I heard that the older flashes for film SLRs do not work properly for digital SLRs, can someone explain why? What is the difference between a flash for film and a flash for digital?

Thanks.

I heard that the older flashes for film SLRs do not work properly for digital SLRs, can someone explain why? What is the difference between a flash for film and a flash for digital?

Thanks.

Where did you hear this from?

I heard the film TTL flash system readed the ligth reflected from the film during the exposure to determine when to shut the light off.
In digital, there is no film, and that's why the e-TTL uses a small pre-flash to calculate the exposure.
But i think you can use a manual flash the old way, and use the lens aperture and distance to control exposure.