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Am unsure how to force an external flash to fire in creative modes (so I can use high ISO in lowlight situation where tripod is impossible). Can pop up built in flash but nothing on manual on how to tell the camera to fire an external one (using 550EX).
Just off the top of my head, I'd try using the manual mode on the flash. Also, if you can fire the builtin flash in your creative modes, maybe the 550 can be slaved to fire separately. Do be careful of highlight burnout. Experiment and make notes on what works best.
> [I use a 20d. but assume that your 300d has similar setting adjustments > available. Try switching from the creative to P mode with your 550EX flash > attached, you should find flash fires regardless of ISO setting.]
Hi! I don't typically post, but I use three 550EX flashes as my main lighting system for group portraits at the weddings I photograph.
I have become very familiar with these flashes. If I don't answer your question properly it's only because I just didn't understand what you might need....so just email me.
I'm not sure if you are attempting to fire this flash while it is ON your camera or OFF of your camera, but I will assume that you are trying to fire it while it is OFF (using the MASTER/SLAVE mode). That is what I will try to explain here, just in case =)
It does not matter which CAMERA mode you are using (creative or Program/Manual, etc) or which ISO setting. What matters is if you have a way to set off the flash externally, so you need a SECOND 550ex flash (or, I set of ALL of my flashes using a wireless Canon transmitter, which is very nice for firing a single light).
Your pop-up flash will not set off the 550 EX. It uses an infrared sensor. If you DID purchase a second flash, you would attach one to the camera (which you would need to set to 'MANUAL' mode) and the other (which you would put on a light stand) would be set to 'SLAVE'.
I am glad they are made with the infrared sensors, because otherwise I would have people with even disposible cameras at the weddings I photograph setting off my lights left and right as I take pictures at the cake table.
If you look on the front of the flash, you will see the infrared sensor. This is the sensor that the flash uses to communicate with other 550 flashes ....so if you DO buy another flash, when you set your flash up on a light stand, it is IMPORTANT that these flash sensors can "see" each other.
If you have questions or if I have totally mis-understood what you're asking about this flash, I can certainly try again to help out. =)
I don't dare to say too much, because for me a "flash photography" is still byword for "utmost confusion", but if you have a spare weekend or two, I would reccomend to browse through a series of articles named *Flash Photography with Canon EOS Cameras* at