Film FP4+ developed in ID11 printed
on Ilford MG RC gloss.
So what is depth of field and how does it work?
Depth of field relates to the area of the image that is sharp. So the subject you focus on in the view finder will be in the middle of the sharpness. How much this extends in front of or behind it, is dictated by the aperture you use. Small F number (large opening) very shallow, large F number (small opening) very wide. The other factor to have a bearing is the focal length of the lens used. For example, with a wide angle 28mm lens you would not require the focus to be exact because the depth of field would be quite considerable in front of and behind the point of focus even at small f numbers (large apertures). But with a Telephoto lens of 200mm the point of focus needs to be precise as the depth of field is quite narrow even at large f numbers (small apertures).
Kodak colour plus negative. scanned
In understanding the way depth of field works you need to know that when you focus on the subject it is at that point the reflected light arrives at the focal plan as fine points of light (sharp). The subjects closer to the lens do not resolve as sharp until they are beyond this point and those further away reach pin sharp before they arrive, because of this they arrive as discs known as circles of confusion. The larger the circles the softer the images appearance. By making the aperture smaller (large F number) you reduce the circles of confusion giving the picture the appearance of full depth of field. (sharp from front to back). The eye considers points of light as large as 0.25 mm diameter as sharp. The same applies to the dot pitch of a computer screen. When it come to the manufacturer of lenses for 35mm format cameras this figure is much smaller 0.08, this is because the maker has worked out that on average a 35 mm negative will be enlarged by twenty times (a print size of 10 x 8.)
The good thing about using a film camera is that you can check on how heavy the points of confusion will be by pressing the depth of field preview button. The advantage I have is I know what to expect from my lenses at particular apertures. This allows me to compose the picture with the amount of soft focus I think will enhance it.
For example the three pictures included with this post.
• Fg. 1 The main reason for the cats paw being out of focus is to add depth and a sense of being very close.
FP4+ developed in ID11 printed on
Iford MG RC gloss
• Fg. 2 The main reason for blurring the background is to exclude a large group of people walking towards me. They did not add anything to the picture I had in mind. By adjusting the aperture to a lager one (small F number) they have been removed making for a much better shot.
• Fg. 3 The depth of field in this picture is very narrow. It took a bit of time in making sure that the whole of the ball was sharp and nothing else.
When taking a picture I consider the 'out of focus' as important as the area to be sharp.