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Depth of Field
What is depth of field, what affects depth of field, and how can we use an
understanding of depth of field to take better pictures? This One
Pager™ Tutorial will make everything clear from near to far.
What Is Depth of Field (DOF)?
Depth of field (DOF) is the distance wherein objects are in focus.
This is the pragmatic explanation.
To be technically correct, DOF is the zone of acceptable sharpness, the area
in front of, and behind, a focused subject that appears in focus.
Circle of Confusion (COF)
Technically, only the subject in focus -- and all other objects at the same
distance -- are in focus; everything else in front and behind are out of focus.
How much out of focus depends on a term called the "Circle of Confusion
(COF, or COC)."
Let's see if we can make that term clear, but if you don't understand it, don't
worry. Many photographers take superb pictures using depth of field to their
advantage never having heard of COF. I never did until I started researching
this topic. And, I find that it just confuses the hell out of most people, so
if you want to skip to the next paragraph, here is your
chance to do so.
OK, back to the COF.
Imagine we are photographing three (3) dots. They are the tiniest dots the
human eye can clearly make out, and of course, we are assuming perfect 20-20
vision and ideal light condition.
So, here we have these 3 dots, arranged one behind the other (with the closest
dot to the right of the middle dot, and the farthest dot to the left of the
middle dot, so a camera can take a picture of all 3 dots).
Now, we focus our lens on the middle dot, which comes out in perfect focus.
The two other dots also appear in focus, but peering closely at the resultant
photograph, we notice, however, that the dot in front of and behind the middle
dot appears as circles instead of perfect dots. I.e., technically, they are
out of focus, but to our naked eyes (at a "normal" distance), they
"appear" in focus.
It is this circle that we call the "Circle of Confusion." So the
COF is the diameter of a dot such that when we view it with the naked eye, it
appears in focus. If this circle gets past this diameter, our eyes tell us it's
out of focus.
Lens manufacturers have to decide what that diameter is going to be and design
their lenses accordingly to be able to resolve a dot within that COF so that
it appears sharp to us. For a more technically detailed explanation of how COF
is determined by lens manufacturers, read Michael Reichmann's excellent tutorial:
Understanding
Depth of Field.
That is about all I am going to say on COF. Do you need to understand COF to
use DOF? Personally, I don't think so. But if you are one of the people who
absolutely must understand COF, its history, how lens manufacturers calculate
DOF based on the COF, etc. etc., just check out the link above or do a search
on Google. There's enough material there to keep a technical mind happy for
many hours.
What Affects DOF
What we really want to know as photographers is what affects DOF so we can
control DOF in our pictures. For a long, long time, photographers have gone
with the following three criteria:
lens aperture
distance from subject
focal length
While the first two are technically correct, the third one has raised somewhat
of a storm of controversy among certain circles. Why exactly, we will make clear
later. Let's look at each of the three criteria in more detail.
Lens Aperture
The aperture is simply the size of the opening that allows light to go through
the lens. It is expressed in f/stops (also referred to as f/value
or aperture value), and a typical aperture range is f/2.8 - f/8, giving
the range from maximum (large at f/2.8) to minimum (small at f/8) aperture.
A small f/value (e.g. f/2.8) indicates a large
aperture.
A large f/value (e.g. f/8) indicates a small
aperture.
So, f/2.8 is a larger aperture than f/8.
Generally, a large aperture gives a shallow DOF, and a small aperture gives
great DOF.
f/2.8
f/3.5
f/4
f/5.6
f/8
large / max
aperture
small / min
aperture
less DOF
more DOF
great for
portraits
great for
landscapes
Putting Aperture into practice:
If you want only the subject the lens focuses on to be sharp, and everything
else to be out of focus -- such as a portrait with the background nicely
blurred -- then you would "open up the aperture," i.e. use a
large aperture.
If you need most of your picture to be in sharp focus -- such as a landscape
scene -- then you would "stop down the aperture," i.e. use a small
aperture.
Shallow DOF
Great DOF
Note how the use of a large aperture throws the flowers in the background
out of focus.
Focus has to be precise.
Using a small aperture
extends the DOF from the foreground
all the way to the background.
49.8 mm, Av, Spot,
1/30 sec., f/3.5,
+0.7EV, Macro, Tripod used
49.6mm, Av, Spot,
1/5 sec., f/11,
+0.7EV, Macro, Tripod used
Minolta DiMAGE A2
In most consumer digital cameras, you may not be able to directly control the
aperture. Many, however, provide a Portrait scene mode and a Landscape scene
mode that basically do what we are after, i.e. use a large and small aperture,
respectively.
As most of you have found out by now, it is quite difficult to obtain a shallow
depth of field with most consumer digital cameras even with the aperture opened
up wide. Why? See the section on focal length below.
Distance From Subject
When you focus on a subject close to the camera, the DOF is less than when
you focus on the subject farther away from the camera.
Putting Distance From Subject into practice:
Step away from your subject to obtain greater DOF, or move in closer
to decrease DOF.
Moving In Close:
Even though we use a small focal length (35mm, 35mm equivalent) and
a small aperture (f/7), by moving in close (macro),
we are able to isolate the subject from the background.
If we had used a larger aperture, we would have obtained even shallower DOF.
Fujifilm FinePix S7000
Focal Length
Set your subject in front (say, about 3m or more) of a bush (or a tree with
lots of leaves, or some other kind of busy background). Use wide-angle and take
a picture. Both your subject and the bush would most probably be in sharp focus.
The background may distract from your main subject -- unless you're aiming
for a Product Shot where you want to situate your subject in his or her
environment, and do want both subject and background to be in sharp focus.
Now zoom in and fill in the screen with your subject's face and shoulders.
Look at the resulting image and you'll probably notice that, though your subject
is still in focus, the bush now appears out of focus, giving a nice blurred
background that does not steal attention from your subject.
Photographers use this technique very effectively to "affect" DOF.
So, a wide-angle lens has greater DOF than a telephoto lens. Most consumer
digital cameras have very short focal lengths and that is why it is so difficult
to obtain shallow DOF, even with the aperture opened up wide.
I promised to come back to the storm of controversy brewing in some circles
that disputes that focal length has any effect on DOF.
The contention is that, for a fixed image size in your screen, the DOF is unchanged
irrespective of focal length used. And they have pictures
to prove it!
However, photography is part technics and part art. The technics part may well
tell us that DOF is the same at all focal lengths for a fixed image size on
screen. What this is saying is that we achieve the same DOF whether we fill
the screen with the face by walking to the subject, or backing off and zooming
in.
The art side, however, tells us that a long lens (i.e. long focal length) reduces
the distance from the subject and thus provides a shallow DOF. Yes, we can achieve
the same narrow DOF by walking close to the subject. However, the results look
very different! Why? Because a long lens also provides a "flattened"
perspective which makes the narrow depth of field much more prominentthan walking closer to the subject using a lens with normal focal length
does. The link above with all the pictures prove it.
Putting Focal Length into practice:
Use long focal lengths to achieve the effect of a shallow DOF.
Long Focal Length:
Bu using a long focal length (380mm, 35mm equivalent), we are able to
"throw the background nicely out of focus"
Minolta DiMAGE Z1
Putting It All Together
Putting it all into practice:
A large aperture (f/3.2), moving in close to your subject (macro) and using
a long focal length (112mm, 35mm equivalent) has thrown the grass into a nice
green blur
Olympus Camedia C-8080 Wide Zoom
A Word On Aperture
Following our rule of thumb on using a small aperture to obtain a great DOF,
does this mean that we should always use the minimum aperture if we want the
greatest DOF?
It sounds logical, but because of diffraction that comes into play when the
aperture is too small, this may affect the quality of the image, giving the
appearance that some objects (especially light sources) are not in focus (but
hey, use this to your advantage to obtain starry effects when photographing
light sources!). So you don't have to stop down to the smallest aperture for
maximum DOF. For example, on 35mm cameras (where the aperture stops all the
way down to f/11 or even f/16), f/5.6-f/8 are usually the best apertures to
use for maximum DOF. On digital cameras, try f/5.6.
Pseudo DOF
A word of caution. Some use post-processing and apply a Gaussian (or other
type of) Blur to achieve a blurred background effect. Depending on your skills,
this may be quite effective but, to a trained eye, this is almost always apparent
and looks "false." So never try to pawn off a processed blur as naturally
obtained shallow DOF. Also, be careful not to apply this blur effect to all
your pictures, but only to a few whose subject matter will benefit from it and
where it won't matter that the blur is processed, not natural.
Summary
Even though the subject you focus on is the only thing technically in focus,
some objects in front and behind your focused subject also appear to our eyes
to be acceptably in focus (thanks to the COF). This zone of acceptable sharpness
is the DOF.
We can increase the DOF (or the appearance of it) by using a small aperture,
moving away from our subject, or using a wide-angle lens.
We can decrease the DOF (or the appearance of it) by using a large aperture,
moving closer to our subject, or using a long focal length.
Our Readers
Write Back
From:
Richard Crossley
Feb
8, 2006
Differences in perspective are not the result of focal
length, but of physical distance, as [we] move the camera from shot to shot.
The truth is that focal length does not affect either
the DOF or the perspective, at least technically. A 50mm shot and a 300mm shot
from the same spot would of course appear different, but that is the result
of magnification. If the 50mm shot is blown up until the image
of the 300mm shot could be cropped from it, they would be identical. The
longer focal length does not change the DOF: it is simply magnifying the out
of focus area.
In the movie making business, it is critical to understand
that, so we don't automatically just run back 100 feet and create audio problems,
and also so that we focus on the framing and desired image size, knowing that
even though our small LCD display may not be giving us that 'soft background'
we want, that when the entire picture is blown up, the true effect will indeed
be seen.
Also, as a filmmaker, I find you miss a large and important
factor in your DOF discussion. Whereas you list focal length as a true determining
factor when in fact it is not, you fail to mention target size at all,
which is a huge factor, and represents the single biggest reason why it is harder
to get a shallow DOF from a digital camera. The relationship of the physical
target size (1/4 inch CCD as opposed to 35mm film) makes a HUGE difference.
[Editor: emphasis ours, and a lengthier version of the
discussion can be found here
]
.
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Page not found – Photoxels
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