© Brooke Clarke 2011
One Time Flash Methods
The light sources used for photoflash (Wiki: Flash Photography
work have changed over time. The early units were chemical
flash powders (Wiki
The popular type had a bayonet base and the professional type had
a standard Edison screw base (the same as a household lamp).
Next came the Flash Cube (Wiki
high volume consumer cameras. The big advantage of these was
that you got four flashbulbs in one unit and it rotated after each
shot allowing for a much shorter time between exposures.
Electronic or Strobe Flash
Unlike the above one time methods where you threw away the flash
bulb, strobe flash has a great advantage in that the light comes
from a tube that can be fired a large number of times.
These used a bank of high voltage
batteries and had a very short recycle time. The batteries
charged a capacitor that has the flash tube connected directly
across the capacitor. The typical operating voltage was in
the 200 to 600 Volt range which was not enough to trigger the
flash tube. In order to start the ionization of the tube a
seperate very high voltage (a few thousand volts)
transformer was used which drove a wire wrapped around the flash
tube. Once started the flash lasted until the capacitor
voltage drained to below the voltage where the tube would stop
conducting. That's to say that almost all the energy in
the capacitor was drained for every flash. If the battery
voltage went down then the flash power would also go down.
It's important the the light from
the flash occurs when the shutter is open otherwise some or all of
the light is wasted.
There are two common types of shutter:
Leaf or Diaphragm shutters (Wiki)
These typically are made as part
of the lens assembly. They are common for view and large
Focal Plane shutters (Wiki)
Tese are positioned just in front
of the film as part of the camera body, so the lens can be made
without a shutter thus at a lower cost. This is common for
35 mm film cameras. For the faster shutter speeds one
curtain starts moving and a short time later the other curtain
starts moving. This way a narrow gap moves across the
film. If a strobe flash went off during one of the faster
shutter speeds only a small part of the film would be exposed,
and that same problem exists for conventional flash bulbs.
So to use flash with a focal plane shutter either the shutter
speed needs to be slow enough so that the shutter is fully open
or a special type of flash bulb is used that has a long peak
light output (called FP bulbs).
The first way to do this was to simply remove the lens cap and
ignite the flash powder then return the lens cap.
When flash bulbs came out most cameras had leaf or diaphragm
and a provision to use a cable release (Wiki
trip the shutter. So one of the first camera flash units was the
Mendelsohn Speedgun (separate
) for use with the Graflex (Wiki
cameras. There were a number of schemes to get the light
synchronized with the shutter opening that all related to the use
of the shutter release cable.
The next innovation was to incorporate a electrical PC flash
into the shutter mechanism. There may be a flash mode switch
associated with the electrical contact. The flash
synchronization modes are:
For Medium type flash
bulbs, the most common and have peak light about 20 to 25
milliseconds after power is applied.
For Fast sync. where the
bulb is at peak light output in about 5 milliseconds.
FP (Front Curtain Sync)
For Flat Peak
flashbulbs. The contacts are closed at the start of the
exposure, also called front curtain sync. A problem with
this is that if there is streaking (say taking a photo of a car
at night where the lights cause a streak) the streaks are in
front of the car.
For Electronic Xenon flash where
the switch closes at the same time as the shutter is fully
opened. For focal plane shutters the shutter speed needs
to be slow enough so that the shutter is fully open, i.e. does
not work at the faster shutter speeds.
Rear Curtain Sync
Modern digital cameras have Rear
Curtain Sync so that the flash fires at the end of the exposure
time thus the streaks are behind the car.
Magnesium Flash-Lamp, Feb 20, 1906, 431/363
Flash-Light Device, Feb 27, 1906, 431/357
Photographic Flash-Light Apparatus, Mar 6, 1906, 431/357
Photoflash Synchronizer, Samuel Mendelsohn
Testing Device for Camera Synchronizers, Samuel Mendelsohn, Oct 26,
Photoflash Synchronizer, Samuel Mendelsohn
Russian w/Google Translator: E640
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