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Hi Wayne,
The OD number refers to the optical density of the film. OD 3.8 is not intended to be used for visual viewing. You would normally use OD 5.0 as it is safer and darker to use for visual viewing and photography. There is a difference between the two - if you use it on a telescope then it must go on the front of the objective. If 5.0 is a bit dark when taking photos then increase your exposure. Make sure the film has no pin holes in it.
Cheers, John W.
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