class notes weeks 2 and 3

Studio                                      

Teacher                                          Week Two                                     Semester Two

History of flash

1839 - limelight: heat calcium carbonate till incandescent
           (low blue content)

1864 - magnesium wire, burn time depending on length

1877 - electric studio lights (dynamo) 2 second exposure

1887 - magnesium powder + potassium chlorate
           (lit with flame or spark, effective but explosive and dangerous)

           preloaded flash caps were safer

1925 - magnesium + oxygen in glass bulb.

1929 - aluminium foil (later zirconium filament) + oxygen in glass bulb
            used till 1970’s

1931 - electronic flash (electric discharge through inert gas)

Flash bulbs

•          magnesium/aluminium/zirconium filament

•            bulbs and magi-cubes

•            some powered by battery

•            1/50th sec shutter delay

•            M (or FP) sync

•            blue glass envelope to bring
   to 5500K

•            single use

•          Harold Edgerton first to use electronic flash1931

•          His first shot was water
   coming from a tap

•            1/500th sec duration

•          Lightning

•             seen as one flash, actually several (3-20, average 4)

•             cloud to ground AND ground to cloud

•             colour temperature 6500K +

•             duration around 1/30,000th sec (0.03ms)

•             1 trillion watts/sec (portable flash unit around 80watts/sec

•          Lightning flash duration

•          although the lightning flash is made of 4 separate flashes
   each one can be graphically represented.

•          How an electronic flash works

•          Turn flash ON

•          battery charges capacitor

•          xenon gas in tube does not
conduct electricity

•          trigger circuit creates electro-
magnetic field around tube
which ionises (excites)
gas inside

•          charge can now pass through
the gas.

•          FLASH

•           

Power source

•          lead-acid batteries
   - rechargeable
   - needs constant maintenance (refilling), acid leaks

•            dry-fit (wet cell, gel not liquid)
   - rechargeable
   - no maintenance or leaks

•            nickel-cadmium / nickel metal hydride  
   - rechargeable
   - no maintenance or leaks

•            mains power supply
   - very quick recycling time
   - more dangerous

Battery type	Min-recycling time	Min number of flashes
Alkaline-manganese Lithium OxyrideTM Ni-MH (2600 mAh) Ni-MH (eneloop*)	4.0 sec 4.5 sec 3.0 sec 2.3 sec 2.3 sec	110 230 125 190 165
Note:   Flash count assumes between 30 -120 sec recycle time             eneloop = Sanyo technology = low self-discharge •             Electronic flash capcitator  •          a device which stores a charge    when connected to a power supply •            can ‘deform’ if not used for a    long period which diminishes    its ability to store a charge •            can be ‘re-formed’ by charging    for an extended period   potentially dangerous because of the high charge stored flash tube •          glass envelope •            positive terminal at one end,    negative terminal at the other •            typically filled with xenon gas    (or krypton or argon) •            thin triggering circuit wire    wrapped around tube •            easily damaged - expensive intensity of electronic flash •          can be measured in terms of peak value given as lumens which in     itself is not of great practical use. Duration •            is given as the time in milliseconds that it takes for the    flash to reach 1/2 its peak value •            is given as the time in milliseconds that it takes for the    flash to reach 1/2 its peak value •            of some use because if the duration is longer than the    shutter speed, the shutter will close before the flash    has finished, causing underexposure output and duration •            Portable/on-camera reduction in flash output     is achieved by reducing duration Blue sky                                  12000-18000K Mod overcast sky                   6500-8000K Average Daylight                    6500K Electronic flash                       5500-6000K Photographic Daylight                        5500K  Flash bulbs                              4000K Tungsten studio lamp              3200K Tungsten household lamp       2500-2500K Candlelight                              1000-2000K    Flash sync and exposure Shutter must be open when flash is triggered. Shutter must not close before flash finishes. Shutter type affects flash synchronisation. Shutter must be open before flash discharge. •       All contemporary cameras have in-built synchronisation so flash does not fire until shutter is open. •       Electronic flash is triggered as soon as the shutter is open •       Flashbulbs are triggered 20 milliseconds prior to the shutter opening to allow them to reach burn-temperature Shutter must not close before discharge has completed. Broncolor Pulso/Primo studio packs •       Maximum output flash duration is 1/250th second •       Keep shutter speed at or below this or underexposure occurs. •       Most studio flash units are not often used on maximum output. All small format DSLR cameras (yours) FP shutters use a ‘travelling slit’ for fast shutter speeds, if flash is triggered, image will only be the area matching the ‘slit’. Fastest FP shutter speed where the shutter is fully open typically 1/250th sec.  Known as synch-speed. Older cameras, slower, 1/125th, 1/60th even 1/30th. Flash triggered when first blind opens shutter, flash said to be synchronised if second blind has not yet started its travel. Contemporary DSLR cameras detect presence of flash unit and prevent shutter speeds faster than ‘synch-speed’ being chosen. Most medium format cameras and large format lenses Leaf shutter synchronises with flash at ALL speeds. Shutter FULLY open at all speeds - but highest speed 1/500 Leaf shutters are useful when combining flash and daylight because of synch with high shutter speed (1/500) Some pro DSLR cameras use 1/400th sec synch speed so the advantage of leaf shutter is now minimal. Portable flash units are rated with a guide number (GN) eg. Canon 580EX has GN of 58             GN =  correct f-number at 1m at ISO100             Allows output of units to be easily compared             Higher guide number = Higher output.             The GN can also be used to calculate exposure when used             on MANUAL mode, if subject distance is known then divide this             into GN for correct f-number             GN / d = f-number  (see next slide for example) Given: Guide Number (GN)   = 22             subject distance (d)     = 2m                         f-number                     = GN / d                                                 = 22/2  = f11 If d = 4m, then;             f-number                     = GN/d                                                 = 22/4  = f5.6 If d = 6m, then;             f-number                     = GN/d                                                 = 22/6  = f3.6 Flash exposure and illumination •          flash intensity decreases as subject distance increases •            correct exposure only achieved for a specific subject distance •            that being the case,       - subjects in the foreground will be overexposed       - subjects in the background will be underexposed Inverse square law •          inverse square law •            if you know the correct f-number to use at a known distance    you can calculate an appropriate f-number to use at any    distance, eg. doubling distance requires 4x the exposure             For a guide number of 64,             GN/subject distance   = f-number             64/1m                          = f64             64/2m                          = f32             64/3m                          = f22             64/4m                          = f16             64/5m                          = f13             64/6m                          = f11   and so on…. Full manual •            capacitor completely discharged each time •            exposure calculated using GN and/or simple distance    versus f-number scale Taking flash photos at night time sporting events •          assume your flash has a GN = 22 •           therefore, @ ISO 100, exposure = f22 at 1m             f = GN / d        f = 22/1 = f22 •           if subject at 10m,       f = 22/10 = f2.2 •           if subject at 100m,     f = 22/100 = f0.22 •           if subject at 200m,     f = 22/200 = f0.11  f0.11…..mmm! calculating your maximum subject distance •          assume GN = 22 and your maximum lens aperture = f4                   GN            = f x d                                                f             = GN/d                      d             = GN/f •           maximum subject distance therefore is;                       d = GN/f                         = 22/4                         = 5.5m •           more than 5.5m, subject becomes increasingly underexposed fractional manual Fractional manual settings •            capacitor partially discharged •            exposure is calculated using GN and/or simple distance    versus f-number scale Simple, accurate but slow to use, on full manual a battery charge does not last long Auto Simple auto •            choose a preferred f-number for lens •            set flash auto setting that recommends this f-number •            sensor on flash head determines when enough light    has reached the subject (via reflected light reading) and    cuts short the flash as required •            remaining charge is stored for next shot •            very easy to use •            often very good results •            exposure is dependent upon subject reflectance    (just like a reflected light meter in a camera) TTL (fully dedicated auto systems nowadays) Flash exposure shutter speed When using flash, shutter plays no role in the level of exposure… The shutter just needs to be fully open… It is the f-number that controls exposure.   Rear curtain flash synchronisation triggers the flash just prior    to the second curtain’s travel.  Some use in creating light    trails behind moving objects when combined with a    slow shutter speed Bulb sync •            FP synch was intended for use with slow burning flash bulbs    that could burn for the entire time it took for the FP shutter    slit to pass over the film. High Speed FP synch - the myth •            Flash unit emits multiple short low intensity flashes to simulate    a continuous light source which is able to expose as the shutter    ‘slit’ travels across the sensor. Studio flash Built in lfash •            low output •            convenient for simple shots or    as a fill light. Portable electronic flash •          powered by batteries •            typically high output •            sophisticated exposure control •            multi-featured and many accessories •            can be dedicated to camera exposure control monobloc •            powered by mains supply (some with battery pack) •            high output •            variable output •            accessories •            convenient for location work •            moderately expensive studio flash •            powered by mains supply •            very high maximum output •            variable output •            many accessories •            extremely versatile •            very expensive strobe flash •            a strobe is a flash lighting system that    provides repeated flashes of light    typically used in theatres    or music venues. Studio flash guide aperture •            portable flash units rated with a guide number (GN)             GN =  f-number at 1m at ISO100   monobloc and studio flash units rated with a guide aperture             GA = f-number at 2m at ISO100 using guide aperture Assume GA = 32 2m = f32 4m = f16 8m = f8 16m = f4 etc  Two methods to determine correct f-number for distance Apply inverse square law when f-number known for 2m (double distance, need 2 stops more exposure) Divide GA by 0.5 x d to determine correct f-number Studio flash output •            output is often given in joules or watt seconds •            1 joule =        work required to continuously produce                         1 watt of power for one second 1 joule in everyday life •            the energy required to lift a small apple one meter straight up. •            the energy released when that same apple falls    one meter to the ground. •            the energy released as heat by a person at rest,    every hundredth of a second. •            one hundredth of the energy a person can receive    by drinking a drop of beer. •            the kinetic energy of a 50 kg human moving very    slowly (0.2 m/s or 720 m/h). •            the kinetic energy of a tennis ball moving at 23 km/h pulso 2 •             Broncolor Pulso 2 (1600J) flash head will produce     1600 watts of power per second •             happily they are also rated with a Guide Aperture (GA) •             f64 at 2m @ ISO 100 (with P70 reflector) •             this allows units to be compared more easily •             lastly, you NEED a flash meter broncolour units Pulso   -           Symmetrical power distribution Primo  -           Symmetrical power distribution _____________________________________ Verso   -           Asymmetrical power distribution Graffit  -           Asymmetrical power distribution