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Basic Explanation of Depth of Field.
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The term ‘Depth of Field’ in photography relates to the
area of an image that appears sufficiently sharp that it
can be called sharp. All other areas are, to a lesser or
greater extent, blurred. Fortunately, depth of field is
something we can - and should - control. Although it is
a technical subject, depth of field should be considered
as a tool to be used as part of composition.
If you are photographing a traditional landscape, a deep
depth of field is usually selected. If you are shooting
portraits, a shallow depth of field is often selected to
isolate the subject from the background.
The main influences on depth of field are:-
- Lens aperture:The smaller the aperture (the larger
aperture number), the deeper the depth of field is.
Conversely, the larger the aperture (the smaller
aperture number), the shallower the depth of field is.
- Camera to subject distance:The effect of camera to
subject distance is that the closer the camera is to the
subject, the shallower the depth of field becomes and
vice-versa.
- Focal length:Focal length affects the depth of field,
but perhaps in a less obvious way than may be at first
imagined. For the purpose of basically understanding the
influence of focal length on the depth of field, the
depth of field gets shallower at any given distance as
the focal length increases. BUT what happens is, if the
subject size is kept the same on the image (i.e. the
subject to camera distance is increased as the focal
length of the lens is increased), the depth of field
stays very nearly exactly the same.
Miscellaneous related information and tips:-
- The term for isolation of the subject using a shallow
depth of field is ‘differential focus’.
- The size a picture is printed at affects the depth of
field too,so expect the depth of field to get a little
shallower if you print a picture really large. You may
notice it if you have only just caught a persons face in
the rear of your depth of field on your screen. Blowing
the picture up may move the subject just outside of the
depth of field - losing you those sharp eye lashes
maybe.
- There is a term in photography called ‘bokeh”, given
to the quality of the blur in the unsharp areas in the
image. It is used to refer to the effect of shallow
depth of field that creates a blur of colour behind the
subject. The depth of field plays a large part in
determining how out of focus the area is and whether or
not it creates a pleasing effect in the image is largely
down to the design of the lens – a good bokeh looks
creamy.
- Depth of field also varies with the sensor size of the
camera used, but to a lesser extent. For example - if
you use a camera phone with a tiny sensor, the depth of
field will be deeper at any aperture (all other factors
being equal) than on your DSLR. You will never achieve
differential focus with a small sensor – in fact, even
if you shoot a crop sensor DSLR you will notice a
difference in the bokeh between it and a full frame DSLR.
- The depth of field is not always half in front of the
focal point and half behind, but varies from lens to
lens and can be as extreme as a quarter in front and
three quarters behind – it is usually around a third in
front and two thirds behind though.
- If shooting portraits, be careful not to use a too
shallow depth of field or you will have to choose which
eye to have in critical focus! Try f2.8 to start with.
- Most lenses are best not used at the extremes of
aperture settings because wide open they often have
their worst sharpness and chromatic aberrations, and
when used stopped down below f11 or f16 can lose
sharpness due to diffraction. The sweet spot for most
non-professional grade lenses is around two to three
stops below maximum so for an f1.4 it could even be f2.8
or f4 before it is sharp, sharpness peaking at f5.6 or
f8 then it usually starts to drop off again after f11.
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Depth of field is particularly relevant to you if you
want to shoot close-up or macro whereby you should
always aim for the deepest depth of field – minimum
depth of field would be f11 here, True macro lenses
perform better close to the subject and it is OK to use
smaller apertures (f22 or even f32) with these lenses.
If the above has whetted your appetite to learn some of
the details behind depth of field, a good place to start
is:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
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