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Technische Daten

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How Canon’s latest flash system operates with EOS cameras

How Canon’s latest flash system operates with EOS cameras

December 2012

By Syl Arena

Canon’s introduction of the Speedlite 600EX-RT flashgun and the Speedlite Transmitter ST-E3-RT in early 2012 brought a wide range of new features to the world of Canon flash photography. Many of these new features are available to users of all EOS digital cameras, whilst a few functions are available only on cameras introduced from 2012 onwards: thus far the EOS-1D X, EOS 5D Mark III, EOS 650D, EOS 6D and the new EOS M mirrorless camera. After examining the new Speedlite features that are available for all EOS digital cameras, I will take a detailed look at the features that become available only when the new Speedlite system is used with the newest generation of EOS cameras.


Identical layout for Speedlite and Transmitter controls

The menu and button layout of the Speedlite 600EX-RT flashgun and the Speedlite Transmitter ST-E3-RT are identical. Learning the operation of one device means that you will know how to operate the other. The only marked difference between the Speedlite and the Transmitter is that the 600EX-RT provides control of radio or optical slaves, whereas the ST-E3-RT unit only provides control of radio slaves.

© Syl Arena

The menu and button layout for the Speedlite 600EX-RT (left) and Speedlite Transmitter ST-E3-RT (right) flash units are identical.

Illuminated dot matrix LCD panel

For the first time, Canon has incorporated a dot matrix panel (172x104 dots) in Speedlite units. Whereas previous generations of Speedlites had static icons that were either on or off, this new LCD creates icons and letters ‘on-the-fly’. This provides the opportunity to have interactive menus and contextual button functions. Additionally, there are three colours to the LCD backlight: green and orange (which can be customised for master/slave usage), and red (which appears as a warning when the thermal circuits are activated).

Interactive menus and contextual buttons

With previous generations of Speedlites it was necessary to assign multiple functions to single buttons. This often meant that the specific button needed for a desired setting wasn’t immediately apparent. With the new dot matrix LCD panel, it’s now possible to have button functions that change depending upon the Speedlite mode (E-TTL, Manual, Multi, Automatic External or Manual External). Additionally, cycling through the menu button (always the rightmost button) reveals the purpose of each button within a specific shooting mode. The synergy of the dot matrix LCD, the interactive menus, and the contextual button functions enables novices to learn how to use a Speedlite much more rapidly than with previous models.

© Syl Arena

In E-TTL mode the button functions are (from left to right): Zoom/Custom Functions, Flash Exposure Compensation, Flash Exposure Bracketing and Sync.

© Syl Arena

In Manual mode the button functions are (from left to right): Zoom/Custom Functions, Flash Power, ‘not used’ and Sync. Flash output is shown in the centre of the screen.

© Syl Arena

In Multi mode the button functions are (from left to right): Zoom/Custom Functions, Flash Power, Total Number of Flashes and Flashes Per Second.

© Syl Arena

In Automatic External (Ext.A) mode the button functions are (from left to right): Zoom/Custom Functions, Flash Power, Flash Exposure Bracketing and ‘not used’.

© Syl Arena

In Manual External (Ext.M) mode the button functions are (from left to right): Zoom/Custom Functions, ‘not used’, ISO, and ‘not used’.

Larger Mode button

The Mode button has been separated from the four buttons below the LCD and made significantly larger. The operation of the Mode button provides further evidence of the benefit of the new interactive menu system. When the Speedlite is not in Wireless mode, pressing Mode repeatedly cycles through E-TTL, Manual, Multi, Automatic External and Manual External. When enabled as a radio master, the Mode button cycles through E-TTL, Manual, Multi and the new Group mode. When enabled as an optical master, the Mode button cycles through E-TTL, Manual and Multi.


The Mode button on the Speedlite 600EX-RT and the Speedlite Transmitter ST-E3-RT is now situated separately from the four buttons below the LCD – this image shows it on an ST-E3-RT unit.

On-camera control via camera LCD

Canon introduced the on-camera access to the entire Speedlite menu with the introduction of the Speedlite 580EX II flashgun in 2007. Operating the Speedlite through the on-camera menu provides more intuitive control as the menu options may be read in any of the many languages present on Canon EOS cameras. Both the 600EX-RT and the ST-E3-RT units can be controlled via the LCD on all 2007 or newer EOS cameras. Look for ‘Flash control’ on cameras with pop-up flashes and ‘External Speedlite control’ on cameras without a pop-up flash. As is discussed in more detail later in this article, with the 2012 camera models, Canon has introduced a Quick View Flash menu scheme that provides even more intuitive control.

© Syl Arena

One of the advantages of radio control is that slave flashes can be enclosed inside softboxes, such as the Westcott Apollo Orb shown here, and then controlled from the LCD of your EOS camera.

2.4GHz radio control

Canon has created the first-ever Speedlite system with two-way radio control built into the units. When compared to the optical (aka: infrared) system used in the previous generations of Speedlites, radio provides greater range – up to 30 metres in congested environments and 100 metres plus in environments without radio interference. Radio also eliminates the line-of-sight requirement, meaning that 600EX-RT flashguns can be used as slaves in many situations that would not work with Speedlites previously, such as the placement of Speedlites inside of softboxes, like the Westcott Apollo, or the placement of Speedlites outside the room to create the effect of real sunlight coming in through a window.

Additionally, this is a two-way radio system, meaning that the slaves communicate with the master. So, if you are shooting multiple slaves and have the beep on the master activated, then the master will not beep until all of the slaves have checked in as recycled. Radio also enables Linked Shooting and remote firing, which will be discussed shortly.

© Syl Arena

When in Radio Master setting the button functions are (from left to right): Zoom/Custom Functions, Flash Exposure Compensation, Flash Exposure Bracketing and Menu 1. Pressing the Menu button repeatedly reveals the additional menus and then cycles back to Menu 1.

© Syl Arena

When in Radio Slave setting the button functions are (from left to right): Zoom/Custom Functions, Flash Exposure Compensation, Group ID Selection and Menu 1. Pressing the Menu button repeatedly reveals the additional menus and then cycles back to Menu 1.

© Syl Arena

When in Optical Master setting the button functions are (from left to right): Zoom/Custom Functions, Flash Exposure Compensation, Flash Exposure Bracketing and Menu 1. Pressing the Menu button repeatedly reveals the additional menus and then cycles back to Menu 1.

© Syl Arena

When in Optical Slave setting the button functions are (from left to right): Zoom/Custom Functions, Flash Exposure Compensation, Group ID Selection and Menu 1. Pressing the Menu button repeatedly reveals the additional menus and then cycles back to Menu 1.

© Syl Arena

Speedlite 600EX-RT LCD screen showing Personal Function (P.Fn) 06 options.

Dedicated wireless button

Fans of the original Speedlite 580EX flashgun embraced the use of a lever to switch the Speedlite into wireless mode as a master or slave. With the introduction of the Speedlite 580EX II, this lever was removed to facilitate weather sealing and the wireless control was linked to the Zoom button, which obscured the access to the wireless system for many novice shooters. Conveniently, on the 600EX-RT, Canon has created a dedicated wireless button that enables direct activation of the Speedlite as a master or slave in either radio or optical mode. If one only uses wireless in radio or optical mode, this button can be customised to go directly in/out of radio or optical via Personal Function (P.Fn) 06.

Expanded channels, Channel Scan and 10,000 wireless IDs

The addition of radio wireless expands the channel selection from the four found in optical wireless to 15. However, some of these 15 channels may be more congested than others as 2.4 GHz is also the same bit of the electromagnetic spectrum where microwave ovens, Bluetooth devices and many wireless routers operate.

© Syl Arena

There are 15 channels in the radio wireless system. The master and slave(s) may either be assigned to one of the channels or the system may be left in Auto.

© Syl Arena

The Channel Scan function indicates which of the 15 radio channels are the strongest. Here, channels 5, 7, 8, and 15 are less strong than the other channels.

© Syl Arena

A new function of radio wireless is that the master and slave units must have the same wireless ID, in addition to being on the same channel. Here the ID is being set to wireless ID 1234.

To find the optimal channel on a radio master, the Channel Scanner (found on Menu 3) will reveal which of the 15 channels have the least interference at a particular location. Once a suitable channel is selected, the master and slave Speedlites must also be on the same wireless ID. Even if there is only one clear channel, the availability of 10,000 unique wireless IDs means that virtually any number of Canon photographers can operate their radio slaves on the same channel without interference.

The Link light confirms that the master and slave(s) are set to the same channel and wireless ID. When the Link light glows red, the master is not communicating with any slaves. When the Link light glows green, the master is communicating with a slave. A green Link light on a slave indicates that it is communicating with a master. An orange Link light indicates that another sub-master is also is use.

© Syl Arena

The Link light shows red on the master and slave when no other units are located or being communicated with.

© Syl Arena

The Link light on a master will glow orange when it is a sub-master (turned on after another unit was activated as a master).

© Syl Arena

The Link light on the master and slave(s) glows green when connected by radio to another unit.

Compatibility with ‘optical wireless’

The 600EX-RT is fully compatible as either a master or slave with previous generations of Canon Speedlites that operate via optical wireless*. This provides a convenient means for a Canon photographer with one or more of the previous Speedlite flash units to start incorporating the 600EX-RT into the mix. For instance, when activated as an optical master the 600EX-RT has all the functionality of a 580EX II in the same mode. Note that it is not possible for the 600EX-RT to operate simultaneously in both radio and optical wireless. It must be either in radio or optical wireless.

* Note: Canon now uses the term ‘optical wireless’ as it more precisely describes how the master Speedlite sends the instructions to the slave Speedlites, via an ultra-fast series of coded pre-flashes from the tube in the flash head. For many years, the term ‘infrared’ was casually used because the wireless Speedlite Transmitter ST-E2 hid a small flash tube behind a thick panel of red plastic.

© Syl Arena

When activated in Wireless mode (as a master or slave), the Speedlite 600EX-RT displays an icon in the upper right corner of the LCD. Here the radio icon is displayed.

© Syl Arena

When activated in Wireless mode (as a master or slave), the Speedlite 600EX-RT displays an icon in the upper right corner of the LCD. Here it's the optical wireless icon.

© Syl Arena

When the Speedlite 600EX-RT is not in Wireless mode, there is no icon in the upper right corner of the screen.

Expanded flash head zoom

The Speedlite 600EX-RT expands the range of the Auto Zoom head from the 24-105mm found on previous generations of Speedlites to 20-200mm. When in Auto Zoom the 600EX-RT will also account for the crop factor of APS-H and APS-C sensors. With the extension of the Wide Angle Panel (WAP), the flash coverage extends to 14mm (which is the same coverage provided by the WAP on previous models).

Light distribution patterns

When the Speedlite 600EX-RT flashgun is operated in ‘Auto Zoom’ three patterns of light distribution can be set by the user via Custom Function (C.Fn) 21. These are:

  • Standard Coverage (C.Fn-21-0): the angle of coverage operates as it has on previous models. A slight bit of fall-off (vignetting) will be seen around the edges of the frame.
  • Guide Number Priority (C.Fn-21-1): the Speedlite concentrates the light at the center of the frame by positioning the flash tube one step forward (i.e. with a 50mm lens, the Speedlite positions the flash tube at 70mm). This can be helpful when telephoto lenses are used. For artistic effects, the benefit is that there is greater fall-off around the edges of the frame.
  • Even Coverage (C.Fn-21-2): the Speedlite spreads the light wider than normal by positioning the flash tube one step back (i.e.: with a 50mm lens, the Speedlite positions the flash tube at 35mm). This avoids vignetting entirely.
© Syl Arena

Custom Function 21 changes the illumination pattern when the Zoom is set to Auto. The options are (from top to bottom): Standard Coverage, Guide Number Priority and Even Coverage.

Colour temperature communication

When paired with an EOS digital camera from the EOS 30D on, the Speedlite 600EX-RT will transmit the colour temperature at which it fired to the camera so that the camera can set a precise white balance. Note that this function requires the camera to be in either Auto White Balance or Flash White Balance.

Improved contact with hotshoe

The accumulation of oil and dust on the pins of a Speedlite’s foot and/or the contacts in a camera’s hotshoe can interrupt the communication between camera and flash. With the 600EX-RT, Canon has incorporated a new metal foot on the Speedlite that literally moves forward and back as the locking lever is engaged. The result is that the five contact pins gently scrape the hotshoe contacts to provide a clear electronic path between camera and Speedlite.


Please click on the window above to watch an animation of how the Speedlite 600EX-RT wipes dust away when coming into contact with an EOS camera hotshoe, thus ensuring a clear electronic path between the camera and the flashgun.

Recycle beep

The new Speedlite and Transmitter provide a beep to indicate with the Speedlite system is recycled and ready to fire. When multiple Speedlites are used as slaves, the beep will not sound until all the slaves have checked in with the master as ready. If desired, the beep can be silenced via Custom Function 20.

Improved thermal detection and continued operation

While the use of an external battery pack will significantly improve recycle time (reductions up to 60% are typical), there is the risk that the flashtube or circuits will overheat during repeated flashes at high power. The original Speedlite 580EX provided no thermal detection. On the Speedlite 580EX II, a thermal circuit would prevent the Speedlite from firing when high temperatures were detected. On the 600EX-RT, Canon has improved the thermal detection and provided a means of continued operation at two levels:

  • Level 1 – the recycle time lengthens to eight seconds, the backlight on the LCD turns to red, and two small ‘heat rays’ appear above the Speedlite icon on the LCD.
  • Level 2 – if the temperature continues to rise through continued usage (which is required during some shoots), at Level 2 the recycle time stretches to 20 seconds, the LCD backlight blinks red, and three ‘heat rays’ appear above the Speedlite icon on the LCD.

Easy-to-change Custom Functions

It is no longer necessary to have the user manual in hand to navigate the Custom Function system, where settings for such features as power saving, measurement increments, etc. can be set. Previously, the Speedlite LCD only displayed the Custom Function number and the number for the option to be set. There was no description of the Custom Function or the options. Now, the interactive menu on the new dot matrix LCD enables the user to set Custom Functions easily on the 600EX-RT and ST-E3-RT. For every Custom Function, there are icons that indicate the options and words that communicate the effects of the available settings.

© Syl Arena

On previous generations of Speedlite flashguns, the options for icons and letters were limited. Here, Custom Function 1 is displayed on a Speedlite 580EX II flashgun’s LCD screen.

© Syl Arena

The dot matrix LCD on the Speedlite 600EX-RT is capable of displaying a wide range of icons and letters. Here the options for Custom Function 1 are displayed graphically (ZZ, power saving) and in words.

© Syl Arena

The new LCD has the ability to change the colour of the backlight. Here, Personal Function 04 is being set so that the backlight colour changes to orange when the Speedlite 600EX-RT is activated as a slave.

Personal Functions

The new Speedlite system introduces Personal Functions (P.Fns) that are specific to each model (whereas Custom Functions typically are available across several models of Speedlite). Via the P.Fn menu, the user can set options for such features as LCD backlight colour, the availability of radio and/or optical wireless, whether the flash will fire during Linked Shooting, etc. Personal Functions must be set directly via the LCD on each Speedlite or Transmitter. It is not possible to set them via the LCD on the camera when the unit is attached to the hotshoe.

On the Speedlite 600EX-RT the seven available Personal Functions are LCD panel display contrast, adjustable in five levels (P.Fn-01); LCD panel illumination colour for normal shooting, green or red (P.Fn-02); LCD panel colour illumination: Master, green or orange (P.Fn-03); LCD panel illumination colour: Slave, orange or green (P.Fn-04); colour filter auto detection, Auto or Off (P.Fn-05); wireless button toggle sequence (P.Fn-06); and flash firing during Linked Shooting, On or Off (P.Fn-07).



An EOS menu screen showing the Flash Quick View menu; in this case the setting is Wireless: radio transmission mode.

While some of the following functions can be achieved through other means on the camera or Speedlite, the full convenience of their benefits are realised only when the new Speedlite system is paired with Canon EOS cameras released from 2012 onwards. The 2012 cameras include the EOS-1D X, EOS 5D Mark III, EOS 650D, EOS 6D and the EOS M.

Flash Quick View menu on camera LCD

With the 2012 cameras Canon has improved this system through the inclusion of a Flash Quick View menu. Now, all the Speedlite menu items are accessed via a single screen, which also enables the user to scan all Speedlite settings quickly. Note that cameras with touch-screen control, such as the EOS 650D and the EOS M, provide full Speedlite control by tapping directly on the touch-screen, which is faster and more intuitive for many users.

© Syl Arena

The on-camera menu systems for the EOS 5D Mark III and EOS-1D X incorporate a new Flash Quick View menu system. Here, an EOS 5D Mark III screen shows (from left to right, from top to bottom): Group Mode, Radio Wireless, Zoom at 24mm, First-Curtain Sync, no Flash Exposure Compensation, no Flash Exposure Bracketing, Channel = Auto, Master Enabled, Wireless ID = 0000 and individual Group Mode settings.

AF-assist beam compatible with 61-point AF systems

Canon autofocus technology took a significant step forward with the introduction of a 61-point, high-density recticular AF system on the EOS-1D X and the EOS 5D Mark III. The 600EX-RT provides an AF-assist beam that covers all 61 AF-points on 28mm or longer lenses. The 61-point AF system and the coverage provided by the AF-assist beam now make accurate focusing in very dim light a reality.

* Note: if you wish to use the 600EX-RT for its AF-assist beam only, select ‘Flash firing – disable’ on the camera’s LCD.

Group (Gr) Mode

For advanced shooters, Canon’s new Group (Gr) Mode provides significant features in four areas:

1 – Five slave groups (A, B, C, D and E) are now available. Outside of Group mode, there are only three groups. The availability of two additional groups, when used with other Group Mode features, enables much wider creative opportunities.

2 – Groups may be turned on/off individually. This provides an easy way to make test shots with only the light of a specific slave group.

© Syl Arena

The EOS 5D Mark III and EOS-1D X DSLRs enable the use of the new Group Mode, which allows individual control of up to five groups, including the use mixed modes and turning individual groups off. Group Mode settings can be dialled in via the LCD on the 600EX-RT, ST-E3-RT, EOS 5D Mark III or the EOS-1D X.

3 – The shooting mode for each group may be set individually in E-TTL, Manual or External Auto. For instance, it is now possible to illuminate a moving subject with Speedlites firing in E-TTL while having the background lit with Speedlites firing in Manual or Automatic External.

4 – For E-TTL groups in Group Mode, flash exposure compensation (FEC) is applied individually to each E-TTL group. Many photographers will find this to be a much more intuitive control system than the A:B ratio scheme used by previous generations of Speedlites.

To access the Group Mode, you must have either a Speedlite 600EX-RT or a Speedlite Transmitter ST-E3-RT activated as a radio master and you must have it connected to the hotshoe of a 2012, or newer, camera model. Control can be provided via the Speedlite or Transmitter LCDs or via the LCD on the camera.

* Note: on pre-2012 cameras, such as the EOS 5D Mark II and the EOS 7D, Group Mode will appear as an option when the Speedlite 600EX-RT or Speedlite Transmitter ST-E3-RT units are activated as a radio master. However, when the shutter is fired the system reverts to E-TTL.

© Syl Arena

Wireless menu showing the A, B, C, D and E Group Mode options on the rear LCD screen of an EOS 5D Mark III DSLR.


An EOS 7D with a Speedlite connected via an SR-N3 Remote Cable that's plugged into the camera's N3 remote control terminal.

Remote Release from a slave 600EX-RT or ST-E3-RT

A 600EX-RT or ST-E3-RT may be used as a remote trigger to activate the shutter on a camera connected to a Speedlite serving as a radio master. For 2012 cameras, the communication passes directly through the hotshoe of the camera with the master Speedlite. For pre-2012 cameras, the SR-N3 Release Cable must be used to connect the master Speedlite or Transmitter to the camera’s N3 remote control terminal. It's not necessary for the unit serving as a trigger unit to be connected to a camera as the release is achieved by pressing the REL button in Menu 2 of the radio slave unit. This enables you to handhold the remote trigger after placing the camera in a remote position.

* Note: Remote Release will fire only one camera at a time. When there are two or more master Speedlites, remote release will fire the camera connected to the master with the green Link light. The other master Speedlites will have an orange Link light to indicate their sub-master status.

Linked Shooting

Linked Shooting is similar to Remote Release except that the shutter activation happens from the camera with the master Speedlite attached. Also, up to 15 additional cameras may be triggered via Linked Shooting, whereas Remote Release only activates one camera. Slaved cameras may have either a 600EX-RT or an ST-E3-RT in the hotshoe.

The EOS cameras released in 2012, and future new EOS camera models, will communicate directly through the hotshoe. For pre-2012 cameras, the ST-N3 Release Cable must used to connect the Speedlite/Transmitter to the camera’s N3 remote control terminal.

To activate Linked Shooting, the 600EX-RT must be in non-wireless mode. Then, the wireless system button is pressed and held down until ‘Linked Shot’ appears on the LCD. The unit will default into a linked slave. To activate it as the linked master, press the wireless system button again. To exit Linked Shooting, press the wireless system button one more time.

© Syl Arena

In Linked Shot mode one master can fire the shutters on multiple cameras. The buttons are (from left to right): Release, Channel, Wireless ID and Menu. Pressing the Menu button cycles through additional menus.

© Syl Arena

In Linked Slave mode the button functions are (from left to right): ‘not used’, Channel, Wireless ID and Menu. Pressing the Menu button will then cycle through the additional menus.

Colour filter (gel) adjusts camera white balance

The 600EX-RT is supplied with a snap-on colour filter (gel) holder and two amber-coloured filters. The function of the filters is to shift the neutral white of the electronic flash to a warmer colour cast of incandescent lights. The lighter of the two filters is intended to be used in mixed lighting when both daylight and tungsten sources are present. The darker of the two filters is intended to be used in tungsten-only environments. Sensors on the bottom of the flash head have the ability to determine which to the two Canon-supplied filters are in use. On 2012, or newer, camera models the white balance will be adjusted automatically when the camera’s white balance is set to Auto or Flash. For pre-2012 camera models, the white balance must be set to ‘Flash’.

* Note: When the filter holder is used with commercially available 75x75mm (3x3”) filters, P.Fn-05 (colour filter auto detection) should be set to ‘OFF’ on the 600EX-RT, and the camera’s white balance should be dialled in manually.

© Syl Arena

The Speedlite 600EX-RT includes a filter holder that snaps onto the head of the flash.

© Syl Arena

The colour filter gel sensor (centre) of the Speedlite 600EX-RT. When the filter holder is used with the two included gels, and the camera white balance is set to ‘Flash’ or ‘Auto’, the sensor on the bottom of the flash head will sense which gel is being used.

Sync speed and High-Speed Sync (HSS)

Canon has indicated that for pre-2012 EOS cameras, the flash sync speed is depreciated by one stop – meaning that on an EOS 7D the 600EX-RT syncs at a maximum speed of 1/125sec and on an EOS 5D Mark II the 600EX-RT syncs at a maximum speed of 1/100sec. However, you should note that this sync speed depreciation is only the case when shooting in radio wireless. Likewise, Canon has indicated that for pre-2012 cameras High-Speed Sync is not possible with radio transmission.

On the 2012, or newer, EOS camera models the native sync speeds of the cameras are fully supported by the 600EX-RT and the ST-E3-RT flash units. The native sync speeds of the cameras are 1/250sec on the EOS-1D X, 1/200sec on the EOS 5D Mark III, EOS M and EOS 650D and 1/180sec on the EOS 6D. Additionally the use of High-Speed Sync in radio wireless is fully supported.

Biografie: Syl Arena

Syl Arena

Photographer Syl Arena became fascinated with photography at the age of eight when an aunt gave him a Box Brownie camera. He later studied at the Brooks Institute of Photography, before earning a BFA in photography at the University of Arizona. Today Syl Arena shoots the people, lifestyles and products of central California for advertising, editorial and corporate clients. He runs a regular blog, PixSylated.com, and in 2009 founded the Paso Robles Workshops. An expert in flash, his best-selling book ‘Speedliter’s Handbook: Learning to Craft Light with Canon Speedlites’ is now on its third edition.