photo:flash
flash technology
see also:
on the net:
basics of using flash off the camera:
Quantum wireless-enabled flashes for Nikon/Canon/Olympus/etc:
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flash photography with Canon EOS cameras:
flash photography with Olympus digital cameras:
flash light meters:
safety of legacy flash units - the issue of high trigger voltages potentially damaging your camera:
concert or event photography when flash is allowed:
polariser film to put on your flash:
flash exposure basics
the amount of light hitting a subject from a flash decreases by the square of the distance, THUS if you double the distance, you reduce the light hitting the subject to a 1/4 which is 2 f stops less light.
if you bounce a flash off a white wall, you must use the total distance of flash to wall PLUS wall to subject as your flash to subject distance AND THEN allow for light loss of about 1 f stop (ie 50% light loss) from bouncing off the wall.
similarly, shooting through a white translucent material such as a flash umbrella will lose perhaps 1 f stop of light depending on material used.
a flash unit can output a variable amount of light up to its maximum output which is measured by its Guide Number (GN).
BUT beware, with modern “zoom” flashes, their stated GN only applies to maximum zoom, thus if using a wide angle or normal lens, the broader coverage required will result in a lower GN.
the Guide Number allows you to determine either:
thus a GN of 45m at 100ISO means that at maximal output using ASA 100 film or ISO 100 setting on your digital camera:
most built-in flashes on cameras have a GN of about 12m at 100ISO which is very limiting but still useful.
There are only 3 main ways to modify the flash exposure effect for a given subject distance
change the aperture (includes putting a filter on your lens such as a ND or polariser)
change the ISO
change the flash output (includes putting diffusers over the flash or bouncing flash off walls, etc)
changing shutter speed mainly effects the ambient lighting mix, although at shorter speeds, it may impact on flash output if the duration is shorter than the flash duration (see below) and there may be some issues with flash sync.
NOTE:
that changing shutter speed will have NO EFFECT unless you choose a shutter speed faster than the X-sync of the camera which is about 1/180th sec (ie. avoid using shutter speeds like 1/500th sec with flash as results will be less predictable) .
You can use this to your great advantage by adjusting the shutter speed you can determine how light or dark the scene is as lit by ambient light and not the flash light. If you want to see outdoors on a sunny day as you would normally expose it and you are using your flash, then just set shutter speed so that it will expose correctly for sunny day when using your set ISO and aperture values.
your effective maximum GN is drastically reduced if you forget that your polarising filter is on - this cuts exposure by 2-2.5 stops and can really confuse you!!!
there are 3 main ways to get correct flash exposure
camera controls the flash
analog TTL - camera has a sensor which detects light bouncing off the film then tells flash to turn off when it is enough
digital TTL - camera instructs flash to fire pre-flashes then determines the correct output for the main flash exposure
some systems may include AF setting to make this more accurate (eg. Canon's eTTL II), but these only work if flash is on the camera and not being bounced, and the lens provides distance information to the camera.
when in a hurry, this is usually the best way to go
great for fill-in flash outdoors to reduce shadows in portaits, etc as camera will meter for ambient light and automatically determine exposure settings and flash output.
nb. TTL = Through-The-Lens metering and thus takes into account any filters used in case you forgot!
flash uses an inbuilt light meter sensor
flash sensor must point at subject and user sets an aperture and ASA/ISO on the flash to match the camera's settings
works well but being a reflective light meter, can be fooled by bright/reflective surfaces, dark backgrounds or nearby objects
don't forget to allow for any filters on the lens.
great when you do not have TTL functionality or flash is incompatible for TTL.
many modern flashes do not have this functionality
manual flash
user sets a manual flash output level (eg. full, 1/2, 1/4, etc), each setting will have their own GN value.
user determines camera aperture & ISO by either:
trial & error
calculations using GN, or,
preferably an incident flash meter placed at the subject and pointing at the flash
favorites include the Minolta IV F or VF light meters, or even a 1degree spot meter if you can't put yourself in the subject's position and you have to use reflective metering with meter aimed at subject.
flash meters can be set to 3 main modes:
ambient - just measures ambient light levels
corded flash - connect a PC sync cord from flash to light meter and press button on meter to trigger flash.
uncorded flash - flash will automatically detect flash firing and display exposure BUT may be tricked by digital flash pre-flashes when using TTL modes.
for set situations, manual flash can in the end be the least confusing as it removes flash output variability as a source of error when things just don't seem to be going right, then you can check for other causes of error:
using your in-built flash on your camera
the built-in flash in most cameras has a maximum output measured by Guide Number (GN) of 11-13 metres at ISO 100 and can be used for:
main flash (although results usually won't be too flattering for portraits)
fill-in flash
as an optical trigger for other flash units (helps if you can power the built-in flash down)
to provide catchlights in eyes when the main light won't do this (also helps if you can power the built-in flash down)
what this means is that if you are using ISO 100 and f/5.6 then 11/5.6 = about 2m and so only subjects at 2m will be able to be exposed adequately when it is on full power. Subjects beyond this distance will be progressively darker, while subjects closer than this will be lighter.
So how do we go about changing these settings?
My favourite is to go fully manual and check the histogram afterwards - if I need more light then I manually adjust them (use M exposure mode on the dial and manual flash mode on the menu). One of the great features with the Oly's which is not often found elsewhere is that you can manually reduce the built-in flash output on the menu system. NB. the flash +/- setting is not functional here as this is only for auto flash.
if you really want to calculate these yourself rather than use trial and error:
the correct aperture to use will be (approx) = GN of manual flash setting x ISO sensitivity / subject distance in metres.
here ISO sensitivity is your selected ISO related to ISO 100 so ISO 200 = 1.4, ISO 400 = 2, ISO 800 = 2.8, ISO 1600 = 4
and on the Olympus E330 (think it is same for E500), the GN in metres at ISO 100 for various manual settings are:
Next, you can just use the camera's auto exposure modes eg. A or P but avoid S.
Here you will be responsible for setting the ISO and in A mode also the aperture, the camera will try to control the flash output for you. You can use exposure compensation to tell camera to adjust the exposure.
BUT if the subject is too far away for maximum flash output to work at the settings you have used, then the subject will turn out dark no matter what exposure compensation you tell the camera to use.
Now, although the built-in flash is really handy, for best photos, you should get a 2nd flash that you can use off the camera to give different lighting effects such as more flattering bounce lighting, etc, but this is quite a bit more difficult for most to get their heads around. But if you need more flash output then you may need to resort to this.
the Olympus E330 and E500 cameras have a menu setting which allows flash compensation (flash +/-) to be ADDED to the general exposure compensation value (+/-):
If you have the flash +/- and +/- setting set to ON, then the camera will ADD the exposure compensations in BOTH the flash +/- setting AND the main exposure +/- setting.
If you have the flash +/- and +/- setting set to OFF, then the camera will ONLY use the exposure compensations in the flash +/- setting AND IGNORE the main exposure +/- setting when it determines the auto flash exposure.
The auto flash works even when dial is set to M (manual exposure) as long as the flash mode is set to a non-manual flash mode (eg. AUTO, redeye or Slow), but when dial is set to M mode, the main +/- exposure compensation is not available and thus whatever setting you have the flash +/- and +/- setting set to, it behaves as if this is set to OFF.
One advantage of having the flash +/- and +/- setting set to ON is that you can rapidly change your flash exposure using the +/- button when using an exposure mode that is not M on the dial.
Thus say you take you pic in Aperture priority mode (A on the dial) and flash set to AUTO and the picture turns out too dark, you can check the histogram and see where the right edge is, then go back to picture tqking mode and adjust the +/- button (by rotating dial while holding it down) to a + value (eg. +1 EV) and re-take the photo.
If the histogram on the second photo has not moved more to the Right, then you can assume the flash is not powerful enough for your aperture, ISO settings and subject distance, THUS you need to change these (or remove that damn polarising filter).
flash photography technology
Using flash to "stop" the action:
TTL flash systems and modes:
this requires dedicated connections between flash & the camera, unless the flash is in-built into the camera
1st developed by Olympus on its OM-2 camera of 1976, with film SLR cameras, light passed through the lens and reflected by the film is continually measured by the flash sensor during exposure until the correct exposure has been achieved.
TTL for digital cameras works a little differently as you cannot use light reflected of the “film” or sensor anymore so alternative metering is used via a pre-flash measurement of a lower power flash burst prior to the main flash.
current best, least complex TTL system appears to be Nikon's i-TTL.
accurate TTL fill-in flash for daylight photos is best on Nikon's i-TTL or Canon's ETTL II. Olympus does not seem to support it yet.
standard TTL flash:
user sets ISO, aperture or shutter speed with option to set exposure compensation for ambient light levels separate to flash exposure compensation, and camera sets TTL flash output to give even illumination but not fill-in.
camera detects when sufficient light has reached camera after shutter has opened & then turns off the flash output.
to achieve fill in, user may be able to set flash exposure to minus 1 stop for example so that ambient light is exposed properly while flash is under-exposed 1 stop but enough to fill in the shadows.
Olympus OM TTL-auto:
Canon A-TTL:
Canon's 1st attempt (with T90 film & EOS cameras) but very poor and no longer used as it really only determined if subject was out of range, the pre-flash didn't actually meter the exposure.
fires pre-flash when half press shutter release, unfortunately if flash is tilted or swiveled, the main flash fires as the pre-flash which is very distracting
auto-aperture TTL:
balanced TTL fill-in flash:
camera attempts to set flash output to fill in the shadows but not overpowering the ambient light, most use pre-flashes to determine exposures needed but these have problems:
may cause subject to blink & be caught in mid-blink
may falsely trigger normal optical flash slave trigger units
can confuse handheld flash light meters
Nikon matrix-balanced TTL fill-flash:
camera adjusts ambient light exposure via its matrix sensor and adjusts TTL flash so that flash is not over-powering for the ambient light and provides fill in of the shadow areas.
Nikon allows either spot or CW metering.
Nikon 3D Multi-Sensor Balanced Fill-Flash (D-TTL):
DX camera analyses likely subject distance from its AF sensors and using its 3D matrix metering system via 18 preflash pulses, determines the correct flash output to create a natural balance between main subject and ambient background lighting.
DX master flash must be on camera for D-TTL to work, in this case camera should be set for spot metered TTL as camera will not know where flash is in relation to camera-subject.
with type G or D CPU lens: full 3D functionality
other CPU lenses: distance information not included thus not 3D
non-CPU lenses or in spot-metering: standard TTL only
Nikon D70 / D100/ D1 series with SB-50DX, SB-80DX, SB-600, SB-800, Sigma EF 500 DG Super SA-N
Nikon i-TTL (see below)
Canon evaluative TTL (e-TTL):
introduced 1995 with Canon Elan II/50 camera
fires a low-power preflash of known brightness from the main bulb to determine correct flash exposure. It measures the reflectance of the scene with the preflash just before the shutter opens using usual light metering, then calculates proper flash output to achieve a midtoned subject, based on that data.
“E-TTL is also generally superior to TTL and A-TTL when it comes to fill flash. The E-TTL algorithms are usually better at applying subtle and natural fill flash light to daylight photographs. E-TTL exposure is also linked to the current AF focus point, which in theory results in finer-grained exposure biasing than most multiple-zone TTL flash sensor systems.
More abstractly, E-TTL is a very automated system and isn’t well documented for the user. For instance, Canon have never published details on the E-TTL auto fill reduction algorithm. It takes a bit of experimenting to figure out how the system is likely to respond. And there’s relatively little user selection or choice in operation modes. Most flash units don’t, for instance, let you manually choose TTL, A-TTL or E-TTL flash metering at will.
All current Canon digital cameras with hotshoes - both the interchangeable-lens SLR cameras and the point and shoot digital cameras - support E-TTL (or both E-TTL and E-TTL II) and do not support either TTL or A-TTL. Even Canon digital cameras with internal popup flashes are E-TTL only. This means that only Canon EX flash units or third-party flash units with E-TTL support can be used with Canon’s current lineup of digital cameras. Older E and EZ flash units will not work correctly - no automatic through the lens metering is possible. You can get manual-capable EZ flash units like the 540EZ to fire in manual flash mode but this requires external flash metering.
Unfortunately, E-TTL has been a particular problem for digital EOS users. Many users report serious problems with wildly varying exposure when using an E-TTL flash unit with their Canon DSLRs. The main problem appears to stem from the way in which E-TTL on these bodies biases flash exposure heavily to the focus point. For this reason some digital EOS users have given up on E-TTL and gone back to the old-style autoflash units. Others routinely set their lens to manual focus once focus has been achieved, since the camera uses a centre-weighted average metering pattern for flash metering when in manual focus. The EOS 10D has revised E-TTL algorithms which rely on centre-weighted average metering for E-TTL flash, even if the lens is set to autofocus mode.”
“There are times when TTL metering may be more desirable than E-TTL. A common example is a studio setting where analogue
optical slave units can be fooled by the E-TTL preflash. The 550EX, MR-14EX and MT-24EX let you disable E-TTL via a custom function, but they’re the only Canon Speedlites with this ability. All other EX flash units (220EX, 380EX, 420EX) will always operate in E-TTL mode when mounted to an E-TTL-capable camera, even if the camera is also capable of supporting TTL and even though they’ll work in TTL mode just fine on a type B camera.”
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complex to use with multiple flashes
cameras: all type A EOS cameras;
flash guns:
with e-TTL, Canon also incorporated technology pioneered by Olympus to provide synch at fast shutter speeds:
Canon e-TTL II (2004 see below)
introduced with the EOS 1D Mark II then 20D/350D to remedy the problems of the e-TTL system.
E-TTL II does not require any changes to either the flash units or lenses used with an E-TTL II camera - the changes are all basically internal to the camera body, hence an e-TTL compatible flash should be compatible with e-TTL II.
the biggest benefit of E-TTL II is that flash exposure is no longer linked to the active focus point.
it analyzes all metering zones before and after the preflash for improved flash metering:
in addition it may use distance information if available
flash guns: all Canon EX series - see above under e-TTL.
cameras which support E-TTL II:
EOS 1D mark II, EOS 30V/33V/7S/Elan 7N/Elan 7EN, EOS 20D/20Da, EOS 350D/Rebel X Digital/Kiss N Digital, EOS 400D/Rebel XTi Digital/Kiss X Digital, 1D mark IIN, 1Ds mark II, 1D/1Ds mark III, 5D.
Olympus digital TTL-auto:
not compatible with film-based OM TTL auto
uses pre-flash in either spot-metered, CW or iESP exposure modes
does not appear to use AF-measured mode
Metz flashes require SCA 3202 adapter (M3 for the E-1, and M4 version for E-300) AND a digital-enabled Metz flash such as Metz 54MZ4i, 45CL4 digital, 76MZ5.
Minolta P-TTL - Sigma EF 500 DG Super SA-N flash
Sigma S-TTL - Sigma EF 500 DG Super SA-N flash
Minolta ADI:
wireless TTL flash:
Powering your flash:
see also:
most flashes are powered by 4-6 1.5V AA batteries which usually give a 5sec recycle time
these can usually be replaced with cheap, rechargeable NiMH batteries but these have problems:
unpredictable, shorter life than NiCd, a single cell failing will stop the whole pack
ideally, should remain as sets and not mixed with different brands, age of battery, etc.
some manufacturers (eg. Metz) have their own AA rechargeable battery packs
alternatively, you can purchase separate generic lead acid power packs which reduce recycle time to 50%, and provide more shots:
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Al doesn't like AA batteries, NiMH or Li ion batteries.
He has made a lead acid black box for wedding photographers, etc using Canon/Nikon/Vivitar flashes;
recycle 3sec, 800-1000 shots; 90 days at 90% charge off a charger;
designed to be on a charger all the time when not in use.
two outputs; 6.4V peak; 2.7lbs;
eg. power a Canon 580EXII via a Quantum
MB-2 cable which leaves the battery door open for the cable.
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for 9V flashes (eg. Metz 45 series);
lead acid battery with fuel gauge; 2 output jacks for separate flashes;
holds it charge for months
300 full shots per charge, recycle time 3-5sec
standard recharge 12hrs; fast charger 2hrs;
940g
with the advent of AA NiMH batteries, these heavy packs are losing favour but are safer for your flash.
high voltage packs via AC port (usually need AA's in to power LCD, etc still):
low voltage NiMH power packs via strobe's battery terminal:
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with 16 NiMH AAs in the QUICK-RECYCLE Module, the BVS PULSAR is about 1.6X as fast (1/3 to 1/2 LESS recycle time) as the Canon CP-E4 with 12 NiMH AAs (8 + 4 in the flash helping to drive external power).
“The QUICK-RECYCLE module in the BVS PULSAR uses two proprietary techniques to accelerate current flow beyond what is possible with a large high resistance HV battery. Plus it does this through the flash battery port, so the flash internals are removed as a limiting factor. This is not the case using HV port which is slowed by normal AA power flow to the flash.”
“it compensates for the lower voltage of NiMH batteries, pushing the pack voltage slightly higher than a lithium AA battery.
photo/flash.txt · Last modified: 2017/11/15 09:31 by gary1