Photography is about creating images with light. For indoor, night, fill light, or certain special effects, using electronic flashes to generate light becomes an important component in modern photography. Canon flash units are called “Speedlites” by Canon. There are currently three shoe mount Speedlites (220EX, 430EX II and 580EX II) as well as two specialty macro flash systems (MR-14EX and MT24EX). While Canon consumer and prosumer DLSRs (such as the Digital Rebel Series and the EOS 40D/50D) have a small built-in flash, the pro series cameras such as the EOS 5D, 5D MkII and the 1D and 1Ds series cameras do not and so require the use of an external flash, often a hotshoe mounted Speedlite.
The intro image demonstrates the size differences amongst the 220 EX, 430EX II, and the 580EX II.
Flash illumination is produced by an electrical discharge though a glass tube filled with gas (often Xenon) at low pressure. This is a short burst of light, typically lasting 1/1000s or so. The amount of light which can be emitted is controlled by the size of the flash tube and the energy of the discharge. If full flash power is not needed, the discharge is “quenched” early. For example if the full flash takes 1/1000s and you only want 1/10th of that amount of light, the discharge is stopped after only 1/10000th second. The automatic metering system in the camera determines when the flash needs to be stopped to give the correct exposure. The color temperature of most flashes matches daylight, so they can be used for outdoor work to fill in shadows without the results showing “mixed lighting” effects.
Canon Speedlite and Flash System Terminology
“Speedlite” is the term used by Canon for its flashes. Not to be confused with “Speedlight”, the term used by Nikon for its flashes!
TTL stands for “Through The Lens” flash metering. In this mode (used only with film cameras), the light reflected from the film is measured by sensors in the base of the camera. When enough light has been received the flash is cut off.
A-TTL stands for “Advanced Through The Lens” flash metering. In this mode a weak pre-flash is issued, either from the flash tube when in bounce mode or from an IR emitter in normal mode. The intensity of the returning signal is used to estimate subject distance and to select an aperture for program operation. Final exposure is determined by measuring the amount of light reflected off the film, cutting off the flash when enough light has been measured. Again this mode is only available on film bodes, not digital.
E-TTL stands for “Evaluative Through The Lens” metering. In this mode the flash tube emits a pre-flash and the camera’s evaluative metering system is used to determine flash exposure. This not only allows more accurate flash metering, but also enables FEL (flash exposure lock) to be used, since the exposure is determined prior to the main flash firing. In addition E-TTL is required for digital SLRs because the digital sensor does not reflect light in the same way as film does and so TTL and A-TTL metering cannot be used. Only the “EX” series speedlites support E-TTL metering. The earlier “EZ” series speedlites only support TTL and A-TTL and so a cannot be used for auto flash operation with any EOS digital SLR.
E-TTL II is much like E-TTL except that the focus distance returned by the lens may, in some circumstances, be used in determining flash exposure. Since E-TTL II is a camera body based modification of E-TTL, all E-TTL capable speedlites can also operate in E-TTL II mode when used with an E-TTL II compatible body.
FEL stands for “Flash Exposure Lock”. It’s available on most both film and digital SLRs made after the EOS Elan II in 1995
FEB stands for Flash Exposure Bracketing. This is a function built into some high end speedlites (such as the 550EX, 580EX and 580EX II) which allows three images to be taken with different flash exposure compensation settings. It’s like AEB (auto exposure bracketing) for continuous lighting.
FP actually stands for “Focal Plane”, though some people think of it as “Fast Pulse” or “Flash Pulse”. In FP mode the flash tube emits a series of very fast flash pulses so that it appears to the film or sensor as a continuous light source rather than a very fast flash. This allows the use of fast shutter speeds, though it does reduce the effective power of the flash.
This is the fastest shutter speed at which the first curtain of the shutter fully opens before the second curtain closes. At speeds faster than the sync speed, the shutter actually forms a slit which moves across the image. The faster the shutter speed, the narrower the slit. If you make a flash exposure at 1/500s when the maximum sync speed of the camera is 1/250s, half the frame will be black. If you want to shoot using flash at 1/500s with that camera, you have to use FP mode (see above) if the speedlite supports it.
Maximum Sync Voltage
When a flash charges, a voltage appears between the center pin and the edge contact on the hot shoe foot. To fire the flash, a circuit in the camera connects these two contacts. The maximum sync voltage is the highest voltage that the camera can connect without damage. This also applies to the voltage across the PC connector socket. On many earlier Canon EOS bodies this was 6 volts, but on more recent models this has been increased to 250 volts. Canon speedlites conform to the 6 volt standard, but some 3rd party flashes (especially general purpose flashes not designed for EOS use) can have sync voltages of up to 250 volts, and a few are even higher.
Unfortunately Canon seem to hide the value of the maximum sync voltage for their SLRs somewhere in small print in the manual and sometimes they don’t list it at all, so n official comprehensive list doesn’t exist. What’s worse is that if you call their tech support you’ll often get a “6v” answer even if it’s actually 250v, and I assume that’s because the support reps like to play it safe if they aren’t sure. As far as DLSRs go, as far as I can tell, the D30, D60, 10D and original digital Rebel all have a 6v limit, while the Digital Rebel XT, the EOS 20D and newer DSLRs have a 250v limit. Despite some internet postings to the contrary, it appears that the PC socket and hot shoe sync voltages are the same.
Canon Digital SLRs safe up to 250 volts: EOS-1D Mark II N, EOS-1D Mark II, EOS-1Ds Mark II, EOS-1D, EOS-1Ds,EOS 30D, 20D, 5D,EOS Digital Rebel XTi, XT (400D/350D),EOS D6000/D2000, Kodak DCS560/DCS520 (circa 1998),EOS-DCS series (circa 1995).
I would assume that the 250v limit also applies to more recent DSLRs such as the 5D MkII, the 1Ds MkIII, the 1D MkIII, the EOS 50D, the Rebel XS and the Rebel XSi
Canon Digital SLRs safe up to 6 volts: EOS 10D, D60, D30,EOS Digital Rebel (300D)
Canon 35mm SLRs safe p to 250 volts: EOS-1V, EOS-1N, EOS-1, EOS 3
Canon 35mm and IX240 SLRs safe up to 6 volts: EOS 650, 620, 630, RT, 850, 750, 700, EOS Rebel Series, EOS Elan Series, EOS 10s, A2E, A2, EOS IX, IX Lite,T90
Second Curtain Sync
In normal flash operation the sequence of operation is as follows:
The first shutter curtain opens fully.
The flash fires (which may take only 1/10000s)
The shutter stays fully open for the rest of the exposure
The second shutter curtain moves across and stops the exposure.
In second curtain sync the sequence of operation is as follows:
The first shutter curtain opens
Nothing happens during the rest of the exposure until…
Just before the second curtain starts, the flash fires
The second curtain moves across and ends the exposure.
What’s the difference? Well say you have a moving car at night. In normal operation (first curtain sync) you get a sharp flash exposed image with the car lights trailing off ahead of the car during the remaining time the shutter is open. This doesn’t look right. What looks better is if the light trails are behind the moving car, and that’s what you get when using second curtain sync. You get the light trail during the regular exposure, plus the sharp flash image at the end rather than the beginning.
Not a Personal Computer connection as you might think, but a flash connector called a “Prontor/Compur” socket. “Prontor” and “Compur” are names of early shutters. Some think PC stands for “Push Connector”, but it doesn’t! PC flash connections are used for external flashes such as studio strobes. It’s actually an awful connector because the plug either falls out of the socket or gets stuck in it. It’s just a push fit, but were stuck with it now. There is a variant called a “Screwlock” PC connector and socket which gives a more positive connection because there’s a locking ring that screws into a compatible socket. Most of the Canon SLRs and DSLRs with a PC socket have “Screwlock” sockets.
Guide Number – GN
Guide Number is a measure of how much light a flash emits. It can be used to manually set the correct aperture for proper exposure based on the distance between the flash and the subject. To find the aperture to use you divide the GN by the distance. So if the flash is 10ft from the subject and the GN is 40 (ft at ISO 100), you would set the aperture to f4 at an ISO 100 setting. The Guide number doubles when the film speed is increased by 2 stops, so that a GN of 40 (ft at ISO 100) is equivalent to a GN of 80 (ft at ISO 400). Guide numbers assume a direct path between the flash and the subject. If the flash is bounced off a wall or ceiling, all bets are off and the effective GN will be much lower.
Type “A” and “B” SLR bodies
The type “A” and “B” designations are sometimes used by Canon to differentiate between SLR bodies that support E-TTL (type “A”) and those that do not support E-TTL (type “B”). There are some additional subgroups, but basically it’s a measure of whether some sort of E-TTL is supported or not. All current EOS SLRs (both film and digital) are type “A” and all digital SLRs (except for the very early DCS3 and DCS1 models) are type “A”. All film bodies released before the EOS Elan II are type “B” and don’t support E-TTL.