 |
| Fg 1. 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 28 mm 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 200 mm the point of focus needs to be precise as the depth of
field is quite narrow even at large f numbers (small apertures).
 |
| Fg 2. Kodak colour plus negative. scanned from print. |
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.
 |
| Fg 3. 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.
