Question about High-Speed Photography Using Flash

[Nicodemus]

Okay. So I'm trying out some high-speed photography (for water droplets, etc) using a setup that I think is recommended elsewhere on this forum. I use a pitch-dark studio, set the camera to have an exposure of about ten seconds, then in the middle of the exposure click the flash. Here's the problem: it's waaaay over-exposed, practically completely white. I've tried messing with ISO settings, but it doesn't seem to help. I'm a major newbie at photography, so don't know much about using a flash. For practice (thank God) I've been using a digital camera, so I'm not wasting film, but I can't figure it out. Any ideas what I'm doing wrong? Thanks.

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[ferny]

If it's over exposed that's because there's too much light getting to the sensor (as you'll already know). To reduce the amount of light getting there you can reduce the amount of light present (turn down the flash or move it further away), reduce the sensitivity of the sensor (drop the iso to a lower number), restrict the amount of light reaching the sensor (increase the f number) or a combination of the three.

[Big Mike]

What aperture is the camera set at? Aperture size will control the amount of light getting to the sensor from the flash. Try different settings, I'd go with mid range to small aperture to maximize your DOF. F8-F22 something like that.

As Ferny said, you can set the ISO low and move the flash farther away. Light falls off exponentially as the distance is increased.

[Soulreaver]

You can put something at the same point you will take the picture and let the camera adjust the settings when its using the flash.Then you use those settings when making a shot of the drop.
You will have to use minimal light for that.

[Silkfire]

How do you set the ISO speed?

[Big Mike]

How do you set the ISO speed?

Any decent digital camera will have the option to set the ISO. HOW, depends on the make & model.

Lower ISO number is less sensitive to light so needs more light. Higher ISO is more sensitive to light, so faster shutter speeds are available but at the cost of more digital noise.

[Hertz van Rental]

If you are trying to freeze motion with a flash:
You will have more success if you have a flash that, in manual mode, alows you to control the output. That is to say, full power, 1/2 power, 1/4 power etc.
How the electronics of a flashgun control the light output is to quench the flash when it has emitted enough light. This means that if, for example, the flash duration at full power is 1/1000th of a second then the flash duration at half-power is 1/2000th of a second and so on.
The same control method is used in Auto mode - the sensor measures the amount of light the flash is giving out (by measuring the light reflected from the object) and quenches the flash when it has emitted enough light. With this method there are a number of variables that can fool the flash into thinking it has emitted more or less light than it needs to and exposures are wrong.
In manual mode you can use a flash meter (or shoot a series of pictures at different exposures - 1/2 stop intervals say) to determine exposure.
Using the lowest power setting will give you the shortest duration flash to really freeze movement. You can still get considerable blur on a shot using full power.
To get really good high speed shots you need a very short flash duration with a smallish aperture. This means you need a high ISO number. Kodak do (or used to do) high speed recording film (b/w) that starts at ISO1000 but can be rated at ISO5000.
Flash duration range should be stated in the flashgun's technical info.
The more powerful the flashgun, the longer the flash duration so you may be better off using several small flashguns slaved together.
Hope this helps.

[paul rond]

aren't you guys talking about the same thing as HIGH SPEED TRIGGERS??

at ... high speed trigger

[Nicodemus]

aren't you guys talking about the same thing as HIGH SPEED TRIGGERS??

Yes...I've been checking out that thread, too, but I'm trying to do this very "on-the-cheap" and low-tech.
My little experiments have become slightly more successful...thanks for all your suggestions. I've been mostly pointing the flash at different angles and farther away. I'll post some examples soon.

You can put something at the same point you will take the picture and let the camera adjust the settings when its using the flash.Then you use those settings when making a shot of the drop.

I can't let it adjust to the settings it would use when using flash, and then take a picture without flash using those settings, if that makes any sense. It's not the greatest digital, so I just have to hold the button down halfway, and then turn off the lights and open the shutter. I AM able, however, to adjust things like the ISO (between 50 and 400, anyway). One of the biggest struggles now is keeping the damn thing in focus.

What aperture is the camera set at?

So, unfortunately, I have no control over the aperture, except by setting up a shot with the lights on and holding the release halfway down. Huh...I wonder if it would help if I shined a bright light straight into the camera while pressing the release halfway down. That would probably give me a smaller aperture. I should probably just do this all with my nice SLR camera and not worry about flushing 25 bucks straight down the toilet...but I'd like to be fairly good at it already first.

[Nicodemus]

You will have more success if you have a flash that, in manual mode, alows you to control the output. That is to say, full power, 1/2 power, 1/4 power etc.

The flash I'm using has a simple lo/high button and a sliding bar for various ISO speeds. I believe (if I understand the manual correctly) that I am using the lowest setting with the fastest possible flash.