A large depth of field means that everything in your picture, starting from a few
feet to way in the distance, will be in focus. A small depth of field means that
only the object you have actually focused on will be sharp. Anything in the frame
that is in front or behind will be out of focus. Therefore, it's only when you have
a small depth of field that there will be any issues with focus.
There are four conditions that can result in your picture having a small depth of
field. They are:
Being close to your subject, especially so if you are using the macro setting on
Zooming in to take a shot with a long lens.
Using a very fast shutter speed.
When it gets dark.
Depth of field is small when the aperture in your camera is large. The aperture is
a hole in the middle of your lens that can be made smaller or larger to let in the
right amount of light for correct exposure. The reason why low light and fast shutter
speeds reduce depth of field is because they both require a wide aperture.
Some cameras will tell you the size of the aperture in the form of an f/no. Confusingly,
when this f/no. is small, the aperture is large and vice versa. A large aperture
(i.e. a small f/no.) will give you a narrow depth of field.
If your camera indicates an f/no. of around 5 or less, then you are dealing with
a narrow depth of field. If your camera doesn't indicate an f/no. (and many don't)
it doesn't mean that it's not setting one, it's just not telling you what it is.
For that reason, the rest of this will concentrate on the circumstances that naturally
result in a narrow depth of field and what, if anything, you can do about it.
Moving in close
When you are focusing on something some distance away, even a narrow depth of field
can be measured in feet or metres. But if you focus in very close then even a large
depth of field may only extend a few inches or centimetres. This is what happens
when you use the macro setting but it can also occur when you are at or near your
camera's minimum focusing distance without using macro. This is typically 3-4 feet.
Doing this in dim light would probably guarantee a narrow depth of field.
All cameras have a minimum focusing distance and your manual should tell you what
it is. If your subject is closer than this, then it will be out of focus. When you
switch to macro, you now also have a maximum focusing distance, anything beyond that
will be soft. Because with macro your lens is very close to the subject, the depth
of field is minimal. A minimum aperture will help this but this will probably require
the use of extra lighting or a very slow shutter speed and a tripod.
The further you zoom in, the less depth of field you will have. The change of depth
of field that occurs when you get close or zoom in are natural optical effects. In
fact, to a large extent, they compensate for each other. This means that, if you
place your subject further away but then zoom in to the same size shot, the depth
of field will hardly change. In these circumstances, you can only increase your depth
of field by forcing your camera to use a smaller aperture.
If you have an "aperture priority" mode, use this and set a high f/no. A "landscape"
program mode will also usually attempt to increase your depth of field. Although
sometimes, this may disable the macro setting.
When you increase the f/no. your camera will automatically compensate by setting
a slower shutter speed. This will probably be the limitation for hand held shots.
If you have a tripod and a static subject you could let the shutter speed go as low
as you like. This is one reason why a tripod is so useful in landscape photography.
Fast shutter speeds
If you set a fast shutter speed then your camera will compensate by using a low f/no.
thus reducing your depth of field. If you use a "sports/action" program setting,
this is likely to be the case. It does this to maintain correct exposure. In this
situation, the one thing you can do to increase your depth of field is to increase
the sensitivity of your camera to light. You do this by setting a higher ISO number.
Your camera will now use a smaller aperture at the same shutter speed and therefore
more depth of field.
When the light goes down
In bright, sunny conditions you can work close to your subject, zoom in and use a
fast shutter speed without having to concern yourself too much with depth of field.
However in darker conditions, doing any of these things will result in a restricted
depth of field.
There is nothing wrong with using a restricted depth of field. In fact, many photographers
deliberately set their cameras to give them a narrow depth of field. It can be used
to great effect to emphasise or isolate your subject. The only problem is that,
if you have a small depth of field then you have to be very accurate with your focusing.
When your focusing is even slightly out, this will be painfully obvious when you
look at your pictures on a monitor. The trouble is that it probably won't be noticeable
in your camera's screen or viewfinder. You can all too easily think that you have
taken a great shot only to discover that it's out of focus when you get home.
The fact is that some cameras are better at focusing than others. It may be a good
idea to take several shots to give your camera the best chance. However good your
camera is at focusing, it is very likely to be less able to cope as it gets darker.
As these are also the conditions that cause a reduced depth of field, this is one
of the most common reasons of out of focus shots.
What are you focusing on?
Important as depth of field is, it doesn't actually cause out of focus shots. They
are caused by being focused on the wrong thing and unfortunately, this is remarkably
easy to do. When you press the shutter, before your camera takes the picture, it
takes an exposure reading and focuses the lens. This occurs when the button is about
halfway down. You should be able to hear and feel this happening.
At this point the vast majority of cameras will focus on whatever is dead centre
of the frame at the time, whether it's your subject or not. A classic example is
when you are taking a picture of two people side by side. It is very likely that
what is in the very centre of frame will be the background and not your subject.
Of course this will happen anytime your subject is off centre.
Because there are many times when you should place your subject off centre it is
worthwhile developing the technique of focusing before you take the picture. It involves
pressing the shutter very gently and learning to recognise when it has set the focus.
This becomes easy with a little practice. Your subject should be right in the middle
of frame at this point.
The second part of the technique is to hold the shutter in this halfway down state
whilst re-framing for your final shot. Press the button the rest of the way down
when you are ready to take the picture. With a little practice this is very easy
to do and for most photographers, becomes second nature.
The sequence above shows the process. On the left, the camera focuses on the centre
of frame, indicated by the black cross. Pan the camera, till the centre is on your
subject and focus. Hold the shutter button steady, re-frame and take the picture.
There are cameras, principally D-slr types, that allow you to choose the way your
camera operates in this respect. You can designate a separate button for focusing
and select a different point in the frame to focus on. That does make things a little
easier but you still need to use a variation of the above technique.
The vast majority of automatic focusing systems use contrast, rather than some means
of measuring distance, to determine focus. This allows them, for instance, to focus
through glass. It also means that you can help your camera by focusing on an edge
with good contrast rather than a flat featureless surface. This is especially true
in dimly lit conditions.
If there is a face in the frame then the eye is the best place to focus, for aesthetic
as well as technical reasons. When there are no faces, or you don't want them in
focus, then you need to find a sharp edge on your subject. Ideally, this would be
a diagonal line because a lot of cameras only focus on horizontal or vertical lines,
Try these techniques if your camera is struggling to lock the focus whatever the
situation. If they don't work then your only solution, if you have the option, is
to focus manually. Unfortunately, only D-slrs are designed to make this as easy as
possible. The viewing screen on most other cameras is not ideal for carefully checking
Focusing using one of these screens is not all that easy but the best technique is
to rock the control back and forth through the best focus point. Make smaller and
smaller adjustments as you go and soon you will achieve best focus. This definitely
takes a bit of getting used to, but each time you do it, the process will take less
and less time.