Optical Slave

watt100 wrote in post #8585273
seems to work on my model, when I set custom function 7 on my 450D "only external flash" emits the preflash does not fire on Tv or Av mode

You cannot turn off the preflash that is used to set exposure on the built in flash on any Canon DSLR.

You cannot tell just by looking. The preflash is very quick and subtle.

But an optical slave, the title of this thread, will have no problem distinguishing this pre-flash as a distinct flash and fire the connected flash before the exposure flash (not enough time to recycle the power capacitors).

The only way to turn off the exposure setting pre falsh is to use the FEL (Flash exposure lock) button. But this is only turns it off temporally and you must accomplish a pre exposure first. Hardly a very satisfying or robust way to turn off the exposure pre flash.

Enjoy! Lon

FlashZebra wrote in post #8585705
You cannot turn off the preflash that is used to set exposure on the built in flash on any Canon DSLR.

You cannot tell just by looking. The preflash is very quick and subtle.

But an optical slave, the title of this thread, will have no problem distinguishing this pre-flash as a distinct flash and fire the connected flash before the exposure flash (not enough time to recycle the power capacitors).

The only way to turn off the exposure setting pre falsh is to use the FEL (Flash exposure lock) button. But this is only turns it off temporally and you must accomplish a pre exposure first. Hardly a very satisfying or robust way to turn off the exposure pre flash.

Enjoy! Lon

all I can say is it works on my camera, I swear! I can distinctly see the popup preflash when half-pressing the shutter and the AF assist beam is enabled, (Tv mode), when I change the custom function "only external flash emits" there is no preflash in Tv mode with the popup flash

watt100 wrote in post #8587758
all I can say is it works on my camera, I swear! I can distinctly see the popup preflash when half-pressing the shutter and the AF assist beam is enabled, (Tv mode), when I change the custom function "only external flash emits" there is no preflash in Tv mode with the popup flash

That's not pre-flash. That's a short flash to assist auto-focus in low light scene. Enable it again and see if you will not see that early flash when you press half way down. The custom function allows you to disable onboard flash's AF assist beam. That will disable both the IR and that AF assist flash.

Pre-flash goes out immediately after the shutter is pressed all the way down. And you can not disable ETTL on Canon's onboard flash. Therefore, you can't disable pre-flash.

liupublic wrote in post #8587913
That's not pre-flash. That's a short flash to assist auto-focus in low light scene. Enable it again and see if you will not see that early flash when you press half way down. The custom function allows you to disable onboard flash's AF assist beam. That will disable both the IR and that AF assist flash.

Pre-flash goes out immediately after the shutter is pressed all the way down. And you can not disable ETTL on Canon's onboard flash. Therefore, you can't disable pre-flash.

:p I wasn't pressing the shutter all the way down!
that AF beam was prematurely setting off the external flash, If this is the case maybe the YN460 is ignoring the preflash. I've only had the flash for several days, obviously I need to do some more experimenting!