Flashes & lighting
The battle between depth of light and depth of field Reference guide
Reference guide for The battle between depth of light and depth of field. 18 pages in English. Read the original PDF, download or print a copy without registration.
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- Norman Enterprises
- Model / document
- The battle between depth of light and depth of field
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- Reference guide
- Language
- English
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- 18
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- Flashes & lighting
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Page 1 · Read text
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'\=- \-MA-w)-
\
THE
BATTLE BETWEEN
DEPTH OF LIGHT
AND
DEPTH OF FIELD
(TUE INVERSE SOUARE LAW)
' BY BILL NORMAN
NORMAN ENTERPRISES, INC.
2601 Emoire Avenue. Burbank. CA 91504 (818) 843-681 1View original page 1Page 2 · Read text
THE
BATTTE BETWEEN
DEPTH OF LIGHT
AND
DEPTH OF FIELD
The "Inverse Square Law" is perhaps the most basic principle
in lighting, but we have found that many photographers are not
completely sure about its application to their photography.
If you photograph large groups, you know that it is often
difficult to provide enough depth of illumination to proprerly expose
the subjects in the back row. If you are a wedding photographer, you
may have had lighting problems such as the white wedding cake in
the foreground being over exposed when the bride and groom are
properly exposed. Perhaps you have time to "dodge" in printing and
correct these problems in your darkroom, but profit is affected when
custom work is required. Most photographers today use color
laboratories for their film processing and printing. Many labs can
provide this same custom work, but if you want to take advantage of
lower-priced automatic machine prints offered by most labs, it is
important that your subjects be evenly illuminated because these
machines do not have the abilitv to "dodge" and "burn-in" your
prints.
The purpose of this booklet is to familiarize you with several
principles of lighting. This will allow you to better use your artistic
talent to produce quality photographs economically.5z 7,--"*View original page 2Page 3 · Read text
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SQUARE LAW?WHAT lS TH/
/tNVERsE
"Light falls off as the square of the distance." This statement
is the Inverse Square Law, but you may ask, what does this have to
do with inverse? The word inverse is not even mentioned in the above
statement! The answer is that the remaining amount of light (illum-
inating the subject) is equal to the inverse square of the distance from
the light to the subject. In other words, if you place a light source five
feet from the subject, the amount of light that remains to illuminate
the subject equals the inverse square of five feet, or l/(5)'z. This is
l./25 of the amount of light at the source.
Let's assume that the correct f/stop for a particular flash unit
and film speed in this situation is f/8 (see fig. 1 ).
Flg.1
...5'......._ n o
*,=r tt tP I b ituminar,"r?ly:11'i3:l AView original page 3Page 6 · Read text
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HOW CAN DEPTH OF LIGHT
BEEXTENDED?
There are several ways to obtain greater depth of light:
1. Reposition the subjects or the light so that the distance
from the light to the subjects is closer to the same
footage as illustrated in fig. 3.
For candid work, this is perhaps the easiest and best
solution. However, in many cases, the most desirable
subject composition is sacrificed in order to provide
adequate illumination. Also, moving the subject could
cause undesirable shadows for proper subject illumina-
tion.
2. Feather the light away from the nearest subject so
that the illumination is directed toward the farthest
subject as shown in fig. 4.View original page 6Page 7 · Read text
Fig. 4
t/? Kfi
This can be a good solution if its application is practical
in your particular lighting situation.
3. Move the light farther from the subject. If we move
the light back to ten feet, the illumination on the first
subject will be the inverse square of 10 feet (1,/102),
which is 1/100. The illumination at the second subject
will be the inverse square of 12 feet (l/12'z) which
is I/1,44 (see fig. 5).
Fig. 5
1/z 1 I elop relationship
If the illumination at the second subject were l/200,
it would be I f,zstop less than at the first subject. How-
ever, 7,/144 is about Vz flstop less than at the first
subject. Therefore, we have extended the depth of
light.
tView original page 7Page 8 · Read text
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BUT what if the shooting space was not large - enough to enable us to move the light back as far as
10 feet? See solution number 4.
4. Bounce the light into an umbrella or bounce flat as
shown in fie. 6.
Mike Butkus Digitally DN:ou=butkus.org,Date:cn=Mike2024.05.06signedButkus,email=mike@butkus.org,by21:48:50Mikeo=ButkusButkus-04'00'camera manuals,c=US
3'fiom light to umbrella
+ 7' lrom umbrella to subiect
= 10' from light to subjecl
If the light is three feet from the umbrella and the
umbrella is seven feet from the subject, the light is
3*7, or 10, feet from the subject. Therefore, the same
depth of light is achieved as placing the light at ten
feet from the subject. In this situation the umbrella
could be called a "wall extender" because it enables
the light to be ten feet from the subject with only seven
feet of working space.
5. Use an additional light to help illuminate the second
subject. This.can be a good solution. It requires your
skill in light placement and technique so that the effect
' of additional light will not be detrimental to the overall
lighting of the photograph.
In actual practice, you may use a combination of these sug-
gestions to achieve the desired results. But, there is one potential
problem that has not been mentioned thus far whsn the solution
-to extending depth of light results in less illumination on the subjectsView original page 8Page 9 · Read text
(as in solutions 2, 3,4 and possibly 5), our hypothetical f/8 aperture
setting can no longer be used. This can result in a depth of field
(sharpness) problem. The extent of this problem depends basically
upon the focal length of the lens and the subject-to-camera distance.
Doubling the light-to-subject distance will result in an
exposure loss of 2 f/stops. Therefore, in solution number 3, we must
usef/4 rather than f/8. In solution number 4 we will lose more than
2 f/stops because of the losses in the umbrella or bounce flat and also
due to the larger area of coverage obtained by using this technique.
You may wish to refer to our booklet "Why Umbrella."
THE PROBLEM OF
DGDCh Gnutrno
If you focus on the first subject, the second subject may be
out of focus as illustrated in fis. 7.
Fig.7
out of focua
If you focus on the second subject, the first subject may be
out of focus (see fig. 8).View original page 9Page 10 · Read text
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Fig. 8
If you focus for the area between the subjects they may both
be out of focus as in fis. 9.
Fig. 9
.."&
Ff
,ffi
..''& :''sffiw"
*1 ,8,;f_ ry'
8:"'
both out of
The smaller the lens opening, the greater the depth of field.
Therefore, the basic objective, aside from reposing the subjects, is to
again stop down to f/8 (or perhaps smaller). Assuming that we do
not wish to use a film with faster emulsion speed, we simply need
more light at the subject. Four times as much light will be required to
regain the 2 f /stop loss in order to use f/8 again.
. Doubling the watt-second level of your flash unit will produce
one f/stop more light with a given lamphead. Therefore, if you are
using your flash unit at 50 watt-seconds, the 2 f/stop increase couldView original page 10Page 11 · Read text
be regained by increasing the energy level to 200 watt-seconds (as
diagrammed below in fig. 10).
Fig. 10
50 watt-rccond8 .
to I 100 watt-seconds (
to 200 watt-seconds , I
If your flash unit does not have the capacity to produce four
times the output, you could consider:
1. A more efficient reflector for the lamphead. Generally
speaking the quality of light will be harsher and - produce more contrast by using a reflector of this type.
2. Using several flash units into a single umbrella or
bounce flat in order to increase output (see fig. 1 1 ).
Fig. 11
100 watt-seconds. Tolalillumination is equal
) to a single 200 watt-second
100 watt-secondr light source into umbrella
t
I
3. Using one or more of the techniques described on
pages 6 thru 8.
You may end up in a compromise or in acquiring additional
equipment, but at least you understand the problem and its possible
solutions.
10View original page 11Page 12 · Read text
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WHAT IF THE MINIMUM DEPTH OF LIGHT
AND THE MINIMUM DEPTH OF FIELD
rs REQUIRED?
Thus far we related the inverse square law to a hypothetical
I situation where maximum depth of light was required. However, you
t probably encounter just as many situations in your daily work where
minimum depth of light is desirable. For example, when photograph-
ing a "low key" portrait, you don't want the main light to illuminate'
the background, nor do you necessarily want the background to be in
focus. You probably don't even want very much depth of light and
depth of field on the subject. Assuming that you are using a suitable
background for "low key" work, there are several ways to create this
effect:
1. Move the light closer to the subject (see figures 12 and
13).
Fig.12
1 f/stop less
illumination on
background
than on subjecl
t
illustrates that background will become darkert Fig.13
due to the ratio of subject illumination to background
illumination as governed by the inverse square law.View original page 12Page 13 · Read text
Fig. 13
fl,..E
2 f/slops
less lllumination
I thanon backgroundon subjectA8
2. Move the subject farther from the backsround as in
fig. 14.
Fig. 14
A --- -51---- --------- s', KJ
2llalope
less illuminalion on background than on subject
With a light-to-subject distance of five feet and a light-
to-background distance of 10 feet, the background
will receive 2 f/stops less illumination than the subject'
You will probably use a combination of both these techniques
to achieve the desired effect.
When using an umbrella light as the main source in "low key"
photography, you must move the subject farther from the background
than normal because the light-to-subject distance cannot be as short
due to the distance lost between the lisht to umbrella and back to the
light (see fig. 15).
t2View original page 13Page 14 · Read text
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Fis.15 .{>
Wfu t['^q 4'totat distance at tisht source
With the light 2 feet from the umbrella, you cannot place the
subject closer than about 4 feet from the light. Actually 6 feet is
perhaps a minimum distance because you must move the umbrella/
light far enough from the subject to keep it out of the field of view.
With the background 2 feet from the subject, the illumination
at the background is not even I f/stop less than at the subject (refer
to fig. 16).
Fig. 16
6'lrom light to subject
8'from light to background
less than 1 f/stop
Moving the subject 5 feet from the background will provide
the background with 2 f /stops less illumination than the subject as
shown in fig. 17.
Fig.17&b: *\l^s ehv
Background has 2 l/stops less exposure than subfect
13View original page 14Page 15 · Read text
WHAT HAPPENS
TO THE LIGHT THAT FA((o
o TI TI . a.,
Actually. li-qht doesn't really "fall off" (decrcasc in intcnsity),
it simpll, spreads out to illuminate a larger area as it movcs farther
from the source. Fig. 18 illustratcs this.
At five feet from the light, the qrea of illumination is 2-5 times
grcater than at onc foot. Thereforc. you are left with only | /25 of the
light actually illuminatin-q the subject. (If 1,ou are photographing in
foggy (or smog-ey) atmosphere or undcrwatcr, a small percentage of
the light is reflccted and refracted in a way that will sli._ehtly alter this
inverse square law. )
t4View original page 15Page 16 · Read text
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As the light-to-subject distance is extended by a factor of 1.4
times, the area of coverage doubles. Photographically speaking, this
means that you lose one f/stop every time you move the light back
from the subject to a distance equal to 1.4 times its previous distance.
We already know that doubling the light-to-subject distance results
in a2 f/stop reduction, but this 1.4 factor if thoroughly understood
will make a world of difference in your comprehension of "how it all
fits together." Slow down at this point and work at understanding the
significance of this 1.4 factor as diagrammed in fig. 19.
+
I
Fig. 19
1r 2'
.4'. 2.8'
I qf) llll -)))) +t t tt-/ -/ 1.4 1.4 1.4 1.4 1.4 1.4 1.4
x 1.0 L[ 2.0. 2.8 4.0 5.6 8.0 1.4 2.0 2.8 4.0 5.6 8.0 11.0
Notice how this 1.4 factor not only gives us a 1 f/stop light
reduction, but it also relates exactly to the f/stop sequence engraved
on the camera lens holder. The word area is the key to this correla-
tion a 1 f/stop light reduction is caused by doubling the area of - coverage.
"F/stop" relates to the amount of light entering through the
area of lens. As the diameter of the iris diaphragm is increased by the
factor 1.4, its area is doubled, allowing twice as much light to enter
through the lens.
Therefore, both progressions are exactly the same. So, rather
than committing light-to-subject distances to memory, we can use the
numbers that we already know are engraved on the camera lens holder,
15
it*View original page 16Page 17 · Read text
WHY DOESN,T MOVING
THE CAMERA FARTHER FROM
THE SUBJECT AFFECT THE F/STOP?
We all know that unless we are using a bellows extension,
changing the camera-to-subject distance does not affect f./stop as
long as the lighrto-subject distance does not change. But why is this
true? Since from the same subject less light will enter through the
camera lens as the camera is moved further away, it would appear
that we would have to open up the lens diaphragm to compensate for
the loss of light. However, the area of the image (subject) projected
onto the film also decreases by the same amount, thereby counteracting
the loss of light into the camera. This light falls off again according
to the inverse square law. If the camera is moved from 5 feet to 7
feet, one-half the amount of light from the subject will enter through
the lens. Since the area of the image on the film plane is now also one-
half, it requires only one-half the light to produce the same exposure
as shown in fig. 20. Therefore, the f/stop does not need to be changed.
Again we see the 1.4 constant (outlined in the previous section) at
work.
l6View original page 17Page 18 · Read text
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WHAT ABOUT SUNLIGHN
We have outlined the inverse square law as it applies to photog-
raphy when using artificial lighting. Sunlight also falls off as the square
of the distance, but since the sun is 93 million miles away, a few
thousand feet one way or the other isn't going to make any difference
in exposure. The only variances that you will experience in sunlight
are changes in light quality and intensity due to changes in the angle
of sunlight through the earth's atmosphere (basically time of day),
to weather conditions (cloudy vs. crisp sunny days), and to reflections
from the earth's surface (sea, desert, dark soil, etc.). You may want
to refer to our booklet entitled "Syncro Sunlight." This booklet outlines
principles in mixing electronic flash with daylight.
coNcLUstoN,
You have now read about the principles of the inverse square
law. You have the knowledge to utilize them to produce fine photo-
graphs for the satisfaction of you and your customers. But you must
work at applying these principles so that they will become second
nature to you. When this is accomplished, you will have developed
the skill to win the Battle Between Depth of Light and Depth of Field.
Your questions and suggestions are always appreciated at
Norman Enterprises, Inc. for they enable us to improve our products
and to meet your continually changing needs.
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