Using a 5-stop Neutral Density ("ND") filter, and a smaller aperture, one can eliminate the cars entirely from the Golden Gate Bridge. ND filters simply reduce the total amount of light, but as you can see, the "tint" of the image can be altered. It's difficult to get a truly "neutral" filter because of two factors: reciprocity failure (which describes how each of the red/green/blue color emulsions in film react differently to light at slower shutter speeds), and the tendency for color tints to be introduced into glass as it darkens. Compensating for both of these is not always possible, although digital editing can fix any picture. (This photo has not been digitally altered to show this effect.)
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