focal length 90mm ISO400 this has the smallest depth of field and really only allows the middle soldier to be in focus which does not really work as we cannot see much of him.
TOP RIGHT: f/11 1/6sec Focal length 90mm ISO400 In this image we can see the depth of field increase leaving the front and rear slightly out of focus.
BOTTOM LEFT: f/22 0.6sec Focal length 90mm ISO400 In this image the whole line of soldiers are in focus although the first and last soldier are not quite pin sharp
BOTTOM RIGHT: f/32 1.3sec Focal length 90mm ISO400 Some lenses seem to support this extremely small aperture and whilst it seems to bring all the soldiers into focus if you look to the edges of the frame and compare it with the f22 shot you will see a degradation in sharpness caused by the small aperture as a rule I try not to use this 100mm Macro lens above about f22 where it remains sharp.
Exercise 3 demonstrated the importance of depth of field and its relationship to aperture, it is important to select the right depth of field for your subject. For instance a closeup of a flower would probably benefit from a shallow depth of field so that the distracting elements behind the flower are not distracting by being in focus.
A panoramic city scape photo however would benefit from a wider depth of field in order to bring all of the compositional elements of the picture into focus.
The lessons that can be drawn from this exercise build on my coments for the previous exercise that used a fixed aperture, this execise adds a further dimension to the focus part of our workflow. This dimension is the one of different depths of field, in the last exercise the apature was fixed at its widest and thus at its smallest depth of field. In this exercise we were asked to focus on the centre of the composition and slowly change the apature as we took shots. The effect of this was to increase the depth of field in increments. The dilemer now is not just where should the focal plane be set but how much depth of field should be included in the composition. There is no correct answer to this itb will depend on the intent of teh photographer. The whole mood and meaning of a photo can change based on the placement of the focal plane and the amount of depth of field. this two is a vital consideration in my composition workflow