Exposure meters — Photofacts Reference guide

CONTENTS


Chapter                                     Page,
      INTRODUCTION                               r96
     r  A.B.C. OF EXPOSURE                        r97
       Law of Reciprocity-What a Meter Does-Capabilities of
          the Film-Value of Minimum Exposures-Exposure  Latitude
        and Subject Contrast-Factors  Affecting Exposures-British
         Standard Exposure Indices-Lens F-numbers-Exposures  for
          Close-up Work.
     EXPOSURE TABLES & CALCULATORS 2o7
            Difliculties with Tables-British Standard Exposure Tables-
        Using Exposure Tables-Exposure Calculators-Tables  for
            Artificial Light.
          PHOTO-ELECTRIC EXPOSURE METER 2ro   3 THE
       The Photo-electric Cell-Acceptance Angles-Dial Calibra-
         tions-Meter Calculators-Bright Light and Dull Light-
         Using a Meter-Average Brightness and Shadow Brightness.
            EXPOSURE METERS                 223   4 VISUAL
         Extinction Meters-{Eve Sensitivitv-Value  of  Extinction
        Meters-The Exposure Photometerl-The Wedge System-
         Comparison I-amp Calibration-Range Shifts-Photometer
            Scales.
      USING AN EXPOSURE METER        44   5
        General Readings   Shadow Readings  The  Highlight         System    Brightness- Range Exposures    -General Advice--            Artificial- Lighting.                        -
   6 EXPOSURES IN REVERSAL WORK       48
         Highlight Exposures-Incident Light Measurements-Meter
          Calibration-Difficult Subjects.

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   INTRODUCTION


      Anyone who is able to give an appropriate exposure every time
      he takes a photograph is well on the road that leads to gbod
       pictures. Suitable exposure is a vital factor in negative making.
     The term " correct exposure "  is being studiously avoided.
     More often than not, any of half a dozen different exposures
      would be equally correct because of the big latitude of modern
         films.
         Latitude is a very helpful quality. But it is also a hindrance
     as it encourages careless computing of exposures. Latitude
      should be regarded only as an insurance ; an insurance that,
      in the event of small errors of calculation, prevents spoiled
        pictures.
      No apology is needed for the first chapter of this book, which
       deals with the whys and wherefores of exposure of the negative.
       Before we can measure, we need to know what precisely we are
      measuring, and whether we are measuring the right thing.
      The inclusion of a chapter on exposure tables is justified
     by the fact that they are a substitute, if not a perfect one,
       for a meter. Many photographers use them successfully, and
       there are right and wrong ways of using them. A chapter has
      been included on the use lf a meter for the exposure of reversal
       materials, which include colour films-an important matter in
      view of the very small exposure latitude in this branch of
      photography.
                                                       Dunn,      The author's grateful thanks go to Mr. J. F.
       A.M.I.E.E., F.R.p.s. for his helpful criticisms and suggestions,
     and for his kindness in correcting the proofs.
                                    George L. Wakefield.





         196


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A.B.C. OF EXPOSURE
                                             Cltapter r

Exposure in photography has two components ; the intenJty
of the light, and the time for which it is allowed to act. For
most practical pu{poses the " law of reciprocity ", &s it is called,
holds good. Stated mathematically, this reads:       I x T:k
                                                  - which it iswhere I is the intensity of the light, T the time for
allowed to act, and k is a constant.
Law of Reciprocity
  Before the reader closes this book in horror, let us leave out
the algebra and state this important law in a simpler way. Let
us assume that an exposure time of I second to light of roo
units gives a certain result in terms of blackness of a silver image.
By the law of reciprocity all the following exposures would give
the same result:-
              Time (seconds)       lntensity
                                   t lro             rooo
                          +               2o0
                    Jroa             20IO
                         roo                        I
                         and so forth
And that is the law of reciprocity which, while it does break
down occasionally, is true for most conditions of exposure.
  In calculating what exposure to give to the film, we are trying
to find out the intensity of the light falling on it, and having
done so, we adjust the time of the exposure accordingly.
    Within limits we can control the intensity of the light fall-
ing on the film by altering the lens aperture. Therefore, our
exposure meter nearly always gives a number of exposure times

                                                                               197

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       along with the F-numbers to use with them. Thus r/roo second
       at Fz is equivalent to r/5o second at F2.8, r/25 second at F4
      and so on. Which combination of time and lens aperture we
       use does not matter at all from the exposure point of view, but
      the choice made may be determined by other considerations
      such as the shutter speed required to arrest movement in the
       subject or the depth of field required.
      What a Meter Does                                             .f
       So we use an exposure meter to measure the intensity of the
        light on the film. This is done indirectly by measuring the light
        reflected by the subject, but there is a simple relationship be-
       tween the brightness of the subject and the intensity of the image
        in the camera. This is taken care of automatically by the meter.
       The question now arises as to the part of the subject that
      must be measured to give us our exposure. Quite obViously
       every subject consists of a fairly wide range of brightnesses from
      high to low. Surely we do not have to measure every tone?
     The answer to this question is, of course, " No ".
        Capabilities of the FiIm
        Let us consider the film. For the moment, never mind the
       question of its speed. If we take a strip of film and expose it in
       steps so that at one end it receives the smallest conceivable
       exposure, and as we move towards the other end the exposure
      becomes bigger and bigger until it reaches a figure much greater
      than would ever be encountered in practice, the result, after
       processing, will be that qne end of the film will be perfectly
       il.a.; th; other practicd{ly opaque. There will be i  tot ot
       clear steps at the one end indicating that the first few smallest
       exposures were insufficient to affect the film. At the other end
       there will be a number of steps all equally black, showing that,
      above a cretain limit, more exposure fails to give any increase
       in blackness.
       The only range of exposures of any use with this particular
        film is that which does show differences in blackness for every
       variation in exposure. In deciding what exposure to give in
       the camera we must anange matters so that the deepest shadow
       in our subject is given enough time to send sufficient light to
         t98
t

the film to form subsequently a faint deposit of silver noticeabiy
different from clear emulsion. All the other brighter tones of
the subject will be darker in the negative, as they send more
light energy to the film. The time of exposure is the same but
                                                                                      s&1z,the intensity is greater. The brightest tone of the subject os,
the sky in an outdoor scene will be almost black in the negative,
but the film is capable of giving many darker tones than that
representing the sky. In other words we have not exhausted
the tone reproducing capabilities of the emulsion.        t
Value of Minimum Exposrffes
  The foregoing conditions result in the best possible negative
from the point of view of exposure. It will show adequate
separation of the tones of the subject but it will be thin, and
  [hi.t negative is always better than a dense one for the following"reasons:-
   (r) Printing exposure times will be short
                     less than with a dense negative   i"j  Grainin-ess is  (S) Lack of definition due to halation and irradiation will not
        arise
  (+) There is very little danger of clogged-up highlights due
      to their being over-exPosed.
                                                               care  " Expose for the shadows and let the highlights take
of themselves." If we adopt this well-known plan we shall
secure negatives in which the deepest shadow tone is a density
only a little removed from clear emulsion. The tone range
of modern negative materials is so large that very rarely will
the highlight tones be ovgr-exposed.
   If we give an exposure fireater than is necessary to yield just
a faint density in the shadows our negatives will be generally
denser, but unless the exposure is very much more than the
minimum, it is still unlikely that the highlights will be over-
exposed. If less exposure than the desirable minimum is given,
the shadow tones in the negative will be clear film and there
will not be sufficient difference between the darkest subject tones
for them to be reproduced as separate tones in the print. This
is under-exposure.
  In practice we generally aim to give a little more than the
bare minimum exposure, just to be on the safe side. If no

                                                                                199

                                                                      I

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margln of error is left in the direction of under-exposure  the
smallest siip in calculation or setting of sirutter speed and lens
aperture may mean an unsatisfactory negative. All exposure
tables, calculators, and meters incorporate a safety factor.
Exposure Latitude and Subject C&?tstrast
   It is worth considering the magnitude of the exposure latitude
in practical photography. A good film can r.p-doce a scene
in which the brightest highlight is r,ooo times is bright asthe
deepest shadow. This means that with such a subject, if the
exposure is regulated so that the deepest shadow is reproduced
in the negative as a tone only a little removed from ciear film,
then the highest light will be reproduced as a high density but
one still distinguishable from the other dark negative tones.
 A subject with a brightness range of r,ooo to r is rarely
encountered. If one is, and the film can just reproduce it, then
the exposure latitude is obviously nil. one exposure and one
exposure only will ensure good gradation from highlights to
deepest shadow. Give too small an exposure and the shadows
become clear film ; give too big an exposure and the highlight
tones merge together in an even blackness.
                                                               the- The brightness range of the average image projected bylens on to the film is about 3o to r.  This is ioniid.rably  less
than the average subject brightness range, but this reductjon is
due to lens flare. It is however, the image brightness range that
we reproduce. with a range of 3o to r the situation as regards
exposure latitude is rather different. If our film can reproduce
a range of r,ooo to t ,q:  the latitude in exposure is
                       :33 to r'                       t"*
Let's call it 30 to r to make it a nice round figure. If, then, our
minimum correct exposure is found to be say, x f z5 second then
                                     seconds we shall still haveif we give in error,  r lz5 x 30 :  r-r lS
a negative in which the subject tones are correctly reproduced.
If will be very dense, probably very grainy, and certainly less
sharp than it could be, but nevertheless capable of yielding a
good print. Any exposure between the minimum of.rlz5 second
and the maximum of r-r/5 seconds would give equally good
negatives from the point of view of gradation.
200

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  204

This business of exposure latitude is not quite as simple as
 some people think. It depends only partly on the film ; the
 brightness range of the subject has a big influence on it, and even
 the development given to the film has some effect on it.  In
 using an exposure meter, greater care is needed for contrasty
 subjects than for flat ones.
 Factors affecting Exposures
  Having discussed the crucial points of exposure in ne$rtive
 making we can list the factors thal have to be taken into aciount
when computing exposures ; factors which the exposure meter
 takes care of.
   (r) Strength of the light reflected by the subject
   (z) Speed of the film being used
   (E) Then lens aperture
Minor considerations such as the factor of any filter used and loss
of speed caused by the use of an ultra fine-grain developer have
to be allowed for by the photographer.
                                                confusion regard-- Until recently there has been a great deal ofing speeds of photographic emulsions and the photographei has
been faced with several kinds of speed numbers; Scheiner,
D.I.N., H. & D., Weston, G.E., Ilford Speed Groups, and
Kodak Film Speeds, to name just a few.
British Standard Exposure Indices
  This situation has now been altered. A British Standard
has been issued covering emulsion speeds (8.S. r31o-rg47,
" Determination of speefl and Exposure Index of photographic
Negative Material " ) and films are now available markJd with
British standard exposure indices. There is no doubt that before
long, all films of reputable make will be marked with them, and
they can be relied upon to be a true indication of the speed of a
film. American Standard exposure indices are in exact agree-
ment with the British Standard.
  B.S. exposure indices can be in either of two forms  ; arithmetic
for use with meters scaled with speed numbers such as weston,
and logarithmic for use with meters scaled in Scheiner and simi-
lar speed numbers. Although there is no exact equivalence
between various speed numbers except between B.S. and A.S.

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                                                       Dunn,      exposure indices, the table below, compiled by F. J.       A.M.r.E.E., F.R.p.s. will be found useful as a rough guide when
       conversion from one speed number to another is necessary.

      Lens F-numbers
       The final factor taken into account by the scales on a meter is
      the F-number of the lens. This is perfectly straightforward, as
       stopping the lens down from one F-number to the next higher
      one necessitates doubling of the exposure time.
      Exposures for Close-up Worh
      When photographing objects very close to the camera when
       the bellows extension is substantially greater than the focal length
       of the lens, the relative F-numbers engraved on the lens are no
      longer a true indication of the illumination on the film. The
      exposure has to be increased, and the amount of the increase can
     be calculated from the formula:-
       Correct Exposure :  Indicated exposur.                                          " ,ry
      Such a calculation arises in copying and the photography of small
       objects such as stamps and coins.


            B.S.l. Degrees                  l9     22     25     28      3l     34     37
          Schei ner Degrees (British
         and Continental)           20     23     26     29     32     35     38
         D.l.N./t0'                              t3      t6      t9     22     25     28
        Approx. H. & D. (British)   200    400    800          I 600   3200   6400   t2800
          llford Group               B    c    D      E       F    G    H
         \.S.A. (American Standard
           Assoc.)...                 6      l2     25     50     r00    200    400
       American Scheiner De-
          grees                         l5      t8      2a     24     27     30     33




       206

[.-

EXPOSURE TABLES AND
   CALCULATORS
                                                   Cltapter z

      The difference between exposure tables and exposure meters is
        that while a meter rneasures the light reflected by the subject,
         tables can only estimate it. This is their weakness. The amount
        of light reflected by the deepest shadow of a subject illuminated
      by daylight is influenced by many things:-
           (t) Geographical latitude
          (z) Month of the year
         (S) Time of the day
         (+) Atmospheric conditions
         (5) Typ. of subject.
         Difficulties with Tables
         Exposure tables have to take all the foregoing into account
       and while the influence of latitude, time of year, and time of day
       can be foretold fairly precisely, the atmospheric conditions and
       type of subject are rather indeterminate. For example, it may
T     be very difficult for the photographer to decide whether say,
rl      the day is dull, very duJl, or something between the two ; or
       whether a subject is in heavy shade or light shade.
FI         In spite of these difficulties, a set of tables is better than the
       most skilful guessing. They are available in various forms both
         for daylight and artificial light, and the precise methods of using
      them differ also. Instructions are supplied with them which
       should be followed carefully.
        British Standard Exposure Tables

        I
       The British Standards Institution has issued a set of tables,il      R.S.gSS  : 1948. A copy of this costs zf -post free from Britishil
       Standards Institution, z4f 28, Victoria Street, London, S.W.r.
    I                                                                           207
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      These tables are excellent, as one would expect in a British
      Standard and they are based on a new and considerably revised
      assessment of the relationship between variations in the light at
        different times of the day and year and the required exposure.
      They give more satisfactory results than some of the older tables,
       particuiarly early and late in the day. It may be worth describ-
      ing how they are used. They are for use in daylight only.
       Three tables only have to be used for the determination of
      exposures. The first takes into account latitude, month of the
      year, and time of day, and yields a number for the conditions
       existing at the time of exposure. This number is added to a
      second number found from the second table, which covers the
      weather conditions and the type of subject. From the sum of the
      two numbers is subtracted the B.S. exposure index (logarithmic)
       of the film being used, and the answer is used with the third table
      to find the exposure times needed at various lens apertures.
      Nothing could be simpler or much quicker.
        These tables and several others of similar type are logarithmic;
      that is, the addition of 3 to any of the numbers in the tables
     means a doubling of the exposure. Decrease of 3 means halv-
       ing of the exposure.

      Using Exposure Tables
        There is nothing difficult about using tables, but the important
      point is to take care in classifying the weather conditions and
       in particular, the type of subject. It is easy to classify straight-
      forward subjects such as open beach scenes, figure studies in
     an open situation and so f.orth, but when it comes to things like
       pictures in a wood, or pdftraits indoors near a window, errors
     can arise because of the impossibility of assessing the lighting
      conditions accurately. No tables can give a reliable estimate
       of the light which filters through the trees of an unknown wood,
      or of that coming through the window of a room.
      When in doubt it is better to err on the side of over-exposure
      than to risk under-exposure. So if it is doubtful whether con-
       ditions should be called dull, or very dull, call it very dull,
     or something between the two. Similarly if  it is difficult to
     make up one's mind whether a room comes under the heading
      of light interiors or dark interiors, call it a dark one. The evils

        208

\- .*..

of even considerable over-exposure are not so great as those of
 real under-exposure which can result in no n"gatirre at all.
   This tendency to expose generously must not be carried too
 far, especially when using a small camera. The resolving power
 of a film is diminished by excessive exposure and good en-
largements become impossible. The safety factor of the tables
may be of the order of 4,so that there is a good margin.
   If a certain set of tables is used always, experienc. *itt slowly
improve the standard of results secured with them. one soon
discovers any little peculiarities they may have in the classifica-
tion of subjects and so forth. Also, if negatives are consistently
thinner or denser than the photographer likes, inciicateh
exposures can be modified by some factor evolved from
experience.

Exposure Calculators
                                                They- Exposure calculators are merely tables mechanized.have no merit not possessed by tables beyond that of .on-
venience. They range from very simple affairs to complex
arrangements like an engineer's slide-rule. The remarks made
concerning the classification of weather conditions and subject
apply to calculators just as much as to tables. Having found a
calculator which gives fairly consistent results, stick to it and
master it. To change from one to another in the hope of finding
perfection is a waste of time and money and will result in nothing
but disappointment. A number of the calculators recently put
on the market are based on the B.S. Exposure Tables alreidy
referred to, and given idgntical results, but more conveniently.
Tables for Artificiat Light
  Tables and calculators for artificial light are obtainable. In
place of assessment of conditions determined by latitude, time
of year and day, and so forth, we are concerned now with the
number of lights and their wattage, distances between lamps and
subject, types of reflectors, and tone of the walls and ceiiing of
the room. Such tables are generally more helpful than thosJfo.
use in daylight as the variety of subjects in an ordinary room is
more circumscribed than out of doors and can be classified rather
more precisely.

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THE PHOTO-ELECTRIC
 EXPOSURE METER
                                        Clapter 1

 The photo-electric meter is the most popular of all and must take
 pride of place in this book. Unfortunately there is a regrettable
 tendency blindly to accept a meter reading regardless of the fact
 that in nine cases out of ten the meter is not measuring the quan-
 tity we want to measure-namely, the brightness of the deepest
 shadow. Simple methods are suggested in chapter Five for its
 use which help to overcome its deficiencies.
 The Photo-electric Cell
   In Figure r is shown a schematic arrangement of a photo-
 electric exposure meter. When light falls on the photo-electric
 cell, a small electric current is generated and within wide limits
 the current is directly proportional to the light energy. Con-
 nected to the cell is a sensitive microammeter which, by measur-
'ittg the current from the cell, indirectly measures the light
 enerSy.
   In front of the cell is an arangement of baffIes so that it is
 affected by light only f.rom a fairly restricted angle. The
 " acceptance angle " of * good meter is very little larger than
 the angle of view of the normal camera which is about 55.. If the angle of acceptance is very much bigger than this, then
 exposure readings will be influenced by objects not to be included
 in the picture.

 Acceptance Angle.s
  The wider the acceptance angle of a meter the more light
 falls on the cell, the more current reaches the microammeter,
 and the more sensitive the instrument appears to be. what is
 more, if the current output from the cell can be made large in
 2r0

this way, the microammeter can be an insensitive one and con-
    sequently inexpensive. There are very many meters still in use,
    mostly of pre-r939 German origin, with high sensitivity owing
    to their having a very wide acceptance angle-sometimes loo'
    or more.
    Dial Calibrations
    The calibrations on the dial of a meter can be a series of
    arbitrary numbers representing light values, actual exposure
    times at a particular aperture and for a film of given speed, or
    they may be units of brightness such as foot-lamberts or candles
    per square foot. The latter are helpful kinds of scales when it
      is wished to measure brightnesses in absolute units. In photogra-
   phy it is necessary to give exposures which increase geometrically

                     tVder sco/e                         Pholo-e/eclrrc  (e//.

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                                                'tl|rcroammeter



                                             Digitally signed by Mike Butkus
                                  DN: cn=Mike Butkus, o=Butkus camera
                                     manuals, ou=butkus.org,Mike Butkus email=mike@butkus.org, c=US                                        Date: 2022.01.04 19:55:00 -05'00'     Fig.  I
    in order to secure equal increases in density, so the scale on the
   meter is arranged geometrically. Take the well-known Weston
   meter for example. Thiil is calibrated in candles per square
    foot but the scale does not run ro, 20, 30, 4o, 50, and so forth,
   but o.z, o.4, o.8, t.6, 3.2,6.5, r3, 25, So, roo and so on, up to
    r,6oo candles per square foot. This form of scale is secured by
    incorporating a suitable resistance in series with the photo-electric
     cell and the micro-ammeter, but it win be noticed with all meters
    that the scale tends to be crowded a little at one or both ends.
    I'his is unavoidable.
   Meter Calculators
    Most meters have a calculator which serves to convert lhe
    light values on the dial to exposrrre times, It generally includes
                                                                                2ll

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a film speed scale and an F-number scale, but the calculator  is
a matter of convenience rather than necessity. The needle could
point to times of exposure for one F-number and one film speed
and leave the calculations to the photographer.
Bright Light and Dull Light
  The brightness of a subject can vary through a wide range
and to obviate a very crowded meter scale it is customary to
provide two scales, one for bright light and the other for dull
light. This is accomplished quite easily by having a resistance
connected across the microammeter and cell for bright light-the
resistance by-passing some of the current. The operation of a
" push-to-open " switch disconnects the resistance for readings
in poor light. An alternative method is to have a mask which
covers part of the cell for bright light readings and is hinged
out of the way in dull light. Whichever method is employed
there have to be two sets of calibrations on the meter dial and
these are usually printed in contrasting colours to avoid risk of
confusion. To make confusion impossible it may be that the
movement of the mask to cover or ciear the ceil changes one set
of calibrations for the other.
Using a Meter
  The usual method of using a photo-electric meter is to point it
at the subject from the position of the camera. The whole of the
area of the subject embraced by the acceptance angle of the
meter sends light to the photo-cell which integrates it and the
actual measurement madp is of the average brightness of the
subject. As was made clear in Chapter One the basis for ex-
posure should be the brightness of the deepest shadow, so the
meter is measuring the wrong thing, and because it measures
the wrong thing it often gives the wrong answer to the question,
" What exposure should I give? "
  Meters are calibrated on the assumption that the brightness of
the deepest shadow is a certain fraction of the integrated or
average brightness of the subject. The precise fraction is de-
duced from readings taken of the average and shadow brightnesses
of a large number of different types of subjects and is usually
taken to be of the order of r/ro.
2t2

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