I have had difficulty understanding how most flash systems work and was hoping someone could break it down. I'll use my current setup for a concrete example:

  • Canon EOS 60D
  • Nissin Di866 Mark II Flash

Some things I'm trying to understand:

  • Does my flash actually have a wireless receiver in it (Instruction manual seems to point to "yes")?
  • If I use the flash on my hot shoe, does it "transmit" to slave flashes automatically, or do I need a transmitter for that?
  • If the flash has a receiver built in, what typically would be the purpose of a separate receiver with TTL support? (i.e. Pocket Wizard). Keep in mind, I DO understand why you'd use a receiver with a flash that has a simple trigger without all the TTL support.
  • If the flash has a receiver built in, can I trigger that with just any transmitter or do I have to brand-match?
  • If my camera can control the TTL settings, what is the purpose of a 'Master' flash since all flashes can be controlled through the camera itself?
  • Where does IR fit into the whole equation?
  • 2
    \$\begingroup\$ I think there are some good individual questions in here, please separate them. As it stands, your question is quite broad. Some example for such question could be: What's the difference between using the built in pop-up flash and an on camera speedlight as master for off-camera ttl slave flashes? or When to use third party transceivers for remote TTL communications over the built-in mechanisms? \$\endgroup\$
    – null
    Commented Jan 7, 2016 at 18:25
  • 2
    \$\begingroup\$ Here's a good rule of thumb -- can you think of a single question which you could put in the title which would sum it all up? If not, it's probably several questions. \$\endgroup\$
    – mattdm
    Commented Jan 7, 2016 at 19:04

3 Answers 3


IR Systems

Think of it this way. Digital cameras are generally made by companies that do other consumer electronics. So, the first wireless controls for off-camera flash that were built were based on existing IR* remote technology—like how your TV remotes work. This has an obvious advantage for manufacturers of not requiring radio bandwidth allocation in all the countries where they want to sell their product. Optical/IR transmitters and receivers communicate by a series of light pulses, and how those pulses are interpreted are typically by a brand-specific protocol. Third party flash makers, like Nissin, et. al. reverse-engineer this protocol to make flashes that are compatible within those systems.

And your 60D can use its pop-up flash as your transmitter. The basic limitation of using the pop-up vs. an on-camera speedlight to be the optical transmitter is that the pop-up can't be rotated or tilted, has a shorter range, and can't communicate high-speed sync. So, it can limit where you can place your remote flash, and you have to use a shutter speed at or below your camera's sync speed.

Like a TV remote, you have to point your transmitter at your receiver or bounce the signal off of surfaces for things to work. Using the pop-up as your master and placing your flash behind you, or trying to hide the flash behind a door, or use it outside a window to fake sunlight--these are all relatively problematic with an optical system, but easily accomplished with a radio system. And TTL-capable radio triggers do allow for remote power control, which becomes extremely handy if your flash is someplace inaccessible or you don't want to have to keep ripping open your Westcott Apollo softbox to get to the flash to adjust it. But you do have to worry about RF interference, since to get around bandwidth allocation issues, most of the newer radio triggers use the 2.4 GHz worldwide ISM band, which is crowded with lots of things, like wi-fi and Bluetooth.

In addition, with an IR system, the ambient light levels need to be low enough for the signal to register. An IR system doesn't work great outdoors in bright sunlight without bounce surfaces around, and you can lose range and reliability, which is why even manual-only radio triggering is so common for on-location shooters. In studio situations, though, IR triggering tends to work pretty well. And the bonuses of using a proprietary IR system are that a lot of the flash protocol is communicated between the camera and flash, so you have extra-fancy features like TTL communication, high-speed sync, group control, and remote power control. Some radio triggers can only communicate the sync signal, and nothing else. So IR vs. RF can be a tradeoff of features and reliability of signal.

Dumb Optical

In the case of the Di-866 you have both a transmitter and a receiver that can speak (if you got the Canon version), the Canon wireless IR protocol (which Nissin calls Wireless TTL). But you also have other "dumb" optical slave modes (SD and SF), which may be what's confusing you.

These systems are not proprietary and can be used with any flash signal. The sensor in this case simply fires the flash when it senses another flash burst. The SF mode fires on the first flash it sees, the FD mode fires on the second burst it sees. This second-burst mode is important if your on-camera flash is using TTL, because TTL fires out a "preburst" for metering, then the actual flash burst. If you use the SF mode, then your flash fires early. This type of simple optical trigger works in concert with studio strobes, or a manual flash burst from any system of camera (including point and shoots), so can be quite useful.

* BTW, the Canon wireless eTTL system is not, strictly speaking, IR. It's near-infrared but still uses visible light signals from the main flash head.

Canon's RF system

And, as a btw, Canon has a second wireless flash protocol, the RT system, that is RF-based, rather than optical. But the transmitters aren't built into any cameras, yet, so you need at least two units in the system. The 600EX-RT, and ST-E3-RT can be used as transmitters, the 600EX-RT and 430EX III-RT can be used as receivers. There are 3rd-party clones of the 600EX-RT out there, too (e.g., the Yongnuo YN600EX-RT).

Nikon also just announced their first RF-based flash unit (SB-5000), so it looks like we're moving away from the older optical-based systems to RF-based ones.

See also:

  • \$\begingroup\$ Very good all-around explanation. Thank you for covering the gamut, this clears things up significantly. From what I understand of what you said, IR would seem inferior and prone to issues. These types of limitations you describe would make me want to go with an RF system. Is that where a 3rd party TTL Transmitter / Receiver like a Pocket Wizard would come in? Slap one on my hot shoe and one on the base of the remote flash? \$\endgroup\$ Commented Jan 7, 2016 at 19:53
  • 1
    \$\begingroup\$ @RyanGriffith Yup. Exactly. Although, these days, like the RT system, there are also now 3rd party flashes that have a radio receiver built-in: both eTTL/HSS capable (e.g., Yongnuo's YN-685, Phottix Mitros+) and manual-only with remote power control (e.g., Yongnuo's YN-560IV/660, LumoPro LP180R, etc.). There are also a lot of lower-cost options than TTL PocketWizards (which don't play well with the Canon 580EXII). I always recommend the Flash Havoc blog and gear guides if you want to see what's out there. \$\endgroup\$
    – inkista
    Commented Jan 7, 2016 at 20:36

Does my flash actually have a wireless reciever in it (Instruction manual seems to point to "yes")?

Yes. It has both an optical transmitter and an optical receiver. That means that it can be used both as an on-camera Master or an off-camera Slave using the Canon E-TTL optical communication system.

EOS optical E-TTL illustration

If I use the flash on my hot shoe, does it "transmit" to slave flashes automatically, or do I need a transmitter for that?

It depends. If your Slave is E-TTL compatible then it can receive the optical pulses of your Di866. If your Slave is not compatible with Canon's optically triggered E-TTL system, then you will either need a transmitter that does communicate in a way the other flash can understand, or you need a set of triggers with a transmitter on the camera and a receiver on the off-camera flash. Or you could use the off camera flash in dumb Slave mode in which the power is set manually on the flash itself and the main flash on the camera triggers the dumb slave to "fire'. Most dumb slave can also be programmed (with varying degrees of success) to ignore the main camera's pre-flash and wait until the second, main flash to fire.

If the flash has a receiver built in, what typically would be the purpose of a separate receiver with TTL support? (i.e. Pocket Wizard).

The difference between an optical communication system, such as that used by all Canon master flashes prior to the 600EX-RT, and a radio communication system such as that used by most wireless triggers. Radio tends to have a greater range, doesn't require line-of-sight, can operate in very bright environments such as direct sunlight (that gives the optical system a tough time), and has the ability for more than one set of the same type to be used in proximity to one another without interfering with each other. (Think several press photographers all using Canon covering an event for multiple publishers. Or more than one shooter at a wedding.)

If the flash has a receiver built in, can I trigger that with just any transmitter or do I have to brand-match?

You don't need to brand match, but you do need to system match. For instance, a Canon 580EX II, your Nissin Di866, and an Yongnuo YN568EX II should all work together when they are all set to use Canon's optical E-TTL system. If one of the latter two is made for the Nikon system, however, it would only work in dumb slave mode, and not in automatic E-TTL modes. Radio systems tend to be more brand and system specific. For example the Yongnuo 622 TTL radio system is not fully compatible with the Yongnuo manual only RF603/RF605/YN-560 radio system. YN622 devices made since the beginning of 2015 can receive RF603/RF605/YN-560 signals, but not transmit them. On the other hand, the Yongnuo RT radio system is a clone of the Canon RT radio system and respective Canon RT and Yongnuo RT units generally work together.

If my camera can control the TTL settings, what is the purpose of a 'Master' flash since all flashes can be controlled through the camera itself?

Backwards compatibility. The camera hasn't always been able to control the master flash attached to the hot shoe via the camera's internal menu on all Canon models. The master flash was only controllable via the control panel on the flash unit. Until recently this was especially true of the entry level bodies. So older bodies that are E-TTL compatible don't have the external flash controls options included in the camera's menu structure. Likewise, the built-in flash on most Canon bodies until recently was not capable of being used as a Master flash. And Canon professional Full Frame bodies do not even have a built-in pop up flash. You need an external E-TTL compatible Master for them.

Remember, the Canon system prior to the 600EX-RT uses optical pulses to communicate between the Master and Slaves, so there needs to be a strobe or near-IR light source on the camera capable of emitting those pulses.

Having manual control on the flash also makes it possible to use it off camera using a non E-TTL trigger that only can transmit a "fire" command to the flash. Flash power would then need to be adjusted manually on the flash, rather than on the camera. This also allows the flash to be used in non-Canon systems (although only as a manual flash and not as an automatic flash).

Where does IR fit into the whole equation?

If you choose to use a controller such as the ST-E2 that sends commands from the camera's hot shoe to E-TTL compatible off-camera flashes, the instructions will be transmitted in the near-visible spectrum between visible red light and invisible infrared light. The ST-E2 functions as the controller portion of a Master Flash, but doesn't have a light tube to emit a full spectrum flash and only emits to command the Slaves prior to the shutter opening. E-TTL compatible receivers can detect both the near-IR light emitted by the ST-E2 as well as the full spectrum light that includes near-IR emitted by a Master Flash unit.

Most E-TTL compatible Master flash units and near-infrared transmitters also have an emitter that uses red to near-infrared light to assist the Autofocus system in low light environments.

  • \$\begingroup\$ @Mike Sowsun Thanks for the catches! I was definitely thinking of my YN568EX II. \$\endgroup\$
    – Michael C
    Commented Jan 7, 2016 at 19:20

Your flash does not have a built in radio receiver. It has an optical infrared sensor which allows it to work with Canon, Nikon and Sony flashes which use IR pre-flashes to communicate with remote flashes.

The reason people choose radio systems is because they do not require line-of-sight between camera and all the flashes like IR systems do. Additionally the range of radio systems is far greater and there is no chance that ambient condition (bright sunlight) will interfere.

Radio systems in general are not compatible with each other so you have to brand-match. Some systems offer remote power control (and some even TTL and HSS support) but most are for basic triggering only.

Note: I scrapped my previous answer due to my misunderstanding of Canon's TTL system.


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