AVO FILM SPEED RATINGS BOOKLET Product booklet

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            AVO EXPOSU RE METERS
                             Film Speed List
                     The following table of film speed settings has been
                       compiled for the convenience of the user of
               AVO Exposure Meters. While such speeds can
                      be used with every confidence of obtaining satis-
                         factory resr4ts, experience may-indicate that in
                    some cases improvements would result from the
                      employment of a modified film speed setting. The
                     main factors which might call for such a change
                         are :-
                     1. Non-uniformity in the speed standards of the
                              various film manufacturers.
                     2. Age of film and bad storage (if known).
                     3. Errors in the actual meter or the technique
                             of using it.
                     4. Errors in the camera shutter speeds, aperture
                                values, and also differences in the transmission
                             of various lenses.
                     5. The technique used in development and also
                             chemicals involved.
                           Errors in equipment may well roughly cancel
                          out, but should they be cumulative to an extent

calling for a change of exposure level, an altera-                     that the most desirable film densitv can be influ-tion equivalent to one stop is recommended for                      e_nced by the power and throw of the projector,still monochrome materials. Such an alteration                       the size ofpicture and nature ofthe screen, but thewould correspond to a change of 3' on the BS                       factors glven should in most cases suit the projec-scale or to a doubling or halving of the ASA and                       tion equipment possessed by the amateur.H & D rating. The tables of speeds, except forHighlight factors have been compiled from infor-           NOTE l.-H.L. Recommendations for shaded
mation supplied by various manufacturers and                                  conditions.
are intended to be used with the meter when                          If the light reaching the subject is reflectedpointed towards the subject as described in the                    almost entirely from a very wide- angle, directing
instruction books relative to each type of instru-                    the meter towards the sky might not include the
ment.                                                                               full proportion of the incident light falling on the
  Exposure for colour and cin6 is not only more                        subject. - In such cases, it may give more pleasing
                                                                                       somewhat less thancritical than for negative monochrome, but a                         results  if the exposure given is
different criterion of exposure is involved. The                      that shown on the meter.
most effective method of obtaining the desired            NOTE 2.-}l'.L. System for monochrome negativeresults is by means of the Highlight principle                                   materials.
originally propounded by Mr. P. C. Smethurst and                                                                                         _. Before making use, if so desired, of the High-first incorporated in a meter known as the AVO                     light factors quoted for monochrome negative                     METER.SMETHURST          HIGHLIGHT                                        to                                       which                    materials,                                                           which                                                                      marked                                                                     are                                                                                                     thus : O, the                              quoted                                              hereininstrument                Highlight factors          the                                                                      following notes                                                                                                        read.                                                                                 should                                                                                  bestill apply. The AVO Universal Exposure Meter,
Model 2, when used with the opal in front of the                   Those AVO exposure meters fitted with High-
photo cell and employed as directed in the instruc-                         light facilities, may be used for still negatives with
tion book, is strongly recommended  for all colour                     precisely the same instructions that apply to
and reversal cin6 exposures. With other models                       reversal monochrome and colour materials. Since
not incorporating this system the conventional                      negatives are not given standard processing, andreflected light method should be used.                            have in any case a far greater exposure latitude                                                   than either monochrome or colour reversal  Highlight factors are not necessarily propor-                      materials, it is not worth while to take into account
tional to their BS, ASA or H & D ratings since a                  any change in sensitivity less than double or half.
different portion of the exposure curve is involved                The factors quoted in the monochrome negative
in their determination. It should be borne in mind                      section of this booklet are based on the Ilford