And Depth Of Field
Focusing on a single object is easy, but what if
you want both near and distant objects to be sharp? For this you
need to know how to control depth of field.
We all know that to make your subject come out sharp
you need to focus on it. And modern autofocus systems operate with
such speed and precision that this is almost guaranteed. They can
check the focus at many different points in the frame and select
the correct one automatically, and even track subjects as they move
across the frame (predictive autofocus). But what they don't take
into account is ‘depth of field', which is a much more important
measure of a picture's overall sharpness. It's ignored not because
it's unimportant, but because it's too difficult for a machine like
a camera to evaluate.
What is depth of field? In theory, only subjects a precise distance
from the camera (the distance you've focused on) will be perfectly
sharp. In practice, the sharpness falls away slowly, so that objects
some way behind the subject or nearer the camera can still look
quite sharp even if, technically they're out of focus. In other
words, there's quite a bit of leeway before objects start to look
out of focus. This leeway is the 'depth of field', and it has a
big impact on the appearance of the photograph. For example, in
a landscape shot you might want everything sharp, from the wooden
stile just a few feet from the camera to the mountain peaks in the
distance. In other words, you want maximum depth of field. Alternatively,
for a romantic portrait shot you might want to throw a distracting
background out of focus. For this, you would want 'shallow' depth
The point is that depth of field can be worked out
and used to your advantage. You can control the depth of field by
the way you adjust the camera. The camera's autofocus system can
focus on a specific object, but that's about its limit. Only you,
the photographer, can take the next step and control the depth of
Most photographers are aware that wide lens apertures
produce shallow depth of field and small lens apertures produce
greater depth of field. The optical science behind this is complex,
but what it boils down to is that at smaller lens apertures, the
sharpness of out-of-focus objects falls away more slowly. In other
words things are much nearer or further away before they start to
Depth of field is also affected by the focal length
of the lens. The longer the focal length, the shallower the depth
of field. This is one reason why portrait photographers prefer longer-than-average
focal lengths - because this usually blurs the background. Conversely,
wide-angle lenses are useful to landscape photographers because
it's easy to get everything in the frame sharp, right from the foreground
to the distant horizon.
Depth of field also changes according to the distance
of your subject - the focus distance. The greater the focus distance,
the greater the depth of field. Similarly, the closer you focus,
the more shallow the depth of field becomes. This is why depth of
field isn't a problem with subjects some way from the camera, but
a real headache with close-ups. You'll seldom see a close-up which
is sharp right from the foreground to the background, unless the
scene has been set up very carefully by the photographer to be as
two-dimensional as possible.
The fourth factor in depth of field is one which
is seldom examined in any great detail, but which is probably one
of the most important. The larger the sensor, the shallower the
depth of field. It's the sensor's physical size that's the key factor
here. A DSLR offers much less depth of field than a compact digital
camera, and a full-frame DSLR offers even less. This isn’t
strictly due to the size of the sensor but actually the required
focal length of the lens. For example, on a full frame DSLR, a photographer
might use a 50mm lens for a portrait shot. If he were using a digital
SLR with an APS-C size sensor, he would require a 30mm lens to get
the same angle of view. However, as already discussed above, the
wider 30mm focal length will give greater depth of field than a
50mm focal length. This is one of the reasons many portrait photographers
chose a camera with a large image sensor. This is exaggerated even
more with compact digital cameras where the sensor size is about
10 times smaller and therefore the camera uses, for example, a 6mm
lens which gives tremendous depth of field.
All these factors intermingle to make depth of field
a potentially confusing business. In practice, though, the decisions
are generally pretty straightforward. In any one situation, the
camera you're working with is already decided, and you've probably
settled on the camera position (the subject distance) and the focal
length (zoom position) needed to compose the shot nicely. All that's
left for you to adjust the depth of field with is the lens aperture.
You're no longer dealing with just the focused distance
- there's also a near limit' (the nearest distance where objects
will still look sharp) and the `far limit' (the farthest distance
where objects will still look sharp). The distance between the near
limit and the far limit is the depth of field. But how do you work
out what the near limits and far limits are?
With fixed focal length (non-zoom) lenses, it's
all rather easy. The lens has a focusing scale marked on the barrel
and an index mark showing the distance you are focused at. Either
side of this index mark are further pairs of marks, usually colour-coded
so that wherever you're focused, the `f8' markers (for example)
would indicate the near and far limits of the depth of field on
the distance scale. Many lenses include several sets of markers
for different lens apertures.
With these, managing depth of field becomes a lot
easier and you can start to plan your shots much more effectively.
Let's imagine you want to get both a nearby gate and a distant hillside
sharp at the same time. A modern camera will offer to focus on one
or the other, but both are wrong, or at least highly inefficient
choices. Instead, the ideal focus point will be somewhere in between,
so that the gate is just within the near limit of the depth of field,
while the hillside is just within the far limit. Rather than focusing
on one thing in particular, think in terms of zones of sharp focus
and how to make sure all the principal elements in your photography
can be made to fall within it. Modern cameras make this harder.
First of all, you have to disable that sophisticated and expensive
AF system and focus manually instead. Second, zoom lenses don't
have depth of field markings. This is because the depth of field
changes with focal length. With the exception of certain ancient
'trombone' zoom designs, depth of field markings are no longer possible.
A rather neat trick is to set the far limit at infinity;
you'll make sure that distant objects remain sharp, while maximising
the depth of field towards the camera. This is a well-known technique
known as 'hyperfocal' focusing.
As well as telling you the distance to focus at, they tell you the
nearest distance at which objects will still be sharp. You look
at the scene in front of you, work out how near the nearest object
is and look up the lens aperture you need to use.
It's important, then, to understand the significance
of depth of field and how to manipulate it if you want to properly
control the sharpness in your photos. One general rule worth bearing
in mind is that to get two objects at different distances to be
equally sharp, you need to focus at a point one third of the distance
between them. This changes slightly with close-ups, where the focus
distance should be nearer halfway between them.
Many DSLRs have depth of field preview buttons.
Normally, all viewing is carried out with the lens at full aperture,
regardless of the aperture value that's going to be used for the
exposure. The camera stops down to the required aperture when the
picture is taken. Depth of field previews, however, stops the lens
down manually. This darkens the viewfinder image, but also gives
an idea of how much depth of field there will be. Depth of field
preview buttons work, but the viewfinder image is often too dark
for details to be clearly visible, and it's difficult to judge the
exact sharpness levels as a result. With digital cameras you can
simply take the shot, examine it close-up in Playback mode and re-shoot
it if you need to. However, it's less hassle to be able to get the
depth of field right first time.
Instead of relying
on your camera's autofocus and program AE exposure modes , try switching
to manual focus and manual aperture control. Remember that photographs
don't consist of a single plane of sharp focus, but an extended
'zone' of sharpness which you can control. You really can point
and shoot - and not because you've invested in the latest and most
expensive technology, but because you're using a principle that
even the smartest camera has yet to exploit.
Use Of Depth Of Field
Landscape photographers generally weigh sharpness all the way from
the far distance right up to the foreground, and this is where an
understanding of 'hyperfocal distance' proves crucial.
Many street photographers don't bother with focusing at all as it's
too time consuming. Instead, you can use a knowledge of depth of
field to 'zone focus'. Here, you choose a lens aperture and a manual
focus distance which gives enough depth of field for any of your
usual subjects. For example, at f8 everything from 1m to 8m might
be acceptably sharp.
Shallow depth of field is not always a bad thing. In portraits it
helps blur unwanted backgrounds, and it can also be used to make
the main subject appear sharper. Here, these frost-covered railings
appear to have even more 'bite' because their sharpness contrasts
with the soft background. You can achieve shallow depth of field
with longer focal lengths and wider lens apertures.
Of Field And Close-Ups
Depth of field is a perennial problem for close-up photographers,
and the closer you get, the worse it gets, to the point where even
at your lens's minimum aperture the depth of field is only a few
millimetres. Here, for example, the photographer had to settle for
just keeping the bloom's stamens sharp - there was no way of keeping
the nearest and farthest petals sharp too.
When a point in the subject is perfectly focused it should appear
as a point on the image sensor. But when it's slightly out of focus
it is instead reproduced as a small disc. The technical term for
this is `circle of confusion', and you'll find it comes up frequently
in discussions about depth of field. If this circle of confusion
is small enough, the subject will look sharp even if it is slightly
out of focus. The point is that the maximum size for the circle
of confusion varies according to the camera or, specifically, the
sensor size. Compact digital cameras have much smaller sensors than
DSLRs, which means images must be magnified much more to make the
same-sized prints. This in turn means that the maximum allowable
size for the circle of confusion for compact cameras is smaller
than that for DSLRs. For the record, the usual circle of confusion
size used for 35mm film or full-frame DSLRs is 0.03mm, for APS-C
sized DSLRs it's 0.02mm, and for compact digital cameras it's 0.005mm.
The circle of confusion value is part of the mathematical calculations
used to work out depth of field figures.
To find out
the depth of field and hyperfocal distance of any given lens and
aperture combination, see the Depth
of Field Calculator utility.
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