Depth Of Field.
Depth of Field or DOF is often one
of the most confusing aspects of photography for beginners.
This is not because DOF is inherently difficult to understand,
but that it is often explained using abstract concepts and
confusing terminology. As such, it is easy to get yourself
in a muddle.
What is DOF and how do I change the DOF
of an image?
Quite simply, depth of field is the
portion of the picture that is in focus. This is dependent
on four 4 major factors.
These four factors work together to
alter the DOF of any given image. I have listed them below
in the order that they affect most photographers. Each factor
is discussed in more detail later.
1. Lens aperture:
Lenses have an adjustable diaphragm that controls how much
light enters the camera. The hole in this diaphragm is referred
to as the “aperture” and the diameter of this
aperture is expressed in numbers, known as f-stops for example,
f2.8, f8 or f11. The higher the number e.g. f11 rather than
f4, the smaller the aperture (in diameter) and consequently
the greater the Depth of Field.
2. The focal length of the lens:
A basic rule of thumb is: the longer the lens, the shallower
the depth of field. Therefore a 28mm lens on a 35mm format
camera will have a much greater DOF than a 200mm lens when
set to the same aperture.
3. The distance between the
camera and the subject:
The greater the distance between the camera and the subject
upon which you are focussing, the greater the DOF. This is
most apparent in macro photography, where the DOF can be incredibly
shallow.
4. The area of the image recording
element (be it film
or a digital sensor):
The larger the image recording area the less DOF there is
at any given aperture. For example, to get a DOF that extends
from 5m to infinity a medium format camera may need to use
a higher f-stop (smaller aperture) such as f64. A 35mm film
camera, however, may need an f-stop of f22. A standard digital
compact which has an even smaller sensor will require an even
smaller f-stop such as f8.
I’m kind of ok so far, what’s the confusing
stuff?
In my experience the biggest stumbling
block that people have is how to express size as it relates
to DOF, largely because the terminology used is somewhat confusing.
The terms greater, smaller, shallower and larger are used
often in relation to the same thing. The result is that they
quite often mean the opposite of what you think they might
mean, causing confusion. In fact, I’m betting that you
are slightly confused now, but hopefully here is where it
will start to make sense.
1 – Aperture (f-stop):
F-stops are found on a standard SLR
lens or on any digicam that gives you manual controls. They
are listed as a series of numbers that don’t necessarily
make a great deal of sense when you first encounter them,
as there doesn’t seem to be much logic to them or their
values. On a typical 35mm SLR lens these f-stops might be:
2.8, 4, 5.6, 8, 11, 16 and 22. They are often referred to
as f2.8, f4, and so on, up to f22.
On a manual 35mm SLR lens there will
be an unmarked but selectable stop between each of these values,
which is known as a “half stop”. On a modern automatic
SLR or a digital camera they will most likely be two number
values between each full stop, these are 1/3 of a stop for
example f13 and f14 located between f11 and f16.
These numbers are perhaps the most important
factor that you will use to control DOF. Almost certainly
they will be the most commonly used factor.
Small f-stops (those that have a low
number, e.g. f2.8) mean a large aperture. For example, f2.8
means that the aperture is larger (or “wider”)
than the aperture at f8. This “large” or “wide”
aperture creates a “shallow” DOF. F8, or F22,
are often referred to as a “greater” or “larger”
f-stops, and as such have small apertures, creating a “greater”
DOF than f2.8. As you can see, the liberal use of greater,
smaller, bigger and shallower makes it easy to get confused.
One way to remember this is:
SMALLER number (f2.8) = BIGGER
hole = SHALLOWER DOF
BIGGER number (f22) = SMALLER
hole = GREATER DOF
It is even easier of you omit the mention
of the hole:
SMALLER number (f2.8) = SHALLOWER
DOF
BIGGER number (f22) = GREATER
DOF
Most often confusion arises when someone
says “use a big aperture”. It can seem more logical
that a bigger number would mean a bigger aperture but this
isn't the case. However, if they were to refer to a larger
f-stop then it would mean a greater DOF.
A note for digital compact users:
You may look at your digicam and see that it only
goes up to something like f8. Don’t worry; this does
not mean that you won’t get lots of DOF, it just means
that the sensor on your camera is quite small. Consequently,
Factor #4 listed above (“How do I change the DOF on
an image?”) will apply. As you have a small image sensor,
an f-stop of f8 should be all that is required to obtain a
great DOF.
2 – Focal Length:
The concept of the focal length effecting
DOF is quite easy to comprehend. As mentioned before the longer
the length of the lens (expressed in mm) the less DOF it has.
This means that wide-angle lenses, those with a short focal
length i.e. 28mm are doubly suited to landscape work, as not
only do they allow you to fit more into the frame, but they
also give you a greater DOF at any given aperture.
Conversely longer focal length lenses,
such as 135mm or higher, are suited to portraiture, as they
allow you to focus in areas through having a narrow field
of view and less DOF. This allows you to use the DOF creatively
drawing attention to parts of the image such as the eyes of
the subject.
Once you understand this, you can use
the lens and its properties to creative effect, creating the
right DOF for the image you are taking.
3 – Distance from the subject:
The third largest influence on DOF is
the distance between the camera and the object that you are
focussing on. The greater the distance between the camera
and the subject the greater the DOF; the closer the object
is the shallower the DOF. This is why in a lot of macro photography
you will see a DOF that is almost unbelievably small. The
lens is so close to the subject that maybe only an edge of
a leaf or the eye of an insect is in focus.
The opposite is also true if you focus
on an object that is located near the horizon; your image
will have an extremely large DOF. This also leads to what
is known as the hyperfocal point, something that is covered
later in this article.
4 – Film / Sensor size:
Again this is essentially a simple
concept, the larger the film / sensor, the shallower the DOF
you will get at a given aperture and focal length lens. Unless
you switch formats, say from a digital SLR with an APS sized
sensor, to one of the full frame 35mm SLRs (digital or film)
or even to a medium format camera.
This is really a constant unless you
do switch between film / sensor sizes. However, it is useful
to bear in mind if you are not getting the results you want
because a switch of systems may help you out. Smaller film
sizes or sensors, such as those on most digital cameras, will
give you greater DOF. This makes them good for landscapes
and gives you a bit more leeway with sports or wildlife shots,
unless you want a really shallow DOF that is. The downside
is getting lenses that give you a very shallow DOF for portraiture
becomes harder and much more expensive.
A lot of digital compacts have a limited
aperture range some going up to only f8, this is because of
their sensor size, the manufacturers can keep costs down by
reducing the f-stop range and still retain a good DOF.
Taking this all in
This is quite a lot to absorb in one
sitting, but here I’m going to use something that helped
me first get my head around the first three factors and how
they relate to each other and DOF. As mentioned above the
fourth s pretty much a constant and I have included it more
for completeness and as a reference.
My first camera was a manual SLR that
had no aperture priority, shutter priority, or auto focus.
The closest it got to any automation was through the lens
metering, activated by a lever on the side. While this slowed
my photography down, it sped up my learning as I was forced
to understand what I was doing rather than let the camera
do the work for me.
The lens that came with this camera
was a 35mm to 70mm zoom. On this zoom were a series of marks
on either side of the focusing mark on the lens barrel. These
really helped the ideas of f-stop, focal length of the lens
and distance-to-subject sink in.
Adjusting the aperture
In this first
image the lens is at the 35mm setting and is set to focus
on an object located 2m away. Using the lines on either side
of the thicker focus line, we can see how much of the scene
would be in focus. The outer line represents f8; at this focal
length of 35mm everything from under 1m to about 15m is in
focus (giving a total DOF of just over 14m).
In the second image, the aperture is set at f5.6, which
is represented by all of the innermost lines save one, the
DOF would now be between 1.5m and just under 3m, giving a
total DOF of just under 1.5m.
Adjusting the point of focus
In Image three I have adjusted
the focus to 5m but kept the aperture at 5.6. The DOF has
now changed, encompassing everything from 2.5m up to near
the infinity symbol 8. As you can see from this example, when
it is compared to image 2, if the point that I am focussing
on is farther away, then the DOF will be greater at any given
aperture, in this case f5.6.
Adjusting the focal length of
the lens
In Image four, I have adjusted
the focal length to 70mm (as opposed to 35mm in the previous
examples). As you can see from this image the lines on the
lens barrel are almost totally hidden. Set again to focus
at 5m with an aperture of f5.6 (the same as used for image
3) the DOF is from 4m to 8m, this is a big change compared
to 35mm. Here you can see the effect that focal length can
play in effecting the DOF on any given image.
The amount that a lens alters the DOF
depends on the individual lens and its characteristics as
well as the camera format that the lens is connected to. Markings
like this are useful on a lens, especially for the beginner
or when focussing using the normal method becomes difficult.
I do lament the fact that my wide-angle lens that I currently
use on my Canon lacks these markings as well as manual focus
distance markings because they are useful for night photography!
Wasting DOF and the Hyperfocal distance
Hyperfocal distance is a somewhat difficult
concept to implement on a daily basis, because it does involve
maths or a good memory or experimentation. When you focus
on an object, the area that is in focus extends in front of
this object and behind it, this is you DOF. If you focus your
camera to infinity the leading edge of the area of focus (the
edge that is closest to you) is the Hyperfocal distance.
Obviously this distance will vary depending
on what aperture and lens you use. There is a formula for
working this out, or a quick search will reveal plenty of
websites that have tables based for focal length and aperture.
Alternatively you can experiment with your own camera to give
you a rough guide of how much DOF you can achieve. If you
focus at infinity and set your camera to the smallest aperture,
you should be able to work out from the resulting image roughly
how far towards the camera your DOF extends. Refocusing on
that point would mean that the DOF would extend up to infinity
as well as even closer towards the camera. Obviously this
method is less accurate than using maths or a table of distances,
however, it does mean that if you are working on landscapes
or have a long time to set a shot up you can use it in the
field. It is certainly worth remembering for those times when
you need to squeeze out as much DOF as you possibly can.
If you want some more information a
quick google
search pulls up lots of pages.
I will be adding to this article in the future. If you wish
to comment on this article, please email me with this
link. I will be adding in a comments page later. If you
wish to ask me a question email me from this
question link, I will also extend this article by answering
any questions submitted.
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