AVO Universal Exposure Meter User manual

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                    TI{ E



     UNIVERSAL
  EXPOSURE METER

            MODET 2




     Sole Proprietors ond Manufocturers :-
THE AUTOMATIC COIL WINDER &
ELECTRICAL EEUIPMENT CO.. LTD.
Avocet House, 92-95 Vauxhall Bridge Road'
              London. S.W.l

The word "AVO" is our registered Trade Mark

CONTANTS

Description of the Instrument                         5
    Zero Setting                                   6
                                                                                                                                                    t     t't-
Operation of the Instrument.                          7
     Presetting for Film Speed                      7
    Range Selection                               9
    Reading Exposure for Still and Cin6 Cameras  9
How to Use the Meter                            t0
   As a "Reflected Light" Meter                      ll
   As an "Incident" or "Highlight" Meter       l7

Miscellaneous Notes                            22
    Determination of Lighting Contrast.  .        22
    Balancing Illumination                        23
     Sensitivity of Instrument                      23
    Extension of Range                          23
    Care of the Meter                         24
    Note relating to Film Speeds..              24

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    DI]SCRIPTIOIT ON' THE
,       INSTRUMENT
  In order that the user of the "AVO" (Jniversal
Exposure Meter, Model 2 may understand its work-
ing, a brief description is given below of the operatlve
parts of the instrument.
   Electrically, there is a microammeter, the pointer
of which deflects across the scale in accordance with
the intensity of light falling on a self-generating
photo-electric cell, this latter component being fitted
in a recess near one end of the instrument.
  In order to convert the pointer indication intophotographic terms, there is a rotary exposure
computer which moves a mechanical pointer over the
electrical one and provides not only "f" stop and expo-
sure time indications, but also "Light" settings for
some of the recently introduced shutters. The exposure
computer consists of an outer circular scale rnarked
with "f" stop values, and also the "Light" scale. This
being rotatable to either of two positions, it can be set
to a low or to a high range. On the low range, light
falls directly on the photo-cell, but on the high one,
an appropriate degree of light obscuration is intro-
duced by means of a shutter which is automatically
lowered in front of the cell. Adjacent to the "f" scale
is an inner one marked with a series of exposure
times, and also a green spot (the "Light" scale indi-

q?tgr) for use as the index mark on the "Light" scale.
 This scale is carried round by means oithe same
 knob which moves the mechanical pointer. This "time
 scale" which also carries three scales of film speeds,
 cqn- bg preset in relation to the knob to correspond
 with the film speed it is proposed to use.
 .   It follows, therefore, that when using the meter,  " it.is merely necessary to cover the electiical pointer
 with the mechanical one, after which the chbice ofa wide range of "f" values is offered against their
corresponding exposure times in addition to the"Light" setting for use with certain cameras. The
combination of aperture and exposure time to beused is entirely at the discretion of the user.
Generally spealiing, rapid motion across the scene
being photographed, calls for a short exposure time
 in association with_a largish aperture. Aiternatively,
the need for considerable depth of focus (sharpness
of focus from close up to distance) necessitates the
use of a small stop with a corresponding increase in
exposure time.
Zero Setting:
   If all light is excluded from the meter. the electrical
pointer should be on the mark just below the grey
dot at the left-hand side of the-scale. Should iI Ui:necessary aj any time to reset the pointer, this is
best done by-.setting the meter to its high range,
excluding all light from the cell, and then rotatiirg
the screw just above the strap holder.

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   Cleaning or wiping the scale window in dry weather
may produce en electrostatic charge, which may
deflecl the elet:trical pointer. Before altering the
instrument zero, it is therefbre advisable to disperse
any possible static charge by breathing 9n th.e glassindiiator window, thus ensuring that the electrical
pointer assumes its natural mechanical zero position.

        OPERATION
                            prtEat 'utrDresetting: for tr'ilrn Speed
  Before using the meter, it should be set to the
desired film index by holding the knob stationary
and then rotating the inner scale by means of the
small stud until the required value appears opposite
the appropriate mark. The plain line should be
used in conjunction with either the B.S. or A.S.A.
scales, or alternatively, the white-flagged ong for
the H.L. factors. The B.S. and A.S.A. indices shown
in the list are those quoted by the film manufacturers
for their various emulsions. In addition, the recom-
mended H.L. factors are shown for those materials
which are more suitable for the incident light method
of exposure. In either case, these figures should be
used as a guide only, since more or less error is present
in all components of photographic apparatus. Ifthese errori are cumulative, modification to the
film speed setting on the exposure meter willprobably be desirable, but experience alone can
show the best values to use.

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 Bange Seleetion
   The range in use is shown by the position of the
 button on the outer scale. This can be moved to either
 the high brightness position shown by a white spot on
 the case, or to the low brightness one shown by a grey
 spot.
    If, when the meter is set to the high brightness
 range, the needle is so far over to the left as to lie
 between the grey and white dots on the scale, the
 lower range can be used with advantage, and it  is
 merely necessary to rotate the outer scale by means
 of the button. If, on the other hand, excessive pointer
 deflection is present, when on the low range, change
 over to the high one.
Reading: Exposure
                 STILL CAMERAS
   After having obtained coincidence of the pointers,
the choice of exposure should be made from aseries of "f" values which appear opposite their cor-
responding shutter speeds. If the camera has only                 ssf))certain fixed     values, the one nearest to the
desired exposure time should normally be used, but
if, as in most cameras, the "f" value is continr-rously
variable, by means of an iris diaphragm, the exact
setting may be employed. When it is desired to use
the Exposure Meter in conjunction with a camera fitted
with a "Light" scale, it is only necessary to set the cam-
era to the reading given on the Exposure Meter
"Light" scale, the probable camera shutter speed
having been previously selected and set on the camera.
  The final choice of speed and stop can be made and

read on the camera itself, there being no necessity to
have identical speed markings on the Exposure Meter
time scale
               crN6 cnuERAs
                                                     16  For cine exposures the usual silent speed is
frames a second, and a red dot at ll32 sec. on the         1{time scale indicates the normal "f" value to employ.
For cameras running at other speeds, it is convenient
to use the time scale on the assumption that the
shutter is open for half the frame period; -for example,24 frames per second can be read as l/50th sec. with
negligible error.
Filters
   lf a filter is used when taking a photograph,  it
is usually necessary to rnodify the exposure to.suit
the filter-and film. fhe exposure should be determined
by the meter as usual and then multiplied by the filter
factor appropriate for the circumstances.
  IIOW TO USE TIIE NTE'IEB
  The meter can be used in the conventional manner
in which it is pointed from the camera position
(usually) towards the subject, and summates the light
ieflected from it, or aiternativety in the Highiight
(or incident Highlight) method in which the light
ialling on the subject is measured.
  rhe renected ilfft"'fff^?it$iif, o" found more
convenient for general use, and the speed - listincludes the most popular makes of plates and films
with their speeds in both the British Standard

                         10

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      Logarithmic (B S.) and the American Standards
      Association (A.S.A.) systems.
                   INCIDENT LIGHT
      When using the rneter on the Highlight principle, an
      opal which is provided with the meter, should be placed
       in a frame in front of the photo-cell. The meter, on
$'     being set to the appropriate Highlight factor, can then
     be used to give direct indication of exposure. The
'lI     Highlight method of working is definitely recom-
     mended in preference to the other method for all cin6
t     and colour work and the Highlight factors (H.L.) are      given in the film speed list for such materials.
          If lighting circumstances make it impossible to
     use the meter on the highlight principle (e.g. a
      sporting event in the open, being viewed from the
      shade of a covered stand), the reflected light method
      should be used instead.
     AS A "REf,'LECTED LIGIIT'' ilIMTEB
        Since the meter is calibrated in accordance with
       British Standard Specification 138311947, it should,
       after the preliminary film speed setting, give without
      modification the ideal exposure in those cases where
     normal subjects are encountered. Since the metert      takes into account all light falling on the cell, an
     abnormal distribution of the areas of light and'f      shade, can influence the pointer deflection even
         if their tones happen to be the same as in an      average subject. It  is desirable therefore, when
                , photographing abnormal suUects, to modify sligLlly
      the exposure actually given from that indicated. The
      method of dealing with each type is given overleaf.
                                               ll
I

Norrna,l Subieefs
  Such subjects usually include a reasonable area ofhigh light, medium tone and shadow, without
excessive contrast in the important portions of the
scene. The exposure indicated can be greatly in-fluenced by the amount of light which-enteis the
instrument from the sky. Should this area of relatively              ,{"{high brightness be excessive, too short an indicatioil
will be shown unless the meter is tilted well down-
wards. The degree of tilt should be such that if the
meter is further depressed, flo rapid change of        Iexposure results.





                                                       I
                                                         I


                Normal Subiects
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Subieeis in Unusua,Ily Brig:hf or l)ark
Sumoundings
   If most of the field of view is of sky, bright water
or sand, white buildings or snow, the exposure
setting as measured from the camera position might
cause under-exposure to average and dark objects of
relatively small size. Conversely, a small normal
subject against a background of, say, dark trees or
heavily shaded buildings may be over-exposed. In
either case, the background should be subordinated
to the principal subject and the meter reading taken
close up to the principal subject to ensure correct
exposure of small areas.





             Abnormal Backgrounds
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          If it is not possible to approach the subject, take
      readings of similar but more convenient objects-a
      reading taken 6 in. from the photographer's hand can
     be use-d instead of a reading on a distant person's
      face under similar lighting conditions. Occasionally
     even this procedure cannot be followed, in which
      case the meter indication should be taken from the
     camera position but used as a guide only, the actual
     exposure being possibly more than double that
      indicated for the subject against a light background
      or half that indicated if against a dark background.
     Back-Lit Subleets
       In taking a back-lit silhouette, make sure that
       the sun, or strong light source, does not shine directly-
      or by reflection from water into the cell window. If
      normal details of the shadow side are wanted instead,
      close-up readings should be taken. For a "halo"
       effect iequiring-     some subdued detail, give half the      close-up  exposure, or  alternatively double that
      indicated for a similar front-lighted subject.
     Subieels in Exeeedingly Poor  Lig:ht
          If the light intensity is so low that a readable     exposure cannot be obtained in the conventional
     manner, it is often possible to determine it by holding
     a white handkerchief or card in the position of the
e      subject. The meter is directed towards this from a
       'surface.,fewinches distance without creating a shadow on the            The exposure to be given should be ten times
      that shown.

                                  l5

Exposures for Colour pholog:naphy
   While we recommend the incident light method
 which utilises the opal, in preference Io the one
 given. below, there_may_ be some users who prefer
 to adhere to the reflected light method, and foi their
 benefit, we give the following advice:                               4
  As the range of brightness which can be recorded
 in colour is limited, it is essential to consider the
 tone range of the subject. It is preferable, in the first                     a,
 pl?qe, to take readings of ttre lightest part of the
 g.ubject (excluding pnre white) and of the darkest ^part (ignoring black). Do not let the shadow of  the
meter fall on the _ area being measured. Compare
 shutter-speeds at the same lens aperture, for t'heseextremes: if the ratio between them is- Iess than
 19 _to.1, then an exposure half-way between the two
 w^ill give good colour results over the whole subiect.
 If, however, lighter or darker colours are of mijor
 interest, then give, respectively, half a stop less or             Imore than the average exposure. (A ..stop" is the
difference between adjacent lens aperture humbers.
The light intensity reaching the film decreases as the      J
n_umbers get bigger, €.g., f8 gives less exposure than
f5.6.)
      th_. brightness range of the subject exceeds-^lf                                                                                                                                         ',l0   to l, then extra lights or reflectors should be
usgd to add illumination to the darker parts, or
alternStively the illumination to the high lights must
be reduced.

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     AS AT  "INCII)ENT''  OB "IIIGHLTGITT''
                        IfBTNIT
     ICxposure for Oolour and  a,ll Cinc  f ilrns
      When exposing colour or cine films, it is usually
t,f    more important to have the brighter parts of the
    I      subject of the correct transparency than the shadows,
       since such parts are the ones which provide the main
       interest, or in the case of cine, which the eye follows.
 I      In any case, the range of brightness with which such
I      films can deal is so much more restricted than with
         still monochrome, that by pegging the light tones at
       their correct exposure, nothing with which the film is
      capable of dealing is lost, but all tones are reproduced
      to advantage. White appears transparent on the film
      or white on projection, and face tones, for example,
      are natural and matched in successive film shots. In
      order to achieve such results, the light measurement
        is based on the brightness of a sheet of opal which is
      illuminated to the same extent as the subject itself.
          If the subject is almost entirely light or dark in
      tone, i.e. the range is restricted, there is a possible
       benefit in slightly modifying the exposure from that
      indicated, always provided one is not concerned
      with matching any particular portion of the scene
      with its appearance when taken in other surroundings.
     The usual recommendation in such a case is to give
     between half and one stop difference, reducing the
      'exposure for light-coloured subjects and increasing
          it for dark.

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   Excessive lighting contrast should be avoided if at
all possible, this being most important when colour
filni is employed. In sunlight, the avoidance ofexcessive contrast is best achieved if the sun is
directly behind the camera.
  The Highlight system was originated to deal with
the reversat process, but it can also be used with
negative-posifive monochrome or colour, -since thebaiic meter reading is more precise than that
obtained by the reflected light method, whilst the
allowance for an abnormal subject can be accurately
assessed.
  To use the meter by the Highlight method, the opal
should first be removed from the compartment
situated beneath the meter by sliding the top retain-
ing plate outwards, after which it should be inserted
in tlie frame in front of the photo-cell. The inner disc
of the computer should then be set to the appropriate
H.L. factoi for the film it is intended to use.

Beadings  in  l)a,ylight
  In use, the meter is always held pointing upwardsat an angle of approximately 45 deg., and it must
always face the strongest light reaching the subject.
In most cases it will not be necessary to take the
meter to the position occupied by the subject itself,
                                                strengthsince in the open,-positionthe light is the sameat the camera        as at the sqbject, and the
readings in the two cases will be identical.

                           t9

The following rules should be observed:
   A. When the sun strikes the subject directly,
        point the meter at the sun (San Reading).
   B. If the sun is not shining, the meter must
         face the sky in front of the subject, i.e. the
       back is turned on the subject and the nreter
        pointed at the sky (Sky Reading).
   C. If the sun is shining but does not strike the
         subject (e.g. the subject is in shadow), point
         the meter at the sky in front of the subject,
      and shade the sun from the meter while
        taking a reading (Shade Reading).
   D. Under trees, and in some other shadow
        conditions, the light varies very greatly
       from point to point. Under such conditions
       the meter must actually be placed in the
        position occupied by th6 subj6ct to be sure
         that the correct light is read (Subject Position
        Reading).
  In all cases of doubt it is wise to use this position
for the meter during a reading

f,eeornrnendations
  Where the sun is right behind the subject, a sun
reading will give a silhouette effect, with dark
shadows. Should it be necessary to match (say) the
face tones in these shadows with face tones in other
shots taken under different conditions, a shade reading
or sky reading may be taken as described above. If

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     this is done, the background of the picture will
     appear over-exposed and burnt out, but this often
      eiu"t a very happy effect by increasing the perspective
     5t ttr" shoi. If ilid sun is rather to the side behind the
      subject, this latter procedure will^naturally "burn out"
     bv bver-exposure any portion of the face seen by the
     ca*e.u that the sun strikes directly
                                           sometimes      In early morning, or late evening, it
     happens that the sky is very blue, and when-the sun
        iitii o" one side oT the subject a sun reading will
      rtto* the shadows exceedingly dark. This trouble is                                                      incorrect    due to excessive contrast and not to
     exposure, because the brightest- parts of.the picture
      wiit be correctly rendered. A shade reading, on the
     other hand, wili "burn out" the brighter parts of the
      picture, and a reflector is the true solution to the
      irroblem. Snow scenes where ttley colstitute the      irain interest, are usually dealt with by slight red.uction
     of         to the extent of one stop as indicated
       earlier."*posure
     Readings  in  Artifieial  Light
       In artificial light, the meter must always be used        in the subject p5sition, since great variation in light
       intensity witt Ue found when comparatively lgw-power
      iurnpt are being used close to the subject. Following
     Ih"         rulE, the meter is always pointed at thet,     brightest"ot-allight when taking a reading.
       The chief difficulty in artificial light lies.in the very
                                                         case      strong contrast that normally arises, and in this

                                 2l

it is useful to use the meter as a means of findingthe lighting contrast present. If this is excessive,
means should be found of reducing it unless special
effects are desired. As a general rule in artificial
                                                             lesslight it is not advisable to use a shadow light
than one quarter of the accent light.

                trII' CELLANEOUS NOTES
I)etermination  of Lig:trtingl Contrast
   It can frequently happen with both daylight and
artificial light, that the ratio between the most
brightly illuminated and the most poolly lit portions
of a subject are too great for the film to handlesatisfactoiily. This effect is specially m-arl.ed if the
lighter coloured parts arq well lit and the darker
parts poorly lit. The lighting contrast can be deter-
inined by means of the meter used with its opal,
by takin! a reading with the me!e1 facing the light
source which provides the high light, and a second
reading with the meter held close to the shadow side
and ficing the fill-in light. The ratio can be readdirectly on the time scale against any one stop, or if
preferied on the "f" scale against any one time, if
one bears in mind that
   1 stop *   difference represents a 2 to I ratio,
  2stops*     ,,          ,,     ,r4,rl       ,,    '                                                               etc.                        8 ,, I   ,,  3 stops *       ,,          ,,        ,,
          *(or one division on the "Light" scale),

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Balaneing  lllrrrnlnatlon
  The opportunities of balancing illumination in
daylight are generally restricted to small subjects,
i.e. to those which can be placed adjacent to some
natural reflector such as the side of a light coloured
building, or alternatively where a reflector can be
broughi to the subject. The relative positions of
subject and reflector should be adjusted to bring the
lighting contrast down to the desired value.
  With artificial light, reduction of lighting contrast
is usually effected by adjustment of lamps or making
use of reflectors such as white sheets.
                  gvfkos " tt tSensitivity
  As a reflected light instrument, the low brightness
range extends from l/8 of a candle per square foot
to 30 candles per square foot when a uniformly
bright surface is placed in front of the meter. On the
high range the limits are 8 to 1,000 candles per square
foot. The angle of acceptance of the meter is ap-
proximately the same as the average camera. Used
in the Highlight manner, the ooal being on the
surface of the instrument accepts light from ap-
proximately 180 deg.

Extension of Bang:e
    If conditions are so bright that a deflection in excess
of full scale is produced on the high range, it  is
possible to reduce the deflection of the meter to the
extent of one stop by masking either side of the cell

                         23

window by means of one finger.
   ln such a case the exposure given should be half
that shown on the calculator.
   caBE ()1. Trrf,l rlrtl'ltR                                                        r
  Keep the meter in its case and ensure that it        I
 is not subjected to mechanical shock during transit
or use. Leave it on its high range if exposed to bright
 light when not in use. Keep the meter dry and away
from chemical fumes. Ensure that it is kept clean
and free from sand, etc.
           NOTE

                                                                                                                                                                                                            I  Although this instrument is provided with speed          I
 tables, and it is therefore only necessary to know the
              use in order to set the meter, we are type of flm in nevertheless sometimes asked why our instruments,
in common with those of other manu.facturers, Qre not
marked Scheiner or DIN. We do not use these speed
systems, since they both contain certain weaknesses  rwhich have been overcome by the introduction of the    f
B.S. and A.S.A. systems, now coming into general use.
   Whilst there is no exact relationship between any
-film speed systems, apart .from ^8.,S. and A.S.A.,onr European Continental users may like to know that,
as a rough guide, the B.S. Index is roughl.r' one lower
than the equivalent Scheiner rating.


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                                       Part No. P072-77