American Cinematographer - December 1980 - 8

TALKING TECHNICALLY
By DAVID W. SAMUELSON
CALCULATING DEPTH OF FIELD
The difference between

Hyperfocal distance
'n

and boys

is the price of their
toys, they say, and undoubtedly the best toy I've been given for
a long time is a wrist watch calculator.
It set me to thinking of all the calculations that can be done
with it as an aid to cinematography, aside from the obvious
ones, like working out overtime, etc.
A good example is a Focus Split.
A typical scene calls for two objects at different distances to
be held in equally sharp focus. To what distance should the
men

lens be set?

My friend Bill Pollard, who designed the Samcine and Kelly
Calculators, taught me that the split focus distance =
2

x

nearest distance

x

farthest distance

nearest distance + farthest distance

For

example, fora particular two-shot, one person is 6 ft. 8 in.
camera and the other 8 ft. 4 in. What is the split focus

from the

distance?

Firstly, you have to reduce that to common units, like 6.66 ft.
and 8.33 ft, or easier still, 80 and 100 inches. Let's use the
inches

one.

Then you

enter it into the calculator thus:
Example

Key Sequence

Display
80

Touch

Touch

-

Touch

x

Enter

Touch

H-

180

2
x

Touch

2

2

Enter circle of confusion

X

0.011111

Touch

X

100
=

80

0.088888
88.8888

A useful

aspect of this particular calculation, incidentally, is
that the same formula can be used for feet, inches or meters

having to change anything.
one

to know how to work out on a calculator is

(H^,

because not only is it the distance to
ensure that infinity and a distance
half that to which the lens is focussed are equally sharp, but it
is an essential ingredient for all depth of field calculations.

which

BEHIND

a

lens must be set to

x

x stop

value

0.001

(mm) x stop value

h-

Display

100

100

X

100

=

10000

X

10000

0.000129

0.000129

-7-

-s-

Enter stop
Touch =

Example

0.0014
-r

value

1.29
.0014

921.42857

8

8

+

115.17857

100

Answer: 89 inches, near as dammit, or 7 ft. 5 in.

hyperfocal distance

(to square)

Touch 0.000129

80

Another useful

*'Touch =
Touch x

0.005555!

Enter nearest distance

without

x

0.005555!

Touch

x

length (mm)

X

-

Enter farthest distance
Touch
=

PFLIALNME

Key Sequence

+

+

0.000129

(in.)

(Note, these particular formulae for hyperfocal distance make
possible to enter lens focal lengths in millimeters and the
circle of confusion in decimal parts of an inch or millimeters
and get the answer directly in feet or meters.)
The next question to arise is what circle of confusion to work
to? The lower the value, the greater the depth of field; the
higher, the less the depth of field. The answer is purely subject¬
ive. One works to whatever value is acceptable.
For shots using an early zoom lens and/or fog, diffusion or
star filters or nets etc., with soft smoochy lighting and no action
or anything brightly lit in the foreground or background, then
1/500 or 0.002 in. (0.05mm) is adequate, but shooting with
modern sharp lenses with no diffusion, and contrasty lighting,
then even 1/1000 or 0.001 in. (0.024mm) may not be small
enough. There's no magic number, it all depends on what looks
acceptable on the screen in the circumstances.
Taking a compromise figure of 1 /700 or 0.0014 in. (0.35mm)
and a lens focal length of 100mm and a focus distance of 10 ft.
at f/8, the way to calculate hyperfocal distance is:

Touch

100

Iength2(mm)

QjrC|e 0f confUSj0n

x

it

Enter lens focal

100

Iength2(mm)

Circle of confusion

Focal
-

80

Enter farthest distance

PFOILIRANNMET

meters

80

*Touch

MM

Hyperfocal distance
in

Enter nearest distance
+

Focal
-

Answer: 115 ft.
An

interesting thing about the hyperfocal distance is that
having worked out (or looked up) the hyperfocal distance for a
particular focal length lens at a particular circle of confusion
and aperture, it is very easy to work out what it would be at
another stop because it is proportional. In the instance above
at f/4 the figure would be 230 ft. and at f/16 it would be 57 ft. 6
in. In fact, if you note down what it would be at f/1 (14.375 ft),
then for any other aperture you just multiply by the stop
Continued

on

Page 1222

ENTRANCE PUPIL POSITIONS RELATED TO FOCAL LENGTH AND FILM PLANE
COOKE 20-100mm VAROTAL and 25-250mm CINE VAROTAL ZOOM LENSES
American Cinematographer - December 1980

Table of Contents for the Digital Edition of American Cinematographer - December 1980
