long exposures and flash

I'm having trouble figuring this out.

I took a couple of long exposure shots with flash recently and they didn't turn out how I expected them to.

Ok, so this was what the shot was. ----
A relatively dark alley end at night, pointing towards a street. I wanted to take a shot of my two friends in front (illuminating them using manual off camera flash), and then have them walk/run out of the scene and then I would expose the scene for 60-90 seconds to get some traffic blur behind them. I determined my aperture by focusing the lens on my two friends, and then looked at my flash's manual setting for feet/aperture guide and set it accordingly.

I just basically wanted them fully visible and exposed properly in the foreground, with the blurred traffic behind them.

The result: The two figures were see through ghosts and barely visible. This baffles me because if I would've had a much quicker shutter speed (say 1/60), they would've been exposed properly (though obviously no time exposure blurred background behind them). So obviously the very long minute or more exposure caused my ghost images.

What happened? And how would I get two solid figures?

You need to balance your flash lit figures to the ambient light conditions and while doing this, use an EV that gives you the shutter speed that fails to stop the traffic.

You fail to mention the film that you were using...I assume either black and white negative or color negative film. If so your flash exposure was underexposed in ratio to the ambient (unlit by flash) conditions.

Sometimes it is easier to do this with multiple exposures. If you decide to do that, when making your calculations, don't forget to factor in the ambient exposure while the flash fires.

Are your negatives kind of dense? It might be that if you hadn't exposed the background your friends would have looked pretty good, but the additional exposure of the background increased the overall density enough to make your friends ghose. I find that it is usually best to have a dark background behind the subject you are flashing if you wish to avoid the ghost effect. I usually like the ghost effect and let that happen. Here is an example of mine.

Bear in mind, too, that flash guide numbers are specified on the assumption of normal indoor conditions. Outdoors, you will need to expose anything from one to two stops more (depending on whether the background is light, such as a beach, or dark, such as a city street). As others have said, by increasing the foreground (people) exposure and possibly reducing the background exposure, you must eventually achieve the result you are looking for.

Regards,

David

When you multiple-expose a negative the densities add together and what it looks like is that bright bits from one exposure fill in the dark bits of the other exposure. So you have got bright lights from the traffic 'writing over' the images of your friends - hence ghosting. What you need is something to block the light where your friends are standing so that their images don't get overwritten. Your friend's bodies are ideal for this if they can stand still enough for long enough. Just run through the procedure you did before but get your friends to hold still. Their images will be sharp because they are exposed by the light from the flash only. The edges of their images may be ghosted a bit because of their inability to stand completely still.

Thanks for all the responses.

I'll try what you're all suggesting next time, since it really does make sense. I don't know why I didn't think of that while i was shooting. It was 35mm Kodak c-41 black and white I used. I didn't want to use it, but my friend insisted.

Paul- awesome shot example. Do you have any info on how you got it?