What's the difference between a FL-D filter and an ND filter? I see that FL-D is to improve light, but so is ND. What exactly is the difference?

An FL-D filter is a color correcting filter to make fluorescent lighting appear more daylight-ish to your film/sensor. An ND (neutral density) is for reducing the amount of light seen by your film/sensor with no (ideally) color modification, which is why it's termed neutral. Most FL-D filters I've seen have a noticeable color tint to them, while an ND filter is just gray.

FL-D (and other color correction) filters are not used as much in digital photography as they were in film days. Furthermore, there many different types (and hence color casts) of fluorescent lighting today than were available thirty years ago. Unless a fluorescent color correction filter is matched to the color of the fluorescent light, it can have a different color cast result than intended. It's so much easier to control white balance in camera (or in post, if shooting raw images) than it is to deal with filters, in general.

That's not to say that color correction filters are useless. I'm generally an advocate of getting the shot as correct as possible in camera, to reduce what needs to be done in post. There are plenty of situations where it might make sense to use color correction filters. See also: Are there reasons to use colour filters with digital cameras?

ND filters just reduce the amount of light coming into the lens, with (ideally) no color cast. Say you are shooting outdoors on a bright day, and want to capture an image with shallow depth of field. That requires you to shoot your lens with a wide aperture. But that means there's a lot of light coming through a wide aperture. You can't get your ISO usefully much lower than 100, so your only means to control exposure to use a fast shutter speed. But if your shutter isn't fast enough (or for some reason you want a slow shutter, such as trying to create some blur), then you need a way to reduce your exposure. That's where ND filters come in. Think of ND filters as the real-world exposure control slider in Photoshop.

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