Flash darker with slaves than without

I have two simple slave flashes that I bought to use for product photography.

They seem to work fine (I can see them fire when the camera's flash fires), but for some strange reason, the pictures I take with them are darker than the ones I shoot with without having the slaves activated.

Any idea what I am doing wrong?

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One response:

Turn off pre-flash

It sounds as you are trying to use your on-camera flash in TTL-mode along with slaves that are not TTL-compatible. Such a combination will not work, and is guaranteed to give you underexposure.

This is what happens: Your slaves trigger before the shutter opens off the pre-flash from your on-camera flash.

In TTL-mode, the camera uses pre-flash for metering, to determine how much flash power to use for the exposure based on how much light it can see reflected back from the pre-flash. The camera sees the light from the firing slaves along with the light from the pre-flash, which in turn causes the camera to think it is seeing a highly reflective subject. The result is that the camera uses very low flash power. Since the slaves have already fired on the pre-flash, they are not able to fire again when the main flash fires. The result is a darker photo.

For this setup to work, you must prevent-the pre-flash from firing the slaves. You usually do this by setting the on-camera flash to manual (not TTL). If this is not possible, some alternative techniques are described here.

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