Rudy Winston
Rudy Winston

Rudy Winston has over 14 years experience with Canon USA's Pro Products team, and has been responsible during that time for training Canon's staff on new products, creating presentations for customers and dealers, numerous writing projects, and providing technical assistance to professional and amateur photographers.

Speedlite 600EX-RT: Radio wireless basics

March 02, 2012

For the first time ever, a Speedlite has arrived on the market with built-in radio-based wireless TTL flash control. The remarkable new Canon Speedlite 600EX-RT brings to serious enthusiasts and professionals a tool that's been requested for years. In this report, we'll go over the fundamentals of this exciting new technology, and hopefully plant the seeds in readers' minds about some of the almost limitless potential this new system brings to creative photographers.

To start: the new 600EX-RT Speedlite

Canon's new top-of-the-line Speedlite replaces the previous Speedlite 580EX II in the line, and can be used with any EOS digital or even film camera. It brings a host of new features to photographers looking for an advanced Speedlite, above and beyond its built-in radio wireless capabilities. Used on-camera, it's fully E-TTL compatible with all EOS digital SLRs and those Canon PowerShot models having a hot shoe on-camera. And, it'll revert to off-the-film TTL if used with older 35mm film-based EOS cameras. So far from being exclusive to brand-new cameras such as the EOS 5D Mark III, the new Speedlite 600EX-RT can find a home in the camera bags of pretty much any EOS photographer.

Wireless, off-camera shooting with the Speedlite 600EX-RT

This new Speedlite remains fully backward-compatible for existing wireless E-TTL shooting. In other words, someone who already owns a Canon Speedlite system and shoots now with the current "optical" wireless E-TTL can bring the new Speedlite 600EX-RT into their world and not miss a beat. The Speedlite 600EX-RT is completely compatible with Canon's existing Wireless E-TTL, and can be mixed freely with flashes such as the 580EX II, previous 580EX or 550EX, 430EX II or 430EX, and/or the recent and less-expensive Speedlite 320EX or 270EX II. Used with optical Wireless E-TTL, it can work as an on-camera "master" unit, or off-camera "slave" unit, and as a slave unit, it can be freely mixed with other compatible Canon Speedlites.

So conventional, optical-based Wireless E-TTL – what Canon EOS users have worked with up to now – is entirely possible with the new Speedlite 600EX-RT.

But it also breaks ground for the entire photographic industry with its world's-first, built-in radio control for Wireless E-TTL. This is entirely separate from, and in addition to, the just-mentioned "optical" Wireless E-TTL which it's backward-compatible with.

The Speedlite 600EX-RT offers the user a choice of Wireless E-TTL methods: radio-based or optical. The two are entirely separate, and cannot be mixed and matched in the same set-up. For radio-based control, as of March, 2012, the only compatible flash gear are the Speedlite 600EX-RT and the accompanying Speedlite Transmitter ST-E3-RT (introduced along with the Speedlite 600EX-RT). Previous EOS Speedlites, like the 580EX II, are not compatible with Canon's new radio-based systems. And, while it's true that several third-party companies have developed accessories in the past few years to make Canon's current Speedlites radio-compliant, these systems – again – cannot simply be mixed and matched with the new radio-based 600EX-RT Wireless E-TTL set-up.

Limits with the current "optical" wireless flash method

Perhaps the easiest way to begin this is to re-visit the inherent limits of optical-based wireless flash triggering. While the current optical-based system is an incredible piece of technology (Canon had the industry's first truly wireless system for off-camera flash that allowed full TTL exposure control of up to three groups of flashes, when it launched Wireless E-TTL with the Speedlite 550EX and EOS 3 SLR camera, back in 1998), it has several important limitations that photographers have had to be cognizant of:

  • Distance limitations
    Since current "optical" wireless E-TTL relies on a series of rapid pre-flashes to trigger off-camera slave units, these pulses of light have to be relative weak in strength to achieve their extremely rapid duration and avoid perceptible time-lag. Weak pre-flashes limit the effective distance slave units can be from the camera/master unit to less than 30 or 40 feet in most instances; in some cases, they have to be much closer than that.
  • Positioning limitations for slave flashes
    The master unit (either a compatible on-camera flash, or the current Speedlite Transmitter ST-E2), puts out pre-flash illumination that covers about the same angle as you'd see from a full-frame camera with a 24mm wide-angle lens mounted. In other words, if slave units are positioned well to the side of the camera, it's entirely possible they'll see even weaker pre-flash signals, or possibly none at all. This makes it difficult, for instance, to position off-camera flashes behind the photographer.
  • Line-of-sight requirement
    Directly related to the positioning limitation above, slave units with optical wireless E-TTL require a clean and direct line of sight to the master unit. If they're positioned behind objects or people in a scene, and the master unit's pre-flashes aren't somehow bounced or reflected to the receiver area of the slave units, they won't fire. Along with distance, this confines wireless E-TTL to situations where users are in fairly contained areas for reliable operation – and limits creative placement of lights, especially for illuminating backgrounds, as accent lights, and so on.
  • Difficulties in areas with bright ambient light
    Since the sequence of pre-flashes from the master unit to slave units in an optical Wireless E-TTL system has to use very brief, weak but rapid pulses of strobe illumination, when working outdoors in daylight, photographers are up against some strong competition – the sun. If one or more slave units are positioned so that they're receiving direct sunlight on their face, the optical wireless sensor may be effectively blinded and not even see the pre-flashes fired by the master unit. Shading the front face of the slave units, and/or working with the master flash much closer to them, sometimes helps. Regardless, working with Wireless E-TTL in bright sunlight, up to now, hasn't always been a so-called "slam dunk" – even though it is compatible with high-speed flash sync, allowing very fast shutter speeds and flash to co-exist, to tame sunlight exposure.

Photographers have persevered, grown to understand the limits and develop effective work-arounds, and Wireless E-TTL has grown to be used by many advanced SLR enthusiasts and pros over the past decade. And, the fundamentals of Canon's optical wireless system have been essentially copied by nearly all competitive SLR camera makers, and are in one form or another available industry-wide today.

Radio is now a reality

Through it all, demanding photographers have longed for a radio-based wireless flash system that was truly built-in to the Speedlite, not done through add-on accessories. Canon has responded to these very vocal requests, with the world's first radio-based wireless flash system. The benefits it brings to photographers are immense:

  • Up to 98 foot (30m) distance range
    Speedlite 600EX-RT units can be placed up to nearly 100 feet (30m) away from the master unit. In practical use, you may find that 600EX-RT units can be much farther from the camera – Canon's official specs are conservative, and account for possible radio interference from the environment some users may be shooting in.
  • Total freedom of placement
    Radio range is over a full 360˚ arc – so slave units can be positioned anywhere, including behind the camera and photographer – still up to nearly 100 feet away.
  • No "line of sight" requirement
    Radio transmission effectively moves through/around most obstacles, including most types of walls, building pillars, trees, plants, and human subjects. Speedlite 600EX-RT units can thus be freely positioned nearly anywhere in a scene, and still fire reliably as long as they're within the roughly 100-foot working range of the Canon radio-based wireless system.
  • Full E-TTL exposure control remains possible
    The Speedlite 600EX-RT's new radio-based wireless system remains fully E-TTL exposure compatible. E-TTL flash features, such as Flash Exposure Lock, ± Flash Exposure Compensation, and (with limitations) high-speed flash sync remain available.
  • Manual flash and more
    For users who occasionally prefer Manual flash control, this is fully within the new radio-based wireless system's abilities. Manual flash mode, Multi (stroboscopic) flash mode, and even in some circumstances External Auto (non-TTL automatic flash, using an external exposure sensor on the front of the 600EX-RT) are all possible when using one or more flashes off-camera and triggering them via the new Canon radio system. And, it can all be controlled from the camera position, right on the master flash (or new Speedlite Transmitter ST-E3-RT).
  • Control of up to fifteen 600EX-RT Speedlites off-camera
    A total of one to fifteen Speedlite 600EX-RT units can be placed off-camera, and fired using either another 600EX-RT on-camera (as a radio-based "master" flash) or an optional ST-E3-RT Speedlite Transmitter on the camera's accessory shoe. When two or more flash Groups are established, anywhere from one to 14 flashes can be assigned to a group (A, B, etc.). Multiple groups do not have to have the same number of slave units within them. Again, the total number of Speedlite 600EX-RT slave units in a set-up cannot exceed 15, regardless of the number of groups.
  • Flash Ready displayed for each group on the Master Unit's LCD panel
    Because of the extensive communication between the master unit and any off-camera, radio-based slave units, each slave communicates to the master unit when it's fully recycled. When each slave unit in a group is fully ready, a flash-ready icon appears for that group on the master unit's LCD panel. In other words, if any single flash has weak batteries and is slow to recycle, the photographer is aware that that group is not yet ready.
  • Remote firing of a camera*
    Position a camera with either the Speedlite 600EX-RT or accessory Speedlite Transmitter ST-E3-RT on its hot shoe. A second 600EX-RT (or ST-E3-RT) can now be configured to fire that camera, by pressing a button... and the camera can be up to 100 feet (30m) away. Flash on the "slave" camera, triggered remotely, can fire or not fire... it's the photographer's choice. This is like having a 100-foot long remote cord, and not having to worry about anyone tripping over the cord!
  • New: Linked Shooting*
    Take a Speedlite 600EX-RT or Speedlite Transmitter ST-E3-RT, and put it on the hot shoe of an EOS camera. Assign this as a Master unit. Now, do the same – with either an ST-E3-RT or 600EX-RT – on up to 15 other EOS cameras, assigned as "slave units". It's now possible to fire the "master" camera, and have the 600EX-RT or ST-E3-RT fire the "slave" cameras, up to 100 feet away from the master. Flash can fire or not fire during exposure, whichever the photographer chooses. It's a new way to remotely control multiple EOS cameras.

    * Linked Shooting and Remote Release can be done with Speedlite 600EX-RT or ST-E3RT directly mounted onto Canon EOS 5D Mark III, and EOS-1D X. With all previous EOS models having an N3-style remote control socket, optional Canon cord SR-N3 is required for Linked Shooting or Remote Release (one end connects to Speedlite or ST-E3-RT, other end to camera's N3-type remote socket).

Whether in the studio or especially on-location, this new system promises to open up some incredible new doors for creative shooters. A lightweight, travel-friendly package of several Speedlite 600EX-RT units, possibly a Speedlite Transmitter ST-E3-RT (which is an optional accessory), and a few stands, swivel-type umbrella adapters and light modifiers suddenly empower the photographer to quickly and easily achieve a variety of different looks in a scene. Whether it's shooting fashion, corporate, commercial, events, weddings, photojournalism, or even amateur sports like basketball or ice hockey, the new radio capabilities of this system make it possible to put Speedlites in places they really couldn't go before. And to get results that were difficult or impossible to get before.

Compatibility with current and previous Canon EOS cameras

While Canon's engineers have developed a remarkable new system of radio-based wireless transmission with the new Speedlite 600EX-RT, there are a handful of limitations photographers will need to be aware of when using it with existing Canon EOS camera bodies.

When combined with the new EOS 5D Mark III or EOS-1D X cameras, none of these limitations listed here apply – these newly-developed cameras incorporated full compatibility for the new radio-based wireless flash system in their design.

  • Flash sync speed: When used for radio-based Wireless E-TTL with any Canon EOS camera introduced prior to the EOS 5D Mark III or EOS-1D X, the maximum available sync speed for flash will be one stop slower than normal. (EOS 5D Mark II or original EOS 5D – 1/100th second; EOS 7D, 60D, 50D, etc – 1/125th second; all previous EOS Rebel models – 1/100th second.)

    This only applies when using radio-based wireless control of off-camera 600EX-RT Speedlites. If a 600EX-RT is simply used on-camera (alone), or if it's used for optical-based Wireless E-TTL in any way, the camera's normal, full flash sync speeds are available.
  • No high-speed sync: For radio-based wireless E-TTL, the only cameras that allow hi-speed flash sync will be the EOS 5D Mark III and EOS-1D X (as of March, 2012). It's not possible with any other previous EOS camera bodies, when combined with the radio-based wireless flash system.
    (Hi-speed sync is possible for both E-TTL and manual flash exposure if a single 600EX-RT is used on-camera with previous EOS models, as well as during optical-based wireless E-TTL.)
  • E-TTL automatic flash not possible (via radio) with the following cameras:
    • EOS-1D and EOS-1Ds (original versions only - all "Mark..." versions are OK)
    • EOS-1v; EOS-3
    • EOS Elan IIE (EOS 50E)
    • 35mm film-based EOS Rebel models
      (Rebel Ti/EOS 300v and Rebel T2/EOS 300X are OK)
  • Radio-based wireless flash operation is not recommended with PowerShot cameras
Radio-based Wireless Flash options:

Using Canon's new radio-type Wireless Flash technology, users can operate either in the same fashion they were used to with the previous optical-based Wireless E-TTL system... or, when combined with Canon's newest SLRs, in the incredibly flexible "Group" mode. Here are the possibilities:

  1. Wireless E-TTL – "E-TTL" appears as flash mode on Master Unit
    This offers the same options as Canon's existing optical-based Wireless E-TTL, but photographers get the benefits of radio-based transmission instead.

    "ALL"every off-camera slave unit fires at even power

    "A:B" and "A:B C" – Two or three, respectively, groups of slave units fire at flash ratios assigned by the shooter, at the camera position (set on the "master" unit's LCD panel). The same six-stop range (8:1 thru 1:8) adjustment is possible for groups A & B. If "A:B C" is selected on the master unit, the "C" flashes are adjusted up to ± 3 stops, compared to total output of the A & B groups.
  2. Manual wireless flash – "M" appears as flash mode on Master Unit
    Anywhere from one to fifteen Speedlite 600EX-RT units can be set as "slave units" and placed up to 100 feet (or more) from the camera. Flash power is set on master unit, manually, from full power (1/1) thru 1/128th power, in 1/3-stop increments. Ratio possibilities – controlled from the camera – include:

    "ALL" – every off-camera slave unit fires at same manually-set power level (which is set on master unit's LCD panel... full 1/1 thru 1/128th power)

    "A - B" – two separate flash groups, each set independently on master unit's LCD panel, over same full 1/1 thru 1/128th power range.

    "A - B - C" – three separate flash groups, all manually controlled from the master unit. Each adjustable over same 1/1 thru 1/128th power range.
  3. Multi mode wireless flash – "MULTI" appears as flash mode on Master Unit
    Manual flash exposure, with adjustable "stroboscopic" repeating-flash effects. One to three flash groups, firing at reduced power (1/4 thru 1/128th power), and at firing rates from 1 to 199 Hz (flashes/second).
  4. NEW – GROUP mode – "Gr" appears as flash mode on Master Unit
    This is the headline feature of the new Canon radio-based Wireless E-TTL system. It requires an EOS 5D Mark III or EOS-1D X (as of March 2012) camera body to perform; for technical reasons, it's unfortunately not possible with previous EOS SLRs.

    One to five separate flash groups (A, B, C, D, E) can be activated, using the LCD panel of the master unit. Each can be assigned a totally independent method of operation. For instance, Group A could be a main light, and work in E-TTL mode at (for example) +1.3 stops power level. Group B could be a fill light, again with E-TTL, but independently controlled to be at minus 1/3 stop. Group C could be a background light, using the External Auto Sensor on the Speedlite 600EX-RT. Group D could be a hair light, firing at 1/64th Manual power. And Group E could simply be turned off.

    It's all controlled from the master unit (again, either a camera-mounted Speedlite 600EX-RT, or a Speedlite Transmitter ST-E3-RT). A group's operating mode (E-TTL, External Auto, or Manual flash) can be changed on-the-fly, again from the master unit. E-TTL power is adjustable, independently, for each group, over ±3 stop range (1/3-stop increments). Manual power is adjustable over the full 1/1 to 1/128th range.

    Combined with an EOS 5D Mark III or EOS-1D X camera, unprecedented power and flexibility in wireless flash is now at the photographer's fingertips, when using the new Canon Speedlite 600EX-RT, with or without its accessory companion Speedlite Transmitter ST-E3-RT.
Technical stuff:

As mentioned before, Canon's Speedlite 600EX-RT and Speedlite Transmitter ST-E3-RT represent a "closed" system. That is, they cannot be combined with other third-party, radio-based accessories sometimes used with existing Speedlites which convert them to radio-type transmission. And again, Canon's new radio-based wireless flash cannot be combined with traditional optical-based wireless flash in the same image.

Other technical points:
  • Radio Channels:
    Up to 15 channels are available to EOS shooters using the new radio-based wireless flash system. Unlike the previous optical-based system (which offered Channels 1 thru 4), the RT system's 15 channels are not only to allow two or more EOS photographers to work in the same area. The multiple channels allow one or more shooters the choice of selecting a bandwidth that offers the clearest signal in a given environment, considering other types of radio devices that may be active in the vicinity, or other interference. So it's entirely possible for two or more EOS shooters using RT wireless flash in the same area to be working on the same radio channel.

    Channels can be automatically selected (master unit selects the strongest available channel, 1 thru 15), or manually dialed-in by the photographer, using controls on the master unit. With the "SCAN" function, a button press shows a graphic on the master's LCD panel, with relative strength of channels 1 thru 15, to assist users who want to manually choose a channel.
  • Radio ID:
    Here is where two or more EOS photographers using the RT wireless flash system can prevent setting off each other's lights – if they're using the same channel. Pressing the button with the heading "ID" above it brings up a screen to set a 4-digit ID number. The ID for master unit and all slave units within a photographer's set-up must be the same (any number from factory-default 0000 to 9999 can be set). This allows use of a single channel, if it happens to be the sole channel getting reasonable radio performance in a given environment.
  • Radio Transmission:
    • IEEE 802.15.4, ARIB STD-T66
    • Primary modulation: OQPSK
    • Secondary modulation: DS-SS
    • Sending frequency (center): 2405 to 2475 MHz

In one move, Canon has revolutionized the way photographers work with flash. The use of portable Speedlites has exploded over the past decade, along with the advent of digital SLRs, as photographers realize the advantages of speed, compactness, ease of set-up, travel and more that they provide. Now, the single remaining thing photographers have begged for – for years – is here. Radio-based, wireless flash triggering that's all self-contained within a portable Speedlite.

And, Canon's new RT system doesn't confine the photographer to exclusively automatic flash, or exclusively manual flash, when used with today's digital SLRs. Users can work in the ways they're accustomed to working up to now, or can push the limits and explore lighting possibilities on-location that they never dreamed of before. It's a supremely flexible system, and one that cries out for creative shooters everywhere to get more creative than ever before with their lighting.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

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