|
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
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