Strobe Test.......and some questions.........

I shot off a couple of rolls of slides today to test out various positions for my Ikelite substrobe 50's. I was shooting macro with a 105mm lens. I shot the rolls above water, on my coffee table. Most of the shots were at f16/125 (Velvia 100). I was surprised that many of the shots were about a stop over-exposed, since this combo works well for me when I'm diving. Does water suck up that much strobe power even though the subject is only a few inches from the lens?

Thanks,

Marta

Yes, one foot (30.48cm) of water sucks up about one f/stop of strobe light.

<snip> I was surprised that many of the shots were about a stop over-exposed, since this combo works well for me when I'm diving. Does water suck up that much strobe power even though the subject is only a few inches from the lens?

Yes. In a gross over-simplification, just under half the light disappears.

Far more important than water sucking up light is the spreading of the beam with distance. Camera (well strobe) to subject distance is the dominant factor. My guess is that you were closer on land than underwater.

On land when something looks only a few inches away it is. Underwater something that looks just a few inches away is actually 30% further away than it appears.

Small differences in camera to subject distances make big differences to strobe power. If you double the camera to subject distance 3" to say 6" you will 1/4 the amount of light from the strobe reaching the subject because of the spread of the beam.

This is the main cause of your over exposure. I suspect you were slightly closer in the land example than your typical "real" not apparent shooting distance underwater.

Alex

Interesting. I had the lens set to minimum focus because part of the test was to see how the strobes reacted with an effective f-stop of f45 or greater. But if I understand you correctly, the minimum focus on land is different than uw because while the actual object I'm photographing appears the same size to my lens uw, it's actually farther away than it would be on land. Would that be correct?