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Steve --
The simple answer is "enter 16."
Filter factors are usually expressed as simple multipliers: for example, 2x, 5x, etc.., in which case, when using more than one, the factors should be multiplied. But occasionally, as is the case with Kodak's Neutral Density (ND) filters, the factors are logrithmic and should be added when more than one is used.
You're working with arithmetic factors (8x and 2x), so the first one reduces the light to 1/8th intensity, then the second filter cuts that intensity in half to 1/16th of the original. Therefore the combined factor is 16x. In log terms 8x = 0.9 and 2x = 0.3; 0.9+0.3 = 1.2 which converts to the arithmetic factor 15.8x or 4 stops.
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