A neutral density filter is supposed to do one thing, cut light without changing color, and the gap between that promise and reality is where cheap filters betray their price. The density rating tells you how many stops of light the filter removes, but the color cast tells you whether it removes those stops cleanly or leaves a tint that you will spend time correcting in post.Density is straightforward on paper. A three-stop filter lets you slow the shutter for silky water, while a ten-stop filter stretches daylight into minutes for long exposures of clouds and crowds, and higher densities simply remove more light. The practical complication is stacking and accuracy, because a filter that claims ten stops may deliver slightly more or less, and stacking two filters multiplies any error in each.Color cast is where filters separate into two classes. Budget ND filters commonly introduce a warm, magenta, or green tint, and the tint deepens as density rises, so a cheap ten-stop filter can shift an entire sky toward purple.
High-quality filters use coated glass and multi-layer
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