OVERVIEW
As with other Canon professional Speedlites, the 600EXRT has a host of custom functions that allow you to adjust specific flash functions to your preference. It also includes seven personal functions for altering options specific to the 600EX-RT. This QuickGuide will list and explain every available option. By convention, Canon numbers each option, with 0 (zero) as the factory default setting. When you change the number to anything other than 0 you change the factory default and customize the setting.

CUSTOM FUNCTIONS
To set a Custom Function (C.Fn):
1. Press and hold in Fn Button 1 <Zm/C.Fn> until the Custom Functions screen appears.


2. Turn the Select Dial to the Custom Function number of your choice.
3. Press the Set/Set Button to access the C.Fn.
4. Turn the Select Dial to the C.Fn setting you want.
5. Press the Set/Set Button to register your setting.
6. Press Fn Button 4 to return to the flash-ready state.

C.Fn-00 – Distance indicator
0: Meters
1: Feet
The default is meters. U.S. users may prefer to set the distance indicator to feet. When set to feet, it will remain so even after you clear all custom functions. You must therefore reset it separately.

C.Fn-01 – Auto power off
0: ON (Enabled)
1: OFF (Disabled)
When set to 0 the flash will automatically “go to sleep” after 90 seconds of inactivity. It does this to conserve battery power. You can “wake” the flash by tapping your camera’s shutter button or the test flash button on the flash. If you set this function to 1 the flash will stay on until you switch it off or the battery runs out of power. Note: Use C.Fn 10 to set the power-off timer for receiver units.

C.Fn-02 – Modeling flash activation
0: Depth-of-field preview button enabled (on camera)
1: Test firing button enabled (on flash)
2: Depth-of-field + Test firing button enabled
3: OFF (Disabled)
Photographers who find the modeling flash feature useful can choose one of three options for how to trigger it. If you find this unnecessary or distracting you can turn it off.

C.Fn-03 – FEB auto cancel
0: ON (Enabled)
1: OFF (Disabled)
If you turn this function off then the flash will not  automatically cancel flash exposure bracketing (FEB) after  3-exposure bracket. Instead, it will continue FEB until you disable it manually. This is useful when you want to
continuously bracket each shot.

C.Fn-04 – FEB exposure sequence
0: 0 — +
1: — 0 +
If you change this function to 1 you change the FEB sequence to “under-exposure normal exposure over-exposure.”

C.Fn-05 – Flash metering mode
0: ETTL II
1: TTL
2: Ext.A (External metering – Auto)
3: Ext.M (External metering – Manual)
Set this function to 1 only if you are using an EOS Type-B film camera. Do not set this function to 1 if you are using an EOS digital camera. External metering will work with either film or digital cameras.

C.Fn-06 – Quickflash with continuous shooting
0: OFF (Disabled)
1: ON (Enabled)
This feature allows you to fire the 600EX-RT when the ready light is green, before the flash is fully charged. It is normally disabled during continuous shooting. If you choose to enable it, the power output will be lower, which in turn will reduce the effective flash  ange. You may need to reduce the flash-to-subject distance, increase the ISO, or both, to compensate.

C.Fn-07 – Test firing with autoflash
0: 1/32 power
1: 1/1 full power
This function controls test flash output when you are in E-TTL or TTL mode. When set function to 1, the test flash will be much brighter but will require more battery power and a longer recycle time. A full-power test flash can therefore be a useful indication of remaining battery power, because the longer the flash takes to recycle after the test flash, the less battery power you have left. 

C.Fn-08 – AF-assist beam
0: ON (Enabled)
1: OFF (Disabled)
Turning the flash’s autofocus assist beam off can be useful if it would otherwise distract or alarm your subjects.

C.Fn-09 – Auto zoom for sensor size
0: ON (Enabled)
1: OFF (Disabled)
When flash coverage is set to Automatic <A>, the 600EX-RT will automatically adjust its angle to coverage to match the field-of-view of your lens and image sensor. If you set this C.Fn to 1, the flash will automatically adjust its angle of coverage to match full-frame/35mm format only.

C.Fn-10 – Receiver auto power-off timer
0: 60 minutes
1: 10 minutes
You may want to reduce the time before a receiver flash unit automatically goes “to sleep” to conserve battery power during multi-hour shoots. If you want the receiver auto power-off timings to be identical for each receiver unit, you must set the same power-off time on each receiver flash.

C.Fn-11 – Receiver auto power-off cancel
0: Within 8 hours
1: Within 1 hour
This function controls the period of time within which you can wake a receiver up from sleep by pressing the test flash button on the sender unit. Once the receiver automatically powers off, you will have to use its power switch to turn it  back on.

C.Fn-12 – Flash recycle with external power
0: External & internal power
1: External power
This function controls whether the flash unit will use the combined power of internal and external batteries or will use only external power. Option 1 conserves the internal battery power essential for controlling the flash.

C.Fn-13 – Flash exposure compensation setting
0: Speedlite FEC button + Speedlite dial
1: Speedlite dial only
Setting this function to 1 allows you to use the Speedlite dial alone to adjust flash exposure. This method is faster than the default but may be more vulnerable to accidental movement.

C.Fn-20 – Audible alert
0: OFF (Disabled)
1: ON (Enabled)
Setting this function to 1 will cause the flash to emit a “beep” when it is fully charged—or, if set on a sender unit ash will also sound a warning if its internal circuitry is beginning to overheat. Although an audible alert is a convenient supplement to visual verification, you may wish to disable if it would be a distraction.

C.Fn-21 – Light distribution during auto zoom
0: Standard
1: Guide number priority
2: Even coverage priority
Setting this function to 1 will increase maximum range and output at the center by slighty reducing the angle of coverage. 
This will cause the edges of the image area to be slightly darker but may be unnoticeable if they are already dark. Setting this function to 2 does the opposite: It will minimize light falloff at the edges by increasing the angle of coverage. This in turn will reduce the maximum range.

C.Fn-22 – LCD panel illumination
0: On for 12 seconds
1: OFF (No illumination)
2: ON (Illumination always on)
You may wish to turn off the LCD panel illumination feature if you shoot only in bright conditions or if it might draw unwanted attention. Conversely, if you shoot primarily in low light, you might want to have LCD panel set to illuminate whenever the flash is on.

C.Fn-23 – Receiver flash battery check
0: AF-assist beam + flash ready lamp
1: Flash ready lamp only By default, the AF-assist beam lamp of each receiver unit will blink when the unit is fully charged. Set this function to 1 if you do not want the AF-assist beam to blink and prefer to use the flash ready lamp alone. You must set this function
on each receiver unit.

PERSONAL FUNCTIONS
To set a flash Personal Function (P.Fn):
1. Press and hold Fn Button 1 <Zm/C.Fn> until the Custom Functions screen appears.


2. Press Function Button 1 <P.Fn> to display the Personal Functions screen.
3. Turn the Select Dial to the Personal Function number of your choice.
4. Press the Select/Set Button to access the P.Fn.
5. Turn the Select Dial to the P.Fn setting you want.
6. Press the Select/Set Button to register your setting.
7. Press Fn Button 4 to return to the flash-ready state.

P.Fn-01 – LCD panel contrast
Press the 600EX-RT’s Select/Set Button, and turn the Select Dial to choose any one of five settings.
P.Fn-02 – LCD panel illumination color – Normal on-camera flash
0: Green
1: Orange

P.Fn-03 – LCD panel illumination color — Master
0: Green
1: Orange

P.Fn-04 – LCD panel illumination color — Receiver
0: Orange
1: Green
Note: Personal functions 2-4 above make it easier to distinguish flash units that have been set up as receivers from those that are not. By default, the LCD panel on units set up as receivers will have orange illumination rather than green.

P.Fn-05 – Color filter auto detection
0: Auto
1: Off/disabled
By default, the 600EX-RT will automatically detect and compensate white balance for the Canon-supplied color filters (AWB or Flash WB only). Disable this function when using 3rd -party filters.

P.Fn-06 – Wireless button cycle sequence
0: Normal >Radio> Optical
This is the default setting. It cycles the operating modes in the following order:
Normal shooting Radio–Master > Radio–Receiver > Optical–Master > Optical–Receiver.

1: Normal Radio
This option eliminates the optical wireless mode and cycles the operating modes in the following order:
Normal shooting Radio–Master Radio–Receiver.

2: Normal Optical
This option eliminates the radio wireless mode andcycles the operating modes in the following order:
Normal shooting Optical–Master Optical–Receiver.

P.Fn-07– Flash firing during linked shooting
0: OFF (Disabled) The flash mounted on the camera will not fire during linked shooting.
1: ON (Enabled)
The flash mounted on the camera will fire during linked shooting.

Whatever setting you choose must be entered into every flash in your linked shooting setup.

TO CLEAR ALL CUSTOM/PERSONAL FUNCTIONS
You can clear all custom/personal functions and restore all default settings by pressing Fn Button 2 <CLEAR>, then Fn Button 1 <OK>. Clearing your custom functions would be useful if, for example, you wanted to loan or sell your
Speedlite 600EX-RT to someone else.