I am trying to take a picture of rain drops or water drops at their frozen point. I met with a lot of under-exposed pictures & as mentioned here want to try the stroboscopic technique. Is there any way for me to fire the built-in pop-up flash manually on a Canon30D ?
I don't see any information on a way to manually trigger the flash, however that shouldn't be necessary. Simply going to a dark area, manually focusing and taking a flash picture (at any exposure length) should generate an image where only the time that the flash was active results in exposure. The flash simply has to be a short enough period to stop the motion of whatever you are trying to capture.
If necessary, you may be able to shorten by reducing flash power if you have manual control over flash power, since many flashes reduce flash power by shortening the flash. This will, however also make the image darker obviously, which means adjusting exposure to get a brighter (and noisier) image.
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You don't need to trigger flash manually, but you can set the power manually. Or you can leave it on TTL and let camera decide how much of flash power it should output. If you are in relatively dark environment, the ambient light will not be recorded even if you use longer shutter speeds so I will recommend not to go faster then 1/250th (min synch speed on 30D). In that way flash will not need to operate in HSS mode and will not reduce power output - it should solve underexposed pictures problem.
And in that case its the flash that freezes the motion (water drop/splash), so shutter speed is not of an issue.
Just be prepared for a lot of shots as you will not be able to catch every drop at the perfect moment.
More info: TTL - Trough The Lens metering, meaning that camera will make a pre-flash (a short burst of flash on low power) to calculate how much of it will bounce back from the subject to camera trough the lens, then calculates how much more should it have to expose properly ans then takes a shot with calculated output of flash.
As AJ mentioned, if the flash burst is too long (movement blur visible in drops), you can manually set flash output to lower settings. Less power - shorter flash burst, less blur. As I am Nikon shooter, I don't know how its set on Canon flashes, but it should be somewhere in the menu, like menu -> shooting settings -> built-in flash mode -> M and then setting the value. Check your manual for exact instructions.
If setting flash power manually you will see that shot is under-exposed, open up the aperture and/or increase your ISO settings.