E-TTL flash

Canon’s DSLR cameras use a pre flash to determine the correct flash exposure, a system known as E-TTL. Way back in time, Canon EOS film cameras used a system called TTL flash where the flash was metered off the film surface during the actual exposure.

Several of the Speedlite EX series of flashes are possible to be configured for either kind of flash metering system. By default Speedlite EX series flashes are set to E-TTL, but a custom function allows them to be changed to the older TTL system.

How to spot E-TTL and TTL

E-TTL and TTL flash metering

Same flash, but C.Fn 05 determines E-TTL or TTL operation


If you’ve been messing with the flash custom functions (C.Fn) then it’s possible to set your flash to TTL, then on most DSLR cameras you get a lot of trouble. Fortunately the LCD display on the flash shows different indications, ETTL for ETTL mode and TTL for TTL mode – kind of obvious.

If you incorrectly set TTL mode; the flash will fire at full power all the time, or not fire at all. You also won’t be able to set wireless flash operation.

The problem is that when the error is relayed via the internet the E is often left out of the ETTL and so TTL and E-TTL become interchangeable in a discussion but they are not interchangeable in use.

So why does this function even exist?

It’s to do with compatibility; Canon DSLR cameras only have E-TTL metering, some* of the film cameras have E-TTL and TTL to allow them to be used with either more recent flashes or older ones. Older Canon flashes from before the Speedlite EX series only provide TTL flash, not E-TTL.

*  Film cameras with both E-TTL and TTL flash capability – EOS-1 V, EOS 3000, EOS 300, EOS 30, EOS 33, EOS 3000N, EOS 3, EOS IX 7, EOS IX, EOS 500N, EOS 50, EOS 50E

E-TTL / TTL confusion solved

If you have a Canon DSLR NEVER set C.Fn 05 to 1 on your Speedlite 600EX-RT, 580EX II or MR-14EX II
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If you have a Canon DSLR NEVER set C.Fn 03 to 1 on your Speedlite 580EX, 550EX, MR-14EX or MT24-EX

Canon Speedlite flashes that can be set to TTL mode

  • Speedlite 600EX-RT – via C.Fn 05, 0 = E-TTL, 1 = TTL
  • Speedlite 580EX II – via C.Fn 05, 0 = E-TTL, 1 = TTL
  • Speedlite 580EX – via C.Fn 03, 0 = E-TTL, 1 = TTL
  • Speedlite 550EX – via C.Fn 03, 0 = E-TTL, 1 = TTL
  • Macro Twin Lite MT-24EX – via C.Fn 03, 0 = E-TTL, 1 = TTL
  • Macro Ring Lite MR-14EX II – via C.Fn 05, 0 = E-TTL, 1 = TTL
  • Macro Ring Lite MR-14EX – via C.Fn 03, 0 = E-TTL, 1 = TTL

External Auto Metering & External Manual Metering

External auto metering

Most photographers would not venture in to C.Fn 05 / 03, but with the release of the Speedlite 580EX II Canon added External Auto and External Manual metering modes – kind of like the old thyristor metering in old flashes.

Canon opted for C.Fn 05 to have four possible values and so some people may be confused when switching back from the External metered modes as to which is the right mode TTL or E-TTL.

Find out more about external flash metering…

Many thanks to Devon based photographer Scot Baston for asking the question that gave me the content to write this post. Scot further added the complexity of Pocket Wizard TT5 and a Sekonic L-478DR meter to the problem. It should have been possible for Scot to set the power of a flash directly from the Sekonic meter, but with the Speedlite 580EX II having C.Fn 05 set to 1, it never could.