(With attention to the Nikon SB-600, indirectly)

Triggering Manual flash mode requires either sync cords, radio triggers, or optical slave triggers on each light.

PC sync cord - A wire cable from camera to the remote flash, to carry the trigger signal from camera shutter. A camera without a PC sync connector can add one with a hot shoe adapter, like the Nikon AS-15 Sync Terminal Adapter. A flash without PC sync connector can add one with a foot mounted accessory (Flash Zebra is a good source, with good descriptions). There have been several other styles of sync connectors (than PC), for example, studio lights typically use either a 3.5 mm, or a larger 1/4 inch phono plug at their end of the cable (PC sync connector is at camera end). Additional flashes can be triggered by optical slaves. This trailing cord to a near light is normally little issue when the camera is on a tripod. Variations can exist, but a PC Male to PC Female cable is an extension cord for the normal PC Male to PC Male sync cable. (PC is Prontor-Compur, brand names of German shutters. The PC sync connector dates from the early 1950s).

Radio trigger - a small transmitter on the camera hot shoe relays the trigger signal via radio to a battery powered radio receiver on the remote flash. Perhaps a receiver on every remote flash, but others can be triggered by optical slaves too. This radio signal may go through walls, around corners, often over great range (~100 meters), and is unaffected by sunlight.

Optical slave trigger - A light sensitive sensor at each remote flash, which triggers the flash in sync when it sees the flash of another manual flash unit. This light path can be reflected from close walls or the photo background, but light is line of sight, normally blocked by obstacles. Slaves have the advantage of being triggered by the full working value of the final flash power, but bright sun can interfere with them, obstacles can block them, and another photographer's flash can trigger your lights. Relatively short range, but is a very popular choice in the studio, where few issues exist.

Methods can be combined, trigger one flash by sync cord or radio, and use optical triggers on the other flashes. However these manual methods are incompatible with the Nikon Commander system.

You CANNOT TRIGGER AN OPTICAL SLAVE WITH A TTL TRIGGERING FLASH. The triggering flash must be in Manual flash mode. Because, TTL flashes fire a weak preflash for the camera to meter (to determine final TTL flash level), which will trigger the optical remote too early (while shutter is still closed), and it will expend itself, and will be unable to contribute again when the shutter opens and trigger is attempted again.

However, there are "digital" optical slave triggers, designed for use with TTL (including S2 optical slave mode in some speedlights), which ignore the first digital preflash (designed for use with simple digital cameras that cannot set manual flash mode to trigger a slave), and then these trigger in sync with the subsequent final flash (but which still cannot work with the much more complex Nikon Commander). These digital slaves have a battery for the timing circuits (delay to wait for second flash). Some have a switch to select "film" mode (S1 mode when applicable), to trigger from the first flash seen, to be usable for Manual mode. The battery allows some of these to work better, regardless of flash sync voltage.

Studio flashes typically all have optical slave triggers built in, which work well in that studio setting. Radio triggers are popular to add, but for an indoor studio situation, optical slaves are simpler. This article is about optical slave triggers such as are found on Ebay. Trigger the nearest one with a manual mode flash any way you can (with sync cord or radio, or use the camera internal flash), and then its flash triggers the other optical triggers in sync (if in range).

The Nikon SB-700, SB-800, SB-900 and SB-910 speedlights have exceptionally fine optical slave triggers built in, selected in SU-4 mode. The Nikon SB-26, SB-50DX, and SB-80DX flashes also have slave triggers built in. Many Chinese flashes today have an optical slave trigger built in too, commonly named S1 and S2 mode (S1 for manual flash, and S2 can ignore a TTL preflash). However some speedlight flashes do not include a slave, and the Nikon SB-600 does not have an internal slave trigger. Such slave accessories can be added to the flash foot or PC terminal, but unfortunately, the SB-600 has a low sync voltage, which can make some optical slaves be problematic. Because, these optical triggers have no batteries. They are instead directly powered by the flash sync voltage itself. This sync voltage is present on the speedlight shoe foot (at center pin), and on any sync cable that may be present.

The issue prompting this discussion is that the SB-600 has a low sync voltage. My Fluke 75 III digital volt meter (DVM, ± 0.3%) reads sync voltage of my two SB-800's as 3.63 volts, which is more than a volt lower than older models (about 5V has been generally typical). A friend says his SB-600/SB-800/SB-900 models all measure the same sync voltage (all same as each other, but his meter said 2V?)   Digital meters generally have auto range but for best accuracy on a mechanical meter, don’t try to measure 5 volts on the 500 volt scale. Start off on a higher safe scale, but then as you see the result, use the lowest safe scale possible, that the reading allows. But it is very easy to believe these models have the same circuitry inside them. So if the SB-600 and SB-800 have the same sync voltage, and this voltage powers the optical slaves, then the premise here is that the SB-800 data below should be applicable to the SB-600 too. Not all optical slave triggers will work with low sync voltage... for example, the Wein Peanut slave below needs 5 volts to work at all, but then it works very well, the best of the lot. Most slaves are more sensitive with the higher voltage. A few slaves (called digital slaves) and those built into speedlights have internal batteries instead of depending on the sync voltage.

Safe Flash Sync Voltage

A concern is that excessively high flash sync voltage in some too-old-to-consider-still-using flashes can damage today’s digital cameras. But sync voltage can also be too low to properly work with some slave units. The slave modes built into flashes should be fully optimum, and you will surely appreciate the features of a modern flash unit.

Flash Sync Voltage is the voltage placed on the sync cable and hot shoe foot by the flash unit. The camera shutter simply shorts this sync voltage to ground as being the signal to trigger the manual flash in sync with the shutter opening. The slave sensor also shorts this sync voltage when it senses a flash, to trigger the unit in sync. It is designed to be shorted, so the limited current is very low. It is manual flash mode, and the flash sees its own sync voltage fall to zero, which is the signal to flash now. This same sync voltage is also the source of power for the external optical slave triggers (those that have no batteries). Today many speedlights have builtin slave modes, which have the 6 volts of the flash batteries always present (and IMO, have great sensitivity for trigger distance). One issue for the slaves without batteries is that a lower sync voltage (less than 5 volts) may reduce their trigger sensitivity. Today, sync voltage is nominally around 5 volts, except these newest Nikon SB models are lower, about 3.6 volts, and this affects the external optical slaves that are powered by sync voltage.

Modern flash design uses sync voltage about 4 or 5 volts. The ISO sync voltage specification is 24 volts maximum. Canon has said sync voltage for some camera models must be limited to 6 volts (see your camera manual for this). But some old flashes will exceed 200 volts sync, that could harm some digital cameras. Nikon DSLR are rated to 250 volts - see your camera manual section about Optional Flashes, for example the paragraph about "Use only Nikon Flash Accessories". You can easily measure the sync voltage with the flash turned on, while not connected to anything. Measure DC voltage from large center pin on speedlight foot, to the metal foot (or to the metal slide contacts on each side of a plastic foot).

The way to know for sure what you have is to simply measure the flash sync voltage yourself. You can measure it on the sync cable, but on a speedlight, the flash foot will be much easier to measure than the tiny PC sync cable contacts. Note that either way, an easy accidental slip of the meter probe while measuring likely will short the center pin to the metal ground to flash it accidentally. No harm done to the flash, that’s how it works, but expect that might happen and guard your eyes — set the flash to low power and aimed away, but NOT placed face down on anything that can be scorched by the hot flash tube (valid concerns).

These images are the foot of Nikon flashes with the Nikon four pin configuration. Nikon, Canon, Sony, Fujifilm, Panasonic, Pentax ... all use the multiple pins for TTL today, and each brand is a different configuration, but each has the center sync pin. My own notion is that any flash with multiple pins on the foot (implying TTL, either for the film or digital eras) likely should have safe sync voltage (but film TTL cannot work on digital cameras). Manual speedlights however have only the one sync pin, and the oldest of those are the concern about voltage. The oldest studio flash power packs can be very suspect too. Here is a list of user reported voltages of old flashes. There are protectors you can put on the hot shoe or on the sync cable to safely use with high voltage sync (Wein Safe-Sync is a popular one), but the price seems better spent towards a modern flash with modern features.

It’s very easy to measure flash sync voltage yourself, to know for sure. With the flash turned on, simply measure the DC voltage between the foot’s red marked center pin and the metal foot (image A). Or if the flash foot is plastic, it must have a metal electrical contact in the shoe groove on each side that will touch the top rim of the flash shoe (image B is a plastic foot to show the metallic ground contact — or some speedlights run a solid metal wire down this shoe groove instead, but measure to the metal in the groove that provides the ground). Use the lowest voltage scale on your multimeter that can still safely read the voltage (start on a high scale and then work down as safely possible). Accidentally shorting this center pin to the metal foot ground with the meter probe will flash the flash. A $10 voltmeter may read off a little, but the safety issue is only the big ballpark that is important — is it less than 10 or 20 volts, or is it more than that? For a reference to check the voltmeter accuracy: A fresh new dry cell battery (carbon-zinc) should measure 1.55V, or a new alkaline or lithium AA cell maybe almost 1.6V at first. A recharged NiMH cell will be about 1.3V.

So, the major point here is that I am assuming my SB-800 tests are also valid for a SB-600, with same trigger voltage of same current group from same manufacturer, should work the same with the same slave trigger. Obviously, the point here is not about use with the SB-800, as the SB-800 already includes what it needs. The SB-800 (and SB-700, SB-900 and SB-910) include a self-powered built-in optical slave (called SU-4 mode), but the SB-600 does not include it.

Sensitivity, Maximum Range that triggered in direct line of sight
Optical Slave Triggering: (In no particular order)	Nikon SB-800 flash 3.63 volts sync		Nikon SB-24 flash 4.93 volts sync		Internet Link Sources  (probably Not permanent?)
	Triggered by 1/128 power	Triggered by Full power	Triggered by 1/128 power	Triggered by Full power
1. Sonia High Range Slave with rotating hot shoe	24 feet	40 feet	24 feet	41 feet	Rotating Hot Shoe Flash Slave Optical Sensor
2. Sonia Slave With Multi-terminal Hot Shoe (Orange) This one is my choice	21 feet	45 feet	23.5 feet	48.5 feet	Sonia Remote Flash Slave Unit With Hot Shoe High Range
3. Sonia Syncro Eye Slave High Range (Orange)	15 feet	30 feet	18 feet	32.5 feet	High Range Peanut Flash Slave Optical Sensor NEW 60'
4. Sonia Optical Slave Sensor, Extended Range (Orange)	7 feet	22 feet	9 feet	27 feet	Extended Range Optical Slave Sensor with Male PC Connector'
5. Sonia Syncro Eye Slave (for Canon) (Green)	8.5 feet	22 feet	11 feet	26 feet	High Range Peanut Flash Slave Optical Sensor
6. Wein Peanut Slave WEPN	won't trigger at SB-800 3.6V sync		29 feet	90 feet	Wein Peanut Slave (Range - 100-Feet)
7. Seagull SYK-3 Optical Slave Trigger	10 feet	24 feet	23 feet	41 feet	SYK-3 Optical Slave Trigger for Hot Shoe Flash

These are the triggers used. I see no difference in appearance of the three orange ones. Of these, #2 was the most sensitive, and it came with the all-metal hot shoe adapter in a small cardboard box, marked Sonia Multiterminal Slave Unit (box did not mention High Range). #3 came foam packed in a plastic box in a cardboard sleeve that said Sonia High Range. The weaker #4 with male PC was bulk packed in a small plastic bag, with no name. No markings on them, but some wide performance variation certainly appears to exist on the orange units.

Optical slaves are normally triggered by one of the working lights, using its full working power. I know using the little D300 flash is a confusion factor, but the low power was to allow comparing the sensitivity in the house. I had a 52 foot span in the house, where I could lay down a measuring tape. But to check the SB-800 internal SU-4 slave triggered by D300 internal flash at 1/128 power level, I had to open front door and cross the street. At 132 feet (shown), my back was at a fence, no more space. Direct line of sight, both the triggering flash and the remote slave sensor were facing either other. The SB-800 internal slave can use the internal battery power, so it is not dependent on the sync voltage. I did not attempt to check it triggered with full power.
With the Wein Peanut triggered with full power of the internal flash, the SB-24 was triggering from the side walk. Both are pretty good stuff, both simply just work, seemingly with any reflection indoors.

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The #6 Wein Peanut is the most sensitive on the SB-24 (of these triggers here), but it is useless on the 3.6V voltage of the SB-800. This Double PC Male Ended Adapter is very convenient to connect it and the other slaves with female PC to a flash or foot adapter with female PC connector. Or, the slave could be connected with a short or long PC Male to PC Male cord, to position the slave in the clear, or to move it to a closer range.

#7 Seagull SYK-3 is very plentiful on Ebay. Only issue is that it works better on the SB-24 than on the SB-800. It has a plastic foot, and a metal 1/4 inch thread on the bottom. It takes a couple of weeks to receive items from Hong Kong, but my experiences have been good.

But #6 and #7 seem most affected by the lower sync voltage.

The #1 Rotating Shoe Trigger had good performance, even at the lower sync voltage. It needs no additional mount or PC sync adapter, but this rotating axis seems a weak point mechanically. It seems strong enough with the flash standing upright above it, but horizontal is a worry. It sags — and does seem to stay together, but it does not inspire my confidence. Since most flash heads rotate anyway, I would have preferred a stronger fixed shoe. Some strong epoxy glue comes to mind (untried).

It has PC connectors on each side, which can trigger other external flashes from the slave, but the side connectors cannot trigger anything internal inbound from a external source. So it could be used as shown at right, with a flash foot PC adapter, with the short PC Male to PC Male cord, allowing the flash to be mounted perhaps more securely. This works fine, but I did not solve that part about fastening the slave sensor. It has to be aimed, but tape or wire surely does it easily.

The metal shoe on this type does short the other three pins on the Nikon flash foot, but which causes no ill effect... The trigger and the flash both work, and work rather well. The shoes have no hole for the Nikon SB-600 pin lock, but that pin is spring loaded, so you can simply drop the pin anyway, and it holds tight. You cannot remove the flash from the hot shoe without lifting the pin. The optical trigger base has a plastic foot, and also a metal 1/4 inch threaded hole to attach to a mount. The extra PC adapter shown has a metal foot and threaded hole.

The three orange ones (#2, #3, #4) all appear the same, indistinguishable, no unique features or marking. However each had widely different sensitivity performance. #4 has a PC Male connector.

The green one #5 is said to work on Canon EX flashes, where the others probably don't. I don't have Canon gear, and am repeating stuff I cannot verify, but the green Sonia slaves are said to be "Canon compatible". Most slaves won't work on Canon speedlights, because Canon's sync voltage does not drop far enough to release the slave. It fires one time, and then locks up. The green Sonia are said to have special circuitry to work with Canon too. Nikon flashes are no issue in this regard, but SB-600 has a lower sync voltage, which sometimes can be an issue.

#2 is $12, and it would be my choice. It includes the multi-terminal PC adapter and shoe to hold the flash, which has two PC female and one PC Male connections, all of which are triggered by the slave in the one PC Male port. This is NOT a universal PC adapter, because the side ports are outgoing only — the slave (or a PC sync cord) must be only on the PC male connector (but it could be used with a PC Male to PC Female extension cord at the front connector). The unique Male PC connector is important though, to allow Slaves with female connectors to connect directly. This #2 adapter has a sturdy all metal body with a shoe foot, and a 1/4 inch threaded hole to attach it to a mount. But it is unknown how much slave performance variation to expect, from unit to unit — very puzzling. I was hoping #4 with the PC Male connector would work out better, since it has the PC Male connection, and can plug directly into PC Female on adapter or flash.

I additionally tested the third one (#3 High Range Sonia, mid-range of these in sensitivity) in a more real situation. I did not set up the real lights and background, but instead, I simply aimed the SB-800 slave flash at a beige wall 7 feet away, to simulate a background light on a background. In this one case (not in above tests), the direct triggering flash was blocked at the slave with a 30x20 inch foam board (detrimental, intentionally counterproductive, hardest possible way, not helpful), simulating the subject blocking a background light behind them (the post does that pretty well too). The slave cell was triggered by the reflection from the "background", in front of the slave flash. The camera was 12 feet behind the slave, and about 5 feet to the side. That reflected path is 19 feet to the background, and 7 feet back to slave. The D300 internal flash at 1/16 power would trigger it here. This internal flash 1/16 power would compare to a typical speedlight trigger at no more than 1/64 power (a SB-800 50 mm zoom triggered it there at 1/128 power).

But a picture of a subject uses more than minimum power, and the subject in right picture (the post) at f/5.6 is with a hot shoe SB-800 at 1/4 power, about 5 stops more power than its minimum 1/128 (and about 7 stops more than internal flash minimum 1/128).

Point of the story: 1/4 power or more from a triggering speedlight is a very realistic actual case (1/4 speedlight power compares to full power from the internal flash). The tiny 1/128 power internal flash is very convenient for comparing, but 1/128 power is an unrealistic limitation for any real situation. Please do not misunderstand the numbers. The point is not that this is difficult to make work. The point is that this is easy to make work. It is only the description that is awkward.

The light path to the slave often needs the wall reflection, and perhaps its range is marginal for extreme cases, but the point is, this is very adequate slave performance for most cases. The optical slave does have the advantage of being triggered from the full final flash power level. My actual studio is normally sideways where the camera is, but I have to move some stuff there. The measuring tape on the floor is some of the technical facilities of my slave testing lab. 😊

The three orange units had rather different performance levels however. I don't understand that. I do notice that those internal photocells are mounted at slightly different positions and angles, probably not good, but I have no clue if that is a factor, or how to predict it. These range measurements can be slightly variable, but I think within a couple of inches. Remember, these are Ebay's cheapest.

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