After seeing this youtube video https://www.youtube.com/watch?v=zh6zr3wKRV0 where famous photographers shooting with cheap cameras I wanted to try what Zack Arias dis in this video. In short, I tried to take pictures with a point and shoot using an external off camera flash in slave optical mode. This external flash was activated by the small point and shot flash after it fired . I believed it possible to obtain images with more incisive light but instead I got pictures with more dull ligh and softly diffused. I can not understand why this has happened.
Optical slave flashes are somewhat finicky because most cameras with a builtin flash have one or more pre-flashes, for red-eye reduction as well as to help with focusing and exposure metering (e.g. Canon's E-TTL). A simple optical slave will trigger on those and then be unavailable for the main flash.
There are two way to tackle this problem:
- Turn off the pre-flashes in the camera's settings. Unfortunately, this may not be possible with some point&shoot cameras.
- Get the optical slave to ignore the pre-flashes. Some can be programmed to ignore a certain number of flashes before they trigger, some even have a mode where they can "learn" a camera's flash profile.
Chances are good that the optical slave you got fired early because your point and shoot camera emitted a "pre-flash"--that is a burst of light from the flash for some purpose other than the main flash burst used to illuminate the scene when taking the image.
Pre-flashes can happen for a number of different reasons (wireless commanding, red-eye reduction, TTL flash metering), and digital cameras may emit more than one. Most optical slaves can only sense when a flash burst occurs, and then trigger the flash they're attached to. Some have the ability to ignore a preflash, but most typically, they can only ignore a single preflash, not multiple ones.
The most common reasons a P&S will emit a pre-flash are for TTL metering. This is where the camera sets off a low-power flash burst of a known brightness level before taking the image, meters it, and based on the results, adjusts the flash's output power automatically. The other main reason for a pre-flash is red-eye reduction. Used together, you're liable to have at least two (if not more) preflashes.
The easiest way to get optical slaves to work is find a way to turn off the preflashes, but this may not be possible with many P&S cameras that don't allow for flash control settings or non-automated control. There is no "turn off the preflash" setting, per se (even some dSLRS don't have the capability to turn the built-in flash to Manual mode). For example, I have a Canon Powershot S90. If I use it in M mode, there are no pre-flashes. If I use it in Av/Tv/P or a scene mode, there will be a single preflash for TTL metering. So, for me to use my Sonia green-based optical slave hooked up to an RF-602TX radio transmitter to set off multiple off-light flashes, I have to have my S90 in M mode.
Your absolute best bet, really to do this, is not to use a cheap P&S camera, but to have a P&S camera with a flash hotshoe. The flash hotshoe would allow you to attach radio triggers to the camera and flash, rather than relying on line-of-sight-limited, range-limited, preflash-tripped optical slaves.
Now, as for "more incisive" light vs. diffused weak light, that has to do with how you're using your flash and whether or not it fired in sync. My best guess is your off-camera flash fired early from a preflash and didn't show up in the image. But you could also have been using it at too low a power setting, too far away from the subject for the light to register, pointed in the wrong direction, or "bounced" to be diffuse, rather than direct. Also realize that for portraits, most of us often find soft diffused light more pleasing and desirable than hard light. YMMV. But simply blasting something with direct flash, even if not from the on-camera axis, still rarely gets the results you want.