EOS Speedlite Custom and Personal Functions

August 12, 2016

Original article posted on July 9, 2013

Virtually every Canon EX-series speedlite, with the exception of the super-compact Speedlite 90EX, offers the options of Custom Functions within the flash itself. As they have done in EOS cameras since 1989, Custom Functions in a speedlite allow the photographer to change default settings and control certain aspects of the way a flash operates. In effect, they can allow the user to personalize his or her flash units, just as the Custom Functions in a camera body allow the photographer to personalize their cameras.

We’ll take a closer look at what the different speedlite Custom Functions do, show the different ways to access them, and hopefully provide a better idea of how they can be used in real-world situations to improve their flash photography.

The various Canon EX-series Speedlites

This discussion relates strictly to Canon’s more recent EX-series speedlite flashes — that is, “EX” is in the flash unit’s model name.  This includes the two macro flashes, the Macro Ring Lite MR-14EX II, and the Macro Twin Lite MT-24EX.  All EX-series speedlites are compatible with any EOS Digital SLR, as well as many film-based EOS SLRs.

How many Custom Functions are within a flash unit varies, from one model to the next.  Predictably, top-of-the-line speedlites like the latest Speedlite 600EX II-RT have more Custom Functions available than smaller, less-expensive units like the Speedlite 320EX.  Fortunately, one bit of good news is that the Custom Function number for a given speedlite Custom Function is pretty consistent from one model to the next.  In other words, speedlite Custom Function (abbreviated “C.Fn”) 10 controls the flash’s auto power off timer, when it’s used as a slave… regardless of whether it’s the Speedlite 600EX II-RT, the 430EX II, or the 270EX II.

Since less-expensive speedlites have fewer Custom Functions, this does mean that the list of Custom Functions is numbered non-continuously… instead of C.Fn 1, 2, 3, 4, 5, and so on, the Speedlite 270EX II’s are listed C.Fn 1, 6, 10, and 11.  At first glance, this may seem illogical, but again, if you remember that this means that C.Fn 6 on the 270EX II means the same thing as C.Fn 6 on the more-advanced 600EX II-RT, it begins to make sense.

Setting Speedlite Custom Functions using the camera’s menu

The actual procedure to set Custom Functions on a speedlite is essentially similar, regardless of the flash model.  There are actually two different ways this can be done.  The first, and arguably the simplest, is to do it through the camera, using the External Flash Control menu screen.

Recent Canon EOS camera bodies have a neat feature that allows nearly complete flash control from the camera’s menu, instead of using the small LCD panel on the back of a speedlite.  Menu commands are spelled-out in plain English, making it a lot easier to understand than simple numbers, or icons on a flash’s LCD panel.  In addition, Canon’s less-expensive speedlites don’t have an LCD panel for control, so flash Custom Functions must be applied using the External Flash Control menu in the camera, with the speedlite attached and turned on.

Cameras that allow setting speedlite Custom Functions via the camera’s menu, as of early 2016:

  • EOS Rebel XS, XSi;  T1i thru T6i;  T6s;  SL1
  • EOS 40D, 50D, 60D, 70D, 80D
  • EOS 7D, 7D Mark II
  • EOS 6D;  EOS 5D Mark II, 5D Mark III, 5DS, 5DS R
  • EOS-1D Mark III, Mark IV;  EOS-1D X, EOS-1D X Mark II;  EOS-1Ds Mark III

Please note that this also requires a recent EX-series speedlite that’s compatible with this sophisticated flash-to-camera communication… earlier EOS speedlites cannot do this, even if mounted to one of the cameras listed above.  Compatible speedlite and camera body are required.  For speedlites, this means:

  • Speedlite 600EX II-RT, 600EX-RT;  Speedlite Transmitter ST-E3-RT;  Speedlite 580EX II
  • Speedlite 430EX II;  430EX III-RT
  • Speedlite 320EX*
  • Speedlite 270EX II*
  • Speedlite 90EX*

* To set Speedlite Custom Functions on these flashes, it’s required to use a compatible camera body and camera’s External Flash Control menu; they cannot be set on the Speedlite itself

Also, please note:  for speedlites with Personal Functions, in addition to Custom Functions, these must be set on the speedlite’s LCD panel.  Even if you have a camera with External Speedlite Control via the flash menu, Personal Functions won’t be controllable there.  

Option 2 — Set Flash Custom Functions on the Speedlite

Using the LCD panel on advanced speedlites like the 430EX series, 580EX series, the 600EX series, and Canon’s Macro flashes, it’s also possible to set Custom Functions.  The process varies slightly from unit to unit, but basically, you do the following:

  • Press and hold down a rear button labeled “C.Fn” for at least two seconds, until the display on the speedlite’s LCD panel changes.  With most speedlites, you’ll see something like, “Fn  01  0” or similar information about the currently-set Custom Function.
  • “Fn” and the two digits that follow indicate the specific speedlite Custom Function number.  “02” would mean speedlite Custom Function 2.  To change this, simply turn the function dial on flashes like the 580EX series, or use the +/- buttons on 430EX flashes.
  • The digit appearing after the Custom Function number tells you the current setting for that particular function.  Like camera Custom Functions, those in a speedlite give you a minimum of two options (such as on/off), and in some cases more.  Option “0” (zero) is always the factory-default setting.
    Press the SET button until the single digit blinks on and off, and then turn the control dial (500-series speedlites) or press the +/- buttons (400-series speedlites) to change the active Custom Function to a different option.

Note that with Speedlite 600EX series, 430EX III-RT, and Transmitter ST-E3-RT, the dot-matrix LCD panel makes flash Custom Function display and adjustment far easier.  If you opt to change this on the speedlite itself, simply press and hold-in the “C.Fn” button for at least two seconds, turn the control dial to highlight the particular speedlite Custom Function you want, press SET;  Turn the dial again to highlight the exact option you want, and press SET again.  (The process is slightly different on the 430EX III-RT.)

Speedlite Custom Functions explained

Here’s a brief and plain-English explanation of different speedlite Custom Functions. Keep in mind: less-expensive speedlite models don’t have each and every one of these available. But if your speedlite does have a particular one, here’s what it does for you:

C.Fn 00:  Distance Indicator Display
(Speedlite 430EX II, 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
If your flash has an LCD panel, it can display the maximum possible shooting distance (in E-TTL) or the distance that will provide accurate flash exposure at the current ISO, lens aperture, and flash zoom setting (in Manual flash mode).  This C.Fn lets you set this in meters — option “0” — or feet (option “1”).
C.Fn 01:  Auto Power Off
(Speedlite 90EX, 270EX II, 320EX, 430EX II, 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
By default, Canon speedlites will turn themselves off (enter sleep mode) after a pre-determined period of inactivity.  The red pilot light goes out at this time.  Flash can be readied for shooting by pressing the camera’s shutter button half-way, or by pressing in the pilot lamp/test button.  If you don’t want to risk delays by going through this wake-up procedure, select option “1”, which disables Auto Power Off, and prevents the speedlite from entering sleep mode.  You’ll need to turn the flash off yourself to conserve battery power.
C.Fn 02:  Modeling flash
(Speedlite 430EX II, 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
A cool EOS feature:  with a single speedlite on-camera, and especially if you’re using one or more flashes off-camera with wireless E-TTL, press the camera’s depth-of-field preview button, and get a one-second, continuous burst of flash to allow you to preview the lighting effect.  C.Fn 02 lets you move this to the speedlite’s test-firing button (option “1”),  fire the preview burst from either the camera’s depth-of-field preview button or the flash’s test button (option “2”), or disable it completely, if you find it annoying (option “3”).
C.Fn 03:  Auto canceling of Flash Exposure Bracketing
(Speedlite 580EX II, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
High-end EOS speedlites like the 580EX II and 600EX series have a built-in Flash Exposure Bracketing feature, which is activated on the speedlite.  (With the 580EX II, for example, press the rear SET button on the flash repeatedly until the icon with stacked rectangles appears;  you then dial-in the difference in exposure you want.)  Three flash images are then taken, and normally, the speedlite then returns to normal operation — FEB is canceled automatically after the first set of three images is taken.  Speedlite C.Fn 03–1 prevents this canceling of flash bracketing, and allows users to continually shoot sets of 3 bracketed flash images, without having to re-set FEB for each new series.  This might be useful, for instance, during product shooting, or when taking group photos at an event.
C.Fn 04:  Flash Exposure Bracketing sequence
(Speedlite 580EX II, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
Like standard ambient Auto Exposure Bracketing, the Flash Exposure Bracketing on select EOS Speedlites by default takes the “normal” exposure first and then follows under- and over-exposed shots. For situations when precise timing of the “normal” exposure isn’t critical (such as shooting inanimate objects or posed subjects under your direct control), you may prefer to shoot flash bracketed shots “in order” (starting with under-exposed shot) — on a browser screen, thumbnail images appear in a visually pleasing sequence. Set this C.Fn to option “1” to do this.


C.Fn 05:  Automatic flash exposure mode
(Speedlite 580EX II, 600EX-RT, 600EX II-RT)
All current and most recent EOS digital SLRs normally use Canon’s E-TTL II method of automatic flash exposure. Manual flash can be set using the flash’s Flash Mode settings, either on the Speedlite or using the camera’s External Speedlite Control menu. This Custom Function in select EOS Speedlites let users deliberately switch to either the older and off-the-film “TTL” type flash mode (for use ONLY with older EOS film cameras like Canon’s EOS A2/A2E, original EOS-1 or EOS-1N models, and so on). There are also options to bypass E-TTL and revert to non-TTL auto operation, which are similar to older “thyristor” units from years past, with the External Metering Auto or Manual settings. In spite of the wording, the latter is indeed auto flash exposure, using an external sensor on the front of the Speedlite – it just means that ISO, apertures and speeds are set completely independently, just like with older and non-dedicated flashes.
C.Fn 06:  Quick Flash when camera is set for continuous advance
(Speedlite 270EX II, 320EX, 580EX II, 600EX-RT)
Quick Flash is a Canon Speedlite feature that allows firing a flash during the recycle period — before a flash has fully recycled from a previous shot. You may have noticed under some circumstances that a Speedlite’s “ready” light first glows green, then switches to red. The red pilot light tells the user the flash has fully recycled, while green indicates there’s enough power to shoot nearby subjects but you’re not yet at full flash power for more distant subjects. Normally, Quick Flash is disabled whenever the camera is set to continuous advance (you won’t see the green “ready” light for partial available power). But for certain types of rapid shooting of nearby subjects, you may want to activate it to allow faster shooting. Two cautions: because full power isn’t available, it’s best for nearby subjects (possibly at higher ISOs and/or wider lens apertures). You can’t go crazy here -- it’s still possible to build up too much heat if you shoot too quickly and repeatedly and don’t give the flash a chance to cool down a bit after a burst or two of rapid shots.
C.Fn 07:  Flash power when test button pressed
(Speedlite 430EX II, 580EX II, 600EX-RT;  Transmitter ST-E3-RT)
To conserve battery power whenever EOS Speedlites are set for E-TTL automatic operation (which is regulated by the camera body), pressing the flash’s test button emits a low power test flash that’s controlled to about 1/32 power. This uses little battery power and the flash recycles from this almost instantly. However, there may be instances when a full power flash burst with the test button can actually be desirable. One example is to allow the shooter to check approximate flash recycle time, thus giving him or her an idea of remaining battery power in the Speedlite (fresh AA alkaline batteries normally take roughly 4-6 seconds to recycle at full manual power, and a noticeably longer wait for the red pilot lamp tells the shooter the batteries are losing strength). Set this C.Fn to option “1” if you want or need a full power manual flash burst when pressing the Speedlite’s test button.
C.Fn 08:  AF-Assist beam on Speedlite illuminating
(Speedlite 430EX II, 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT)
If an EOS camera with a Speedlite attached is set to One-Shot AF mode and its AF system detects low levels of light and/or subject contrast and detail, the camera body will send a command to the Speedlite to illuminate a built-in AF-assist beam. A patterned beam from mid-range and high-end Speedlites gives the AF system some detail to focus upon, as long as subjects aren’t too far away. Normally, this deep red and near-infrared beam comes on when the camera thinks it’s needed. But in situations where it could be annoying, like possibly appearing in other shooters’ pictures, or you have other reasons for not wanting it to illuminate before shots are taken, it can be disabled by setting this option to “1” (activating AI Servo AF is another way to disable the AF-assist beam, too).
C.Fn 09:  Auto flash zoom for different size imaging sensors
(Speedlite 430EX II, 430EX III-RT, 580EX II, 600EX-RT)
Auto zooming of the flash head is built-in to mid-range and upper-end EOS Speedlites. The zoom settings are identified by lens focal lengths and, by design, assume the coverage needed to illuminate the picture area with a full-frame camera at those specific lengths — such as with a 24mm lens, 35mm lens, 50mm lens and so on attached. However, with EOS digital SLRs having smaller image sensors, this would waste light outside the picture area (a 24mm lens on an APS-C sensor camera, for example, gives a field of view equivalent to what a 38mm lens would provide on a full-frame camera). By default, recent EOS Speedlites are able to read the sensor type on newer EOS SLRs and automatically adjust the zoom head to compensate for this. For users who don’t want this variation (such as to ensure that manual flash exposure levels remain constant at specific flash head zoom settings, regardless of the camera being used), set this C.Fn to “1” to disable the automatic correction of flash coverage.
C.Fn 10:  Auto Power Off if Speedlite is set as a wireless “slave unit”
(Speedlite 270EX II, 320EX, 430EX II, 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT)
Normally, if an EOS Speedlite is set as a slave unit, it maintains its ON status and keeps sending battery power to its capacitors (meaning it’s instantly ready for the next picture) for up to 60 minutes after the last shot was taken or the last button on that slave unit pressed. This is good for being ready for the next shot if you anticipate delays during shooting. However, it does drain battery power faster, especially if the slave unit(s) is using only the in-board AAs. For shooters concerned about saving battery power, setting this C.Fn to option “1” on each compatible slave unit will allow it to enter sleep mode after 10 minutes of inactivity. A press of the master unit’s test button will wake the slave flashes up (often requiring a few seconds to build power back up), after which they’ll be ready to shoot again.
C.Fn 11:  Waking up slave units by pressing master unit test button
(Speedlite 270EX II, 320EX, 430EX II, 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT)
Identified by the somewhat confusing term “slave auto power off cancel,” this flash Custom Function lets you define how long a slave unit can stay in sleep mode and still be able to re-awaken if the master flash’s test button is pressed. By default, sleep mode on a slave unit can extend for a long eight-hour period (after which it basically shuts itself completely off). Because a small amount of battery power is required periodically during sleep, setting C.Fn 11 to its “1” setting means you can cut the time before a slave unit switches completely off to one hour instead of eight.


C.Fn 12:  Speedlite use of internal AA batteries if an external battery pack is attached
(Speedlite 580EX II, 600EX-RT, 600EX II-RT)
This is exclusive to those EOS Speedlites that can accept an optional external battery pack, such as the Canon Compact Battery Pack CP-E4N or CP-E4.  Four working AA batteries are always required in an EOS Speedlite, even if a battery pack is attached.  Normally, battery power from the four internal batteries is used along with the external pack to power flash recycle, as well as provide juice for flash functions such as powering the auto zoom head, the flash’s LCD panel, etc.  But using power from the internal batteries for recycle does drain them more quickly.  This flash Custom Function allows users of external battery packs to prevent the internal AAs from being used for flash recycling purposes, thus extending their life and minimizing need to change the internal batteries when the external battery pack may still have power left.  Set Speedlite C.Fn 12 to “1” to do this.
C.Fn 13:  Flash Exposure Compensation via Speedlite dial only
(Speedlite 430EX III-RT, 580EX II, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
When Flash Exposure Compensation is set on a high-end EOS Speedlite, it normally requires pressing the SEL/SET button (580EX or 580EX II) or the ± button (600EX series), then turning the flash’s control dial.  Setting this C.Fn to option “1” lets them accomplish this by turning the Speedlite’s rear dial alone, with no need to press the button first.  And, with the 600EX II-RT, this also allows changing Manual flash mode power by dial alone (full 1/1 power, ½ power, etc.)
C.Fn 14:  Speedlite 430EX II displays maximum shooting distance or active lens aperture
(Speedlite 430EX II)
Exclusive to the Speedlite 430EX II, this C.Fn allows the user to change the flash’s LCD display from a digital indication of maximum available shooting distance (feet or meters, as set in C.Fn 00) to showing the current lens aperture when it’s set for E-TTL operation. In manual flash exposure mode, the 430EX II display always shows the flash-to-subject distance required for “proper” flash exposure at the current power level, lens aperture and ISO settings.
C.Fn 20:  Audible beep tone when Speedlite(s) have fully recycled
(Speedlite 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
Exclusive to the top-of-the-line 600EX series and their companion Speedlite Transmitter ST-E3-RT, this Custom Function allows users to activate a “beep” tone when a flash has fully recycled. This C.Fn is particularly useful during radio-based wireless flash shooting because the system is smart enough to monitor the recycle status of every flash in a group and report it to the master unit on-camera (either 600EX-RT or ST-E3-RT). A lightning bolt icon appears on the master unit when all in a group are fully recycled and the master unit can be set to give a single beep tone when all off-camera 600EX-RTs in all groups have recycled — an easy way to avoid misfires from units not yet being recycled when the shutter button is pressed.  By default, this beep tone is off.
C.Fn 21:  Control spread of illumination during auto flash head zooming
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT)
The 600EX series and 430EX III-RT bring a new feature to fast shooters who rely on the auto zoom head to change flash coverage as a new lens is mounted (or zoom as a camera lens is zoomed to maintain flash coverage). The spread of coverage can be changed using Speedlite C.Fn 21. Normally, a slight amount of light fall-off occurs at the outer edges of an image, which allows a bit more light concentration at the center where subjects most likely are located. But by switching to option “1” (Guide Number Priority), you can further concentrate illumination in the center and give a little more lighting power for distant flash on-camera shots (telephoto shots of sports or wildlife, for example). And by switching to option “2” (Even Coverage), lighting spreads out more during auto flash head zooming and gives more even lighting from the center to corner for subjects like group photographs and others where the outer areas are as important as the center of the frame. This C.Fn has no effect during wireless operation with off-camera slave units.
C.Fn 22:  Button and LCD panel illumination
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
This is for Speedlite 430EX III-RT and 600EX series, and Speedlite Transmitter ST-E3-RT. Presuming operation in low light situations, when the control dial is turned or any button on either of these units is pressed, the LCD panel and all buttons will illuminate for 12 seconds to make settings easier in adverse lighting. But C.Fn 22 allows users to modify this in two ways. Option “1” turns this illumination off completely — ideal for conserving battery power when working outdoors or in bright indoor conditions. Option “2” allows the illumination to remain active whenever the flash is turned on — possibly a good choice for making repeatedly quick changes to flash settings in dimly-lit areas, especially if an external battery pack is attached.
C.Fn 23:  Slave unit ready, indicated by flashing AF-assist illuminator
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT)
Since the first EX-series Speedlites of 1998 with wireless flash capability, Canon Speedlites used as off-camera slave units have given photographers a visible indication that they’ve recycled by flashing the red AF-assist beam on the front of the unit once per second. Speedlite 430EX III-RT and 600EX-RT also do this by default, but for the first time, a Speedlite Custom Function now allows users to disable this (the only “ready” light on the slave unit would be the rear pilot lamp). Option “1” prevents the AF Assist illuminator from flashing to indicate “ready,” if it’s needed to prevent the illumination from appearing in long-exposure images or to conserve a bit of battery power.
Personal Functions: Speedlite 430EX III-RT, 600EX series and ST-E3-RT

Canon’s newest professional items offer additional user-controls called “Personal Functions.” These are above and beyond the Custom Functions already discussed and are set on a separate menu screen on the dot-matrix LCD panel of the Speedlite 430EX III-RT, 600EX series, and ST-E3-RT.

Personal Functions can be thought of more as “set-up” type of functions to configure your Speedlite or transmitter to your preferences. They can be changed at any time by going into the P.Fn menu, but they’re less likely to be changed in the middle of a shoot than some of the previously mentioned flash Custom Functions. As we discussed earlier, Personal Functions cannot be accessed on the External Speedlite Control menu of EOS cameras — you have to use the flash’s menu to view and set them.

An important thing to note:  Unlike the Speedlite Custom Functions, where regardless of the flash model in use, the number of a given function is the same, this isn’t the case with the Speedlite Personal Functions.  P.Fns differ among Speedlites and transmitters which include them.

Here’s a brief run-down of the available Personal Functions:

P.Fn 01:  Contrast of unit’s LCD panel
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
Increase or decrease the contrast on the dot-matrix LCD panel of any of the units listed above. By default, it’s factory-set in the middle of a five-step range.
P.Fn 02:  Illumination color for normal, non-wireless shooting
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT)

Canon has cleverly set the speedlites with radio wireless capability to change illumination color of the LCD panel from green (standard and non-wireless shooting, as well as master unit shooting) to an orange or yellowish shade (when either speedlite is set as a slave unit). This makes them easy to identify quickly, even in total darkness.

With P.Fn 02, if you prefer the LCD illumination color of a non-wireless flash to be orange instead of green, set this Personal Function to Option “1.”

P.Fn 03:  Illumination color for Master unit operation
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
Green is the default light for the flash or transmitter’s LCD panel. Setting this to Option “1” lets you change it to orange whenever it’s set for master operation.
P.Fn 04:  Illumination color for Slave unit operation
(Speedlite 430EX III-RT, 600EX-RT, 600EX II-RT;  Transmitter ST-E3-RT)
The default LCD panel illumination is orange, distinguishing slave units from any master units in low-light conditions. This P.Fn allows changing color of a Transmitter or Speedlite to green when it’s set to slave operation. Sharp-eyed users may wonder why the Speedlite Transmitter ST-E3-RT would have any slave settings — this is for use in its special Linked Shot/Slave mode only.
P.Fn 05:  Auto detection of Canon color filters
(Speedlite 600EX-RT)

Speedlite 600EX-RT is equipped with a clear plastic filter holder and two specific amber gel filters, which can change the color of flash illumination. By default (P.Fn 05 at its “0” position), whenever this filter holder is attached, the color of either mounted filter is detected. And if the camera is set to either Auto White Balance or the Flash White Balance setting, the white balance is now adjusted to accommodate this filter (and, ideally, balance it to the ambient light in the scene if the proper filter is being used on the Speedlite).

For users who prefer to adjust White Balance manually — in particular, if you’ve cut your own gel filters of different colors and installed them in the Canon-supplied filter holder — set this P.Fn to “1” to disable filter color detection. Canon cannot guarantee proper White Balance correction for filters other than those provided with the Canon gel filter holder, even if user-supplied gel filters are an amber color.

P.Fn 05:  Quick Flash Operation
(Speedlite 600EX II-RT)

One difference between the 600EX-RT and newer 600EX II-RT is how Quick Flash is handled.  On the version II, Quick Flash is always active by default, whether the camera is in Single or Continuous drive setting.  Via P.Fn 05, the user has the option to turn the Quick Flash feature off, and ensure him- or herself that full power is always available when they see the ready-light appear in the viewfinder, and on the speedlite.

P.Fn 05:  AF Assist Beam method
(Speedlite 430EX III-RT)

Because of its compact size, Speedlite 430EX III-RT’s AF Assist beam unit can only provide near-infrared, patterned AF Assist when the camera is using the center AF point.  Therefore, the 430EX III-RT’s default for AF Assist is to use a series of rapid pulses of white flash illumination for AF Assist.  (Note that if a Canon color filter is attached to the 430EX III-RT, it will not use the intermittent flash method for AF Assist.)  If you’ve manually selected the center AF point and prefer the more subtle near-infrared type of AF-Assist, setting Speedlite P.Fn 05 to option “1” will enable this.

P.Fn 06:  Limit which wireless flash mode is available when wireless button pressed
(Speedlite 600EX-RT)
Speedlite 600EX-RT not only offers its breakthrough built-in radio wireless flash capability, it also is backward compatible with Canon’s previous “optical” type wireless flash. When the 600EX-RT’s wireless button is pressed, the flash cycles through normal (non-wireless, flash on-camera) operation, radio operation (master or slave) and optical operation (master or slave). With P.Fn 06, users can limit the 600EX-RT to radio-only wireless operation (via Option “1”) or optical wireless (via Option “2”). In either case, it’s possible to toggle back to normal non-wireless operation by pressing the same wireless button.
P.Fn 06:  Flash firing during Linked Shooting
(Speedlite 600EX II-RT)
This speedlite Personal Function has been moved from #7 to #6 on the latest Speedlite 600EX II-RT.  See the description under “P.Fn 07,” immediately below, for information on its operation.
P.Fn 06:  Quick Flash feature active or inactive
(Speedlite 430EX III-RT)
Quick Flash is a feature in Canon speedlites that allows the flash to be fired before it’s fully recycled.  This would allow a photographer to grab quick moments happening near the camera, even if his/her flash hasn’t fully recycled.  You’ll know its active if you take a picture, and then see the flash’s ready light turn green, and then subsequently turn red.  The green ready light means Quick Flash is possible, and that the flash hasn’t recycled fully until the ready light is red.  Personal Function 06 allows users to ensure that full power is available for every flash picture, by disabling the Quick Flash function (if it’s set to the “1” setting).
P.Fn 07:  Flash fires or doesn’t fire when used in Linked Shooting mode
(Speedlite 430EX III-RT, 600EX-RT;  also with 600EX II-RT via P.Fn 06)
Canon’s unique Linked Shooting mode allows up to 15 EOS cameras with a 430EX III-RT or 600EX series speedlite attached to the camera’s hot shoe to fire in concert when a “master” camera with Speedlite or Transmitter is fired (it can also work by just pressing the “release” button on a Linked Shot / Master Speedlite or Transmitter).  By default, the attached speedlite on each “slave” camera won’t fire during actual exposure. With P.Fn 07, each radio-controlled wireless flash in the system can be independently set to fire, if set to “1”.  (Keep in mind this is for a flash mounted to each “slave” camera.  Linked Shooting mode is not compatible with a wireless, multiple-flash set-up with flashes positioned off-camera.)
P.Fn 07:  Direct setting of Flash Exposure Comp or Manual Flash power by turning rear dial
(Speedlite 430EX III-RT)
Normally, adjustments to flash exposure require users to first press an appropriate button, highlight the exposure adjustment on the flash’s LCD panel, and then turn the rear control dial to input any change.  Setting P.Fn 08 to option “1” on Speedlite lets users directly turn the dial to adjust E-TTL flash exposure or Manual flash power.  

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