Generic Flash Primer
Table of Contents
- Exposure Modes
- Off-camera Flash
- Flash Trigger Voltage
- Flash Link farm
This primer is part of a series of articles about using flash on a digital camera.
This segment addresses using so-called generic flash outside a dedicated framework, (i.e. non-TTL auto and manual flash, as well as non-dedicated off-camera flash).
Generic flash units are not compatible with the advanced dedicated flash control systems that are offered by Canon and Nikon. Instead, they provide a more basic functionality, but on many different systems.
While dedicated units are can have their power adjusted by a computer built into the camera, either through a wired connection to the camera body, or wireless by coded light signals, generic units are designed to fire when there is a (short) circuit between the flash's center pin and edge.
As there is no dedicated control system for flash power control, the power output of generic units must be controlled by buttons and sensors on the flash unit itself. Most of the generic units offer at least manual (with adjustable power ratio) and non-TTL auto exposure modes.
For a list of models that can be used for generic flash, please consult:
2. Exposure Modes
In order to have the scene properly lit, with correct exposure and with a good balance between foreground and background light, we need to control the amount of light put out by the flash, and adjust aperture, shutter speed and ISO to match that, as well as the ambient light. In this series, the following flash exposure control modes will be discussed:
- TTL automatic flash mode (i.e. Nikon's i-TTL and Canon's E-TTL) is only available on dedicated flashes. When you are using a TTL automatic flash mode, the correct power to use for flash is determined by measuring the amount of flash light reflected by the entire scene or foreground subject through the camera lens (TTL = Through The Lens).
- Non-TTL Auto flash mode (available on some dedicated and most generic flashes). When you use a non-TTL Auto flash mode, you use a built-in sensor at the front of the flash that measures the average flash light reflected by the entire scene.
- Manual mode (available on some dedicated and all generic flashes). When you use a Manual flash mode, nothing measures the light, you control exposure by setting up the power of the flash by selecting how much (as a fraction of maximum output) light the flash should emit, and selecting a ISO and aperture that will yield the right exposure.
Non-TTL Auto first appeared in the 1970ies and was the preferred method for controlling flash exposure until well after dedicated flash systems built around some form of TTL light measurement debuted around 1980.
In Auto mode (sometimes called Auto Thyristor mode), the flash is told what aperture and ISO the camera is set to, and a built-in sensor on the flash uses that measures the average light reflected from the scene and uses a thyristor circuit to shut off flash power when the sensor indicates that the subject has received enough light for a correct exposure. The measurement is done in real-time, during the actual exposure. Unlike TTL, there is no pre-flash when you use Auto to control exposure.
Because the Auto-mode does not use pre-flash, the delay when you press the shutter button is shorter. The difference is small, but it may make a difference in responsiveness when you shoot sports with flash. Some people has fast enough reflexes to blink when they are exposed to the pre-flash. By using Auto-mode, you can photograph such people with their eyes open.
The exposure metering in Auto mode is simple and predictable when used for flash key (i.e. the flash is the main light). Most photographers learn how to compensate for the errors that the system introduces in difficult lighting situations.
For auto flashes with a head that can tilt and/or swivel, the sensor is placed on the body of the flash, never on the head. This means that as long as the flash is fixed to the hot-shoe, the sensor will measure reflect light along the same axis as the lens. The flash metering system will therefore automatically compensate for any light loss that is the result of (for instance) bouncing the flash light against a ceiling. But if the flash is used out of the hot-shoe, the fact that the flash sensor is no longer aligned with the camera may cause problems. For instance, if you put an auto flash inside a softbox, the flash sensor will measure the light reflected inside the softbox and not the light reflected from the actual scene. To get around this limitation, some auto flashes has a detachable sensor that can be mounted in the camera's hot-shoe while the flash can be put somewhere else.
Unless the flash is made to communicate with the camera (see below), to use an auto flash you need to set the camera up in manual mode (M). You then select a shutter speed equal to the camera's maximum X-sync speed or less, and a suitable aperture and ISO-value. You dial in the same aperture and ISO on the flash and you are ready to shoot.
Some flashes with Auto mode, the Canon 580EX2, and the Metz 54 MZ, are built to communicate with the camera through the hot-shoe, and automatically gets the aperture and ISO-value from the camera. In that case, there is no need to use M mode, you can also use P or A mode, and work with camera and flash just as tightly integrated as if you were using TTL.
Most other flashes of this type allow you to specify aperture and ISO through controls on the flash. Some very simple Auto flashes have only a limited set of fixed settings and require you to adjust aperture and ISO on the camera based upon a guide on the flash or a cheat sheet.
Unless the flash is made to communicate with the camera you dial in flash exposure compensation in auto mode by “lying” to flash it about what aperture you're using. However, if the flash picks up the aperture from the body you need to use explicit flash exposure compensation.
Some argue that modern TTL technology has made Auto obsolete. However, you can still buy cheap and powerful generic flashes using Auto for exposure control (e.g, Cactus KF36). Some dedicated flash units, such as Canon 580EX2 and Nikon SB-900 also have an Auto mode.
When a flash is used in Manual mode, flash output power is not controlled by the camera (TTL) or by the flash (Auto). Instead, the photographer is in control.
Various manual modes exists, but the most common is a mode where the photographer controls the flash by setteing a power ratio on the flash. The power ratios are usually displayed as fractions: “1/1” is full power, “1/2” is half power, and so on.
Not all manual flashes let the photographer set a power ratio. In that case, assume that only a single power ratio (1/1) is available.
After deciding on what power ratio to set on the flash, the photographer need to determine what aperture to set on the camera for a correctly exposed scene. The aperture need to match the power of the flash
The simplest way to accomplish this, is to use a handheld flash meter (e.g. Gossen Digiflash): Fire off a test flash, meter it, and the meter will tell you what aperture to use.
If you regularly shoot complex lighting setups with one or more manual flashes, I strongly recommend that you use a flash meter. With a flash meter, you can meter any setup with all your lights in place and set up with appropriate power ratios. Aftering metering the test flash there is no need to make another reading until you rearrange the lights. A flash meter will work with a multiple flash setup, bounced flash, and all sorts of light modifiers (e.g. brollys and softboxes).
If you don't have a flash meter, it is possible to compute the aperture to use from ISO, distance from flash to subject, and the flash's Guide Number (GN).
The GN is usually only given for ISO 100. Note that if the flash has a zoom head, or a Fresnel lens that may be used to focus the beam, the GN varies with the position of head or lens. Refer to the manual to find the GN at the zoom setting you're using.
To work out the aperture to use at ISO 100, simply divide the GN with the distance to the subject.
For example, at ISO 100 for a flash with a GN of 40m (meters) at full power, and the main subject 7 meters away, you get 40/7=5.7. Round this to the nearest available aperture, in this case f/5.6. This is the aperture you set on the camera.
If you are shooting at a different ISO value than 100, you multiply the guide number at ISO 100 with the square root of the ISO value divided by 100 to obtain a revised guide number. For instance, if the flash has a GN=40m at ISO 100, at ISO 200 the guide number becomes 40 x SQRT(200/100) = 57m. If you still are 7 meters from our subject, you get 57/7=8.1, and should the nearest available aperture is f/8.
Guide numbers are not much used these days. Most photographers prefer to use a flash meter when working with manual flash. But back in the 1970ies, when working photographers still used guide numbers on a regular basis, most photographers used pre-computed guide number tables to show what aperture to use at various distances and ISO-values. The image to the right show such a table. It is from the back of a Prinz Jupiter 2000 flash from 1973 which has a guide number equal to 18 (meters) for ISO 100/21° (known as “ASA” and “DIN” in the 1970ies).
The guide number is still useful if you want to work out the reach of a flash &ndash including the reach of non-manual flashes. In that case, divide the guide number of the flash with the f-number. The answer is the flash to subject distance.
For example, if your flash has a guide number of 40, and your aperture is f/2, flash-to-subject distance for correct exposure is 40/2=20.
If the flash let you set a power ratio, the guide number is reduced by the square root of the ratio you set. For example a flash that at full power has a guide number equal to 40 has at half power (power ratio 1/2) a guide number equal to 40 x SQRT(1/2) = 28.
Similarly, you can get more power by using more flash units. The combined guide number is the guide number of a single flash multiplied with the square root of the number of flashes. For example if a single flash has a guide number equal to 40, firing three of these at the same time gives the combined blast of light a guide number equal to 40 x SQRT(3) = 69.
To work out the aperture to use for manual bounce flash against a white ceiling, first compute an aperture by dividing the flash's guide number with the total distance the light has to travel (i.e the distance from the flash to the reflecting point in the ceiling, plus the distance from the reflecting point to the subject). Then open up at least two extra stops to compensate for light being absorbed by the ceiling. Check the histogram to determine if the exposure is correct, and adjust if necessary.
For manual fill flash, increase the shutter speed until the background is properly exposed. To lessen the effect of the flash on the foreground you may dial down the light output using a suitable power ratio, increase the subject to flash distance, or use light modifiers such as diffusers.
3. Off-camera Flash
For working with multiple off-camera flashes, the dedicated wireless flash control system from Canon or Nikon will often do the job. However, dedicated flash units are expensive, and the built-in flash on most of Canon's DSLRs (the EOS 7D is so far the only exception) can not be used as commander in Canon's wireless control system. The built-in flash on Nikon's entry level DSLRs (such as D60 and D5000) have the same limitation. If the camera's built-in flash can not be used as commander, you also need to buy dedicated wireless transmitters (ST-E2 or SU-800) or an extra dedicated flash unit to control off-camera flash.
In addition to the additional cost of using dedicated flash units, there may be situations where the manufacturer's dedicated system for off-camera flash do not work well.
For instance, when you want greater range, when you are working in a mixed setup where you want to use you dedicated units together with studio strobes or monolights, or in some other environment where it is not possible to use a dedicated system for wireless control, or when you are using a manual lens, you probably will want to use generic wired, plain optical and/or radio off-camera flash instead.
Because generic off-camera flash doesn't involve a pre-flash, you can freely mix all three means of communication. For instance, you can put a radio transmitter on the camera as master, and use this to trigger a few strategically positioned slaves units with a radio receiver. The light from these will then trigger other units fitted with optical receivers. Units in close proximity to each other can be connected to the same optical or radio receiver by wires. By doing it this way, total cost is kept low, and at the same time the photographer can move around freely because no wires attach the lighting to the camera. However, strobes from different manufacturers often have different colour temperatures. Mixing several brands may lead to problems with colour casts.
Using non-dedicated, small flash gun with only manual control for multiple strobe lighting arrangements (as opposed to monolights or studio lights) is sometimes referred to as “strobist style” lighting, after the popular blog Strobist.com, where Baltimore based photojournalist David Hobby champions this type of lighting style.
Generic Wired Flash
Provided that both the camera and the flash has a pc-socket, you can use a pc-male-to-pc-male sync cord, such as the Nikon SC-15 Coiled Sync Cord to move the flash off-camera. If either don't have a pc-socket, see the segment about connections to find a suitable adapter.
For multiple off-camera flashes, you can connect several units to the camera or to other flash units by means of a multi flash pc adapter like the one shown on the right.
However, wired solutions are cumbersome to set up, and also entails the hazard of tripping over wires. If you are stringing together flashes from different manufacturers, trigger voltage safety may be an issue. Therefore, many photographers prefer to use wireless solutions using light or radio signals to trigger the remote flash.
Plain Optical Slave Triggers
A plain optical slave trigger is an electronic device that will trigger an off-camera flash when it “sees” another flash fire. You can buy cheap plain optical slave triggers and attach a compatible flash unit to it to create an optical slave flash. Some flashes even have a built-in optical slave trigger, such as the old Nikon SB-26 or Metz 44-MZ 2. Most studio strobes has a built in optical slave trigger.
However, to use the light from the camera's built-in flash to trigger remote units, you need to prevent (or make the remote units ignore) the pre-flash. Otherwise, the pre-flash that is intrinsic to dedicated flash systems will ruin your shot by setting off the optical slaves before the shutter opens. How you can do this is described in the next section).
If you don't want the master flash to contribute light to the exposure, dial down the power ratio as far as it will go without making your slave flash(es) unreliable. If you want to reduce the master flash output without affecting the range, you can make a makeshift IR-pass filter by taping black unexposed E6 slide film in front of the master flash.
You should not mix TTL exposure control with plain optical slave flash. In this kind of setup you should use both the camera and all the flash units involved in manual mode. Refer to the sections Auto and Manual to learn how to control exposure in these modes.
Not all optical slave triggers work well with all flash units. See the section about off-camera flash caveats to see what you need to look out for.
Preventing pre-flash, etc. from firing slaves
When using plain optical slaves (i.e. a setup where the flash on the camera is used as master to trigger one or more slave receivers that are programmed to fire when they see the light from the camera flash), the slave flash must fire after the shutter opens. If it fires too early, it will be unready when needed to light the scene.
Some cameras strobes the on-camera flash to assist the camera's autofocus system in dim conditions. This will trigger the optical slave pematurely. If your camera does this, you need to disable AF-assist to use plain optical slave falsh.
Also, the pre-flash that is at the heart of a dedicated flash control system must be prevented from firing the slave flashes.
How you do this depends on your camera's make and model.
To turn off pre-flash for Canon compacts, such as the Powershot G5, use manual mode (M). Note that this also disables E-TTL. See the camera's manual for details (e.g. p. 101 in the Powershot G5 manual).
On a Nikon camera, you disable pre-flash by selecting manual flash mode. Note that this also disables i-TTL. NB: Do not use the manual setting in Nikon's commander mode. It will not work.
As far as I know, there is no straightforward way to disable pre-flash for the built-in flash of Canon DSLRs (but check your manual, I may have missed something), but several methods exist that have the same effect:
- Attach an external flash unit to the hot-shoe. You can either use a generic flash, or you can use a dedicated flash set to fully manual. Do not use the manual setting in any commander or master mode, as this will result in the master firing pre-flashes. NB: Not all dedicated flash units offer a fully manual mode.
- Attach an external flash to the camera's pc-connector instead of from the hot-shoe. To use this method, both camera and flash must have the appropriate connector, or you need an adapter.
- Use a special slave flash with a built-in “digital” trigger that can be set to ignore pre-flash, or an external slave flash trigger with this capability.
- Use blank FEL or blank FV lock to fire the pre-flash while covering the flash head in tin-foil or similar).
Blank FEL and Blank FV Lock
The technique known as blank FEL (Canon) or blank FV lock (Nikon) is a way of preventing the pre-flash from affecting slaves controlled by plain optical triggers.
This is most useful on Canon DSLRs where there as far as I know is no straightforward way of disabling pre-flash, but you can, if you want to, also use this technique on a Nikon DSLR.
Here is a step by step receipe for blank FEL or blank FV lock:
- Make sure the built-in (dedicated) flash is popped up and ready.
- Cover the flash completely (e.g. with your hand, tin foil, or a dark cloth).
- Push and release the button to lock flash exposure (FEL/FV lock). The preflash will be emitted, but should not trigger the slave strobes.
- With Canon, you now have about 16 seconds to shoot your picture. With Nikon flash exposure is locked until you push the button a second time.
- Uncover the flash, and depress the shutter button fully to take the picture.
Note that not all DSLR cameras have a dedicated FEL/FV lock-button. With some models, you need to assign this function to a programmable button first.
For more details, and some variations, see Julian Loke's note on blank FEL.
Mixing Dedicated and Generic Wireless Flash Units
People that own a combination of dedicated and generic flash units sometimes want to use all the units they already own at the same time to light a scene. I.e. for frugal, or other, reasons they want to use a mix of generic and dedicated flash units in a particular setup.
If you want to do this, the simplest way to do accomplish it is to “dumb down” your dedicated units by disabling all advanced features such as i-TTL and AWL (Nikon), or E-TTL II and dedicated wireless control (Canon). Make sure that you prevent pre-flash from firing the slaves prematurely. I.e.: Switch everything over to manual and operate the combo by treating all the units involved as generic flashes.
However, there may be situations where you want to retain dedicated functions such as TTL and AWL, and at the same time fire off generic wireless slaves. If you want to try this, here are some suggestions to help you along:
- Use the dedicated optical system (Nikon's AWL or Canon's wireless E-TTL mode) with the camera's built-in flash or a dedicated master flash in the hot-shoe to trigger dedicated remote flashes. Then, connect a radio slave trigger to the camera by means of the the camera's pc-connector, and use radio slave triggers to fire the generic wireless slaves.
- If you want an optical solution, the fundamental problem you run into is that the pre-flash that is intrinsic to dedicated flash will trigger any plain optical slave prematurely. As a workaround, you can lock exposure well ahead of the actual exposure by using FEL (Canon) FV lock (Nikon) to introduce a delay between the time flash exposure is measured, and the time the actual exposure is performed. Your plain slaves will fire when you press the FEL/FV lock-button, but by introducing a delay, they will have time to recharge and therefore be ready to fire again when you press the shutter button. See your camera's manual for a description of how you activate FEV or FV lock.
- As an alternative to using FEL/FV lock to create a delay, you may try to use so-called digital slave triggers that that can be set to ignore the pre-flash. However, while some of these devices cope reasonable well with TTL-mode pre-flashes, they may not be advanced enough to also work with the additional commander-mode pre-flashes. I currently know of no such device that works reliable in commander-mode.
Note: Mixing dedicated and generic flash means that the sophisticated exposure logic that is at the heart of any dedicated flash systems will not “see”, and therefore not be able to take into account, the light thrown by the generic units in the mix. This may, depending of circumstances, throw the dedicated exposure control system off track. But if, for example, the generic units are just used for background fill, this does not matter much.
Radio Slave Triggers
Optical solutions for off-camera flash of both the dedicated and plain variety struggles outdoors, especially in bright light. Optical units also require a clear line of sight between master and slave.
For this reason, many instead opt for radio transitters to control off-camera flash units. Radio will work outdoors in bright light, and requires no line of sight between master and slave. It also avoids the problem, noted below that not all optical receivers are compatible with modern flash units.
The downside of radio is cost. Quality radio systems, (e.g. Pocketwizards), are expensive, but in later years, several low cost alterantives have become available.
The low cost radio slave triggers does not support TTL exposure control. In this kind of setup you must use the camera in manual mode. You control the power of the flash by using non-TTL Auto or Manual power control.
Off-camera flash caveats
Canon EX-series flashes do not work well with some third party hot-shoe slave triggers such as FlashZebra #129, Hama 6967, Kaiser K1501, Seagull SYK-4 and Wein HS. The flash will fire, but only once. The flash need to be power-cycled before it will fire again. The reason for is that the complex electronic circuits in these newer flashes prevents the Silicon Controlled Rectifer (SCR) used in these slave receivers from resetting. If buying such a unit to use with a Canon Speedlite, you should check that is compatible first. The companies FlashZebra and YongNuo supply some optical slave triggers they claim to work with most Canon Speedlites. Of particular interest is the YongNuo CT-301P. This is a cheap 4 channel radio trigger that uses the European “free for all” 433 Mhz band. However, the CT-301P receiver (sold separately for around USD 20) also has an optical sensor, so presumably, it can do double duty as an optical slave trigger.
Newer Nikon Speedlights and the “new” Vivitar 285HV will not work reliably with optical triggers that is powered by the flash trigger voltage. The models I know may cause problems include FlashZebra #129, Hama 6967, Kaiser K1501, Seagull SYK-4, Wein HS and Wein Peanut. These, and many similar units, needs at least 6 volts to work, and will not operate reliable if the flash has a lower trigger voltage. The Nikon SU-4 slave flash controller works fine with all Nikon Speedlights.
Some flashes will not trigger when placed in a plain (two-contact) hot shoe as used by plain optical or radio triggers. This is the case with Nikon SB-400, Nissin Di-466, Nissin Di-622, Sunpak PF30X, Sunpak PZ42x, and Sigma EF-500/530 DG ST for Canon. These flashes are triggered via system-specific pins instead of the central contact of the hot shoe.
Some newer Canon Speedlites, in particular 430EX, 430EX2, 580EX and 580EX2 emits RF noise that interferes with some radio triggers. This makes these Speedlites fire at random with the early (V1 and V2) versions of the Cactus PT-04 radio triggers. The problem is fixed in a later (V4) model. Check out compatibility with your Speedlite before buying a trigger of this type.
If you want to use a generic flash on a DSLR camera, you first need to figure out how to connect the flash to the camera. As noted in the section on flash trigger voltage below, Both Canon's and Nikon's recommendation is that you should only mount one of the manufacturer's own units in the hot-shoe.
Canon's higher end DSLRs (e.g. EOS 10D, 20D, 30D, 40D, 5D, 1D-series) comes with a pc-socket, that can be used to trigger generic auto and manual flash units. The maximum safe voltage rating for the pc-socket can be found in your camera's manual. For all current models, I belive it is 250 volts.
Nikon's higher end DSLRs (e.g. D200, D300, D700, D2-series, D3) comes with a pc-socket, that can be used to connect most generic auto and manual flash units. The maximum safe voltage rating for the pc-socket can be found in your camera's manual. For all current models, I believe it is 250 volts.
To hook up a flash to the pc-socket, you just run a cable from the flash with pc-plug that fits the camera's pc-socket. If you want to use the camera's hot-shoe to hold the flash, I recommend that you convert it into a cold-shoe by putting insulating tape over the contacts to avoid any electrical connection between camera and flash through the shoe.
If your flash does not come with a pc-plug, you can buy a cheap pc-to-hot-shoe adapter, such as the model displayed on the right, to make the connection. You connect the adapter to the camera's pc-socket, and mount the flash in the adapter's hot-shoe.
Most generic flashes will also work if you mount them in the hot-shoe. However, because of the hazards associated with high trigger voltages you should be aware of the risks involved.
Dedicated flashes built for a different system will in many cases not fire from the hot-shoe, but may work fine with the pc-to-hotshoe-adapter.
Most compact cameras and some entry level DSLRs do not have a pc-connector. For these cameras, a possible workaround is to use a cheap hot-shoe-to-pc-adapter such as the adapter shown on the left, in the camera's hot-shoe. You can then connect the flash's or the sync cord's pc-plug to the adapter's pc-socket. Since the adapter is plugged directly into the camera's hot-shoe, you should only do this if you know that the flash is trigger-voltage is safe.
For wireless off-camera use, you can use plain optical or radio slave triggers to fire generic flash units. Using slave triggers also removes any trigger-voltage worries, because the third-party flash is never in physical contact with the camera, only with the optical or radio slave trigger.
5. Trigger Voltage
A matter of much debate is the maximum trigger voltage that is safe for a flash that is to be used in the hot-shoe on a digital camera.
Likewise, if you are slaving flash units or strobes of other units in a wired multi-flash setup, a slave with a too high trigger voltage may damage the electronic circuits in the lead flash, or vice versa.
Before connecting a generic flash unit to your camera, you should always check out the maximum safe trigger voltage in the manual for your particular camera (e.g. the manual for the Nikon D80 says on p. 119):
Use only Nikon Speedlights. Negative voltages or voltages over 250 V applied to the accessory shoe could not only prevent normal operation, but may damage the sync circuitry of the camera or flash.
For connection through a pc-connector, Canon lists the maximum safe trigger voltage in the manual (e.g. 250 volts). However, this number does not apply to the hot-shoe. Canon does not officially give out information on the safe voltage for hot-shoe mounted flash guns (beyond the obvious recommendation that you should only mount one of the Canon's own Speedlites in the hot-shoe).
However, an email from Chuck Westfall (Director, Media & Customer Relationship, Canon USA), posted in this thread in DPreview's Canon EOS 350D/300D forum in April 2005 by Doug Kerr had the following to say about trigger voltages:
The EOS Digital Rebel XT [350D] uses a modified version of the EOS 20D's shutter unit. Consequently, acceptable trigger circuit voltage for both cameras is the same, i.e., 250 volts. Except for the original Digital Rebel [300D], all current EOS digital SLRs (i.e., EOS-1Ds Mark II, EOS-1D Mark II, EOS 20D and EOS Digital Rebel XT) generate their X-sync signals electronically rather than mechanically. This is why they have higher acceptable trigger circuit voltage ratings than earlier models like the D30, D60, 10D and original Digital Rebel [300D]. These older models cannot be modified to achieve a higher trigger circuit voltage rating, since such a modification would require a different shutter mechanism as well as a complete redesign of the supporting circuitry.
I take this to mean that all Canons DSLRs newer than the 350D, as well as all the professional models, can use flash with trigger voltages up to 250 volts in their hot-shoe. However, 6 volts is the safe limit for the D30, D60, 10D, 300D, and Canon's digital compact cameras.
IMPORTANT DISCLAIMER: The above information is believed to be genuine and is reported here in good faith. However, I disclaim any responsibilty for your camera if you hook it up with an oddball flash gun and it fries. If you choose to act on this information, you do so at your own risk.
The ISO 10330 (Photography - Synchronizers, ignition circuits and connectors for cameras and photoflash units - Electrical characteristics and test methods) recommendation says that cameras and flash units should be able to accept trigger voltages up to 24 volts. AFAIK all modern flashguns (from the 21st century) comply with this.
For what it is worth, my Canon 550EX has and my Nikon SB-28 Speedlight both has a trigger voltage of 6 volts. A Nikon SB-600 uses 3.3 volts.
CAUTION: To avoid doing harm to the camera, you should always measure the trigger voltage before using a generic flash or foreign dedicated flash on DSLR camera to make sure it is within safe limits. Some flashes, and in particular vintage editions of the popular Vivitars, may have very high trigger voltages and can damage the camera. (The recently resurrected version, the Vivitar 285HV, use less than 6 volts and is “safe”.)
As for technical protection measures, Wein sells a range of devices (safe-syncs) to protect against excessive trigger voltages, but in my opinion, using a radio trigger. is just as easy and more flexible.
For more information on flash trigger voltages, see this webpage.
6. Flash Link farm
- David Hobby:
- Strobist (Great blog about Nikon flash use and lightning in general by Baltimore based photojournalist David Hobby.)
- David Hobby:
- Strobist: Lighting 101 (Good tutorial on using off-camera flash.)
- David Hobby:
- Strobist: On assignment (Examples of clever use of off-camera flash.)
- Joe McNally:
- Blog (Photojournalist and author of several books about creative flash use.)
- Neil van Niekerk:
- Flash Techniques (Tutorial on flash photography for weddings and portraits.)
- Home Page (Supplier of custom made cords, slave triggers and other flash accessories.)
- Lighten up and shoot (Blog aiming to teach photography and lighting with a little humor.)
- Lighting essentials for photographers (Web site about flash, studio lighting and natural light.)
- Portrait Lighting:
- Home page (Web site and blog devoted to studio lighting for portraits.)