Electronic flash guide Reference guide

LET'S START FROM THE BEGINNING




        Electronic flash is LIGHT! It is more commonly
called the "strobe light". This light is generally produced
by a xenon gas filled lamp which is referred to as a flash
tube or flash lamp.

     The light emitted by the flash tube has three
major characteristics:


                                            use of    EXTREMELY BRILLIANT -  permits the       relatively small lens aperture openings.


                                                unsharp    VERY SHORT lN DURATION -  prevents      photographs due to camera movement.


                                                          the    COLOR SPECTRUM -  approximates that of     sun, can be used with daylight color films.


      These three qualities make electronic flash very useful
   producing artificial photographic lighting.

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                                         .                Figure L  BLOCK DIAGRAM                                         .     TYPICAL AC OPERATED ELECTRONIC FLASH

                In figure l, the incoming AC line voltage is raised
         from I 15 volts to a higher value at circuit A. For electrical
             safety, the transformer will automatically isolate the remain-
             ing circuits from the AC source.

              At circuit B, the high voltage is rectified. Now the
            electrons will flow in one direction only which will enable
              circuit C to operate.

                    Circuit C consists of a high voltage storage capacitor
         which stores the energy that will later be released in the
          form of light at the flash tube, circuit D.

              The capacitor energy will not be released until the
             trigger circuit is pulsed. The camera shutter contacts are used
            to pulse the trigger circuit.

t

There are t\vo basic types of triggff'circuits  - direct
and thyratron type.
      In the direct type, the camera shutter contacts are
connected directly to the trigger circuit and, in some cases,
the excessive current may cause damage to the shutter
contacts.
      In the thyratron type, the camera shutter contacts
are connected to the grid of a thyratron tube or, in more
recent designs, to the gate of a Silicon Controlled Rectifier.
When the camera shutter is tripped, the sync pulse is used
to trigger the thyratron or SCR, and the current which
could cause damage to the shutter contacts is absorbed
by the thyratron or SCR. All Norman Enterprises units use
thyratron type triggering utilizing a Silicon Controlled Rec-
tifier.




      For many years the common term used to describe
the output of an electronic flash unit has been watt seconds.
Unfortunately this is very misleading in that the term 'watt
seconds' is used to describe the amount of energy stored in
the main discharge capacitors and is not directly related to
the amount of actual light output.
      In describing our units, we have chosen to list the
energy storage as well as the more important B.C.P.S.
(beam candle power seconds). B.C.P.S. is the measurement of
the light output of a given lampheiid when operated in con-
junction with a given power supply at a given watt second
level. This measurement is more realistic in that you are
measuring the actual light that will fall on your subject and
affect exposure.

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      The color quality of light from a Norman Enterprises
electronic flash unit very closely resembles normal daylight.
No filtration is required in using the units for exposing color
films.

      For extremely critical work it may be necessary to
use a slight amount of filtration with some transparency
color films.

      With negative color films there is never any need
for light balancing filters as any slight difference in color
balance can easily be corrected in making final prints from
the color negatives.




       Accurate exposure is of prime importance to the
quality of finished photographs. With color films you do not
have as much latitude as in black and white materials, nor
do you have the contrast controls in making finished prints
from color negatives.

         It is very important that you take the time to esta-
blish an accurate guide number that will result in optimum
exposure. Once you have established this guide number, you
will be able to adapt it to different situations so as to be sure
of accurate exposure at all times.

WITHOUT VOLTAGE STABILIZATION                        WITH VOLTAGE STABILIZATION


                                                                                   Ii                   +'-
  o                                             o                                                            L    (1)i,,1'",'   i      =                            T                                          -j"ofn.                                                                    f
                                                                   E                         >-F 4.-  ffi:n'   =j\rF;+-
               SECONDS                              SECONDS

       Voltage stabilization is vitally important to light output
consistency from an electronic flash unit and is best demon-
strated by the two graphs here.

       Figure 3 represents an electronic flash unit that does not
have voltage stabilization. As you can see, the light output
changes in proportion with the AC line voltage. You will also
note that the longer the unit is allowed to charge, the greater
the light output will be.

       Figure 4 represents an electronic flash unit with voltage
stabilization. The capacitor voltage is higher after a given length
of time; however, once this voltage reaches a pre-determined
level it stays the same even if there is a considerable length of
time elapsed before triggering the flash.
     With the short recycling time of Norman Enterprises
flash units, you are assured of extremely consistent light output
on units equipped with voltage stabilization.
       Wherever you are working, you know that you will have
the same light output flash after flash after flash.

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              Most electronic flash units have ready lights that are
         supposed to indicate that the capacitors are charged and the
           unit is ready to flash. In many units the ready light starts in- \           dicating when the capacitors are charged only as much as
  I         7O%. lt such a unit were flashed the instant the ready light
        came on, the resulting exposure would be one stop less than
           the exposure at full charge.
              In our flash units equipped with voltage stabiliza-
          tion, the ready light indicates at the 100% level, thus
         completely eliminating ready light error and insuring con-
            sistent exposure.

              The duration of flash will vary with the number of
         lampheads being used on a given single channel power
           supply. If the flash duration is l/500 second with one lamp-
          head, the duration would decrease to approximately l/1000
          second if two lampheads were employed on the same power
$$          supply.
H            The duration of flash also changes with the watt
$         second level used on the same power supply. If the flash dur-
          ation is l/500 second on full power, it will decrease to
          approximately l/1000 second on half power.
                   Circuit voltage has much to do with flash duration.
          Since 'strobes' were first placed on the market, gteat strides
         have been made in developing low voltage electronic flash
              circuits.
               With our lower voltage circuits the flash duration isI          longer thereby grving more effective exposure to the film
         and virtually eliminating reciprocity failure.
 I             The effective sensitivity of any photographic emul-
 l         sion varies with the level of illumination and the exposure
         time. Generally speaking, with very long or very short
         exposure times the sensitivity of a film is decreased, re-
           sulting in under exposure and color shift.
             For all practical purposes this reciprocity failure
         need not be considered in using Norman Enterprises elec-
          tronic flash units with todav's films.

The ratio between the key light and the fill light in
making portraits is governed by the film used, the effect
desired and the particular taste of the photographer.
      For color portraiture the lighting ratio should gener-
ally be 3 to l. In computing this ratio, you must keep in
mind that the fill light is adding illumination to the high-
light areas as well as the shadows. Let us assume that the
fill light has a value of l. The lieht on the shadow areas
and the highlieht areas would both have a value of I with
the key light turned off. To arrive at a 3 to I ratio we must
add 2 units of light to the hiehlights, or the key light must
be one stop brighter than the fill light.
     An easy way of computing this is to use the f stop
scale as shown here. If the fill light is at 8 feet, we must
place the key light at 5/z feet to arrive at the desired ratio.
(In mathematical terms we could say that the square of the
fill lieht distance is twice the square of the key light dis-
tance.) Using the f stop scale and converting f numbers to
feet, you can easily arrive at any desired lighting ratio.
                 1 STOP


                                                            f 111       il16

                                llt ttot'
       This same system can be used to determine exposure
increase needed for bellows extension. From the lower side
of the scale you can see that if you are using an eight inch
lens and the bellows are extended to ten inches, You would
need to increase exposure by 213 of a stop.

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 A WORD ABOUT MODELING LIGHTS




    We would like to caution you about the
possible effect of modeling lights on the color
balance of photographs.
     When using a camera with a Packard shut-
ter or when using a regular compur type shutter
at slow shutter speeds, the light from the model-
ing lamps may register on the film in addition to
the flash exposure.
      There will be no real problem with color
films if the modeling lights are left on all the
time, but if you make part of your exposures with
the modeling lights on and part with them off you
may see a different color balance in the finished
prints or transparencies.
      There are two simple rules to follow to
prevent this problem. First, always use a shutter
speed high enough to eliminate the effect of the
modeling lamps. As a gsneral rule Vl00 to 11250
second should be used.
       Second, establish a pattern of making all
exposures with the lights on or all exposures with
the modeling lights off.

One problem that plagues photogra-
              phers is correct polarity in sync cord connec-
               tions. For safety's sake, most electronic flash
              units are grounded and in most cameras the
             shutter contacts are also grounded to the
            frame of the camera.
            A very simple way to check PolaritY
                is to attach the sync cord to the camera and
             then touch any exposed metal on the camera
              to the light stand supporting one of the lamp-
              heads. If the unit flashes, you should reverse
             the sync cord plug to achieve correct polar-
               ity. Remember that anodized metal is non-
              conductive and should be considered in mak-
               ing the above check.





       hactically all focal plane shutters are
completely open only dt the slower speeds
and therefore must be used at these speeds
with electronic flash. Generally you should
use the highest speed on your focal plane
shutter that is completely open. This will vary
from camera to camera and you should make
a photographic test if you are in doubt as to
the highest speed that you can use.

SAFETY FIRST





Electronic flash units are equipped with fuses for
safety's sake. lf a fuse blows it is generally an indi-
cation that there is something wrong in the circuit.

Often there is a tendency to install a fuse of higher
amperage than recommended - DO NOT DO THIS!
 lf you have a continuing problem with blown fuses,
return your unit to the factory for checking or take
 it to a qualified electronic  f lash repairman for service.
We strongly urge you not to be a "do it yourselfer".
Unless you have a thorough knowledge of electronics
do not attempt to dismantle your electronic flash
unit if a problem develops. Even in a unit that will
not flash for some reason, there can still be energy
stored in the capacitors and you could receive quite
a severe electrical shock if you are not familiar with
the circuit.