text: Questions about ND Filters. | APUG.ORG
  Questions about ND Filters.
  Discussion in '35mm Cameras and Accessories' started by snegron, Dec 21, 2006.
  I have never used an ND filter, just read about them in books, posts, etc. My understanding is that they are used mostly for high contrast scenes or scenes where there are several F stop differences between foreground and background. How does this work? Are they split in the middle? Are they built to rotate? Can they be used with AF cameras? Do they affect in-camera metering? What are the different grades?
  I've had limited experience but here's a long-winded explanation that might help a little.
  Regular ND filters are solid neutral grey and are used to increase exposure. They are used to get longer shutter speeds or wider apertures for whatever reason the photographer deems appropriate.
  Graduated ND filters are different. They are part clear and part ND with an area of graduation between the two parts. Mostly, they are used to hold back light from a subject within a specific area of the composition. For landscape photography, that's usually the sky or, sometimes, a body of water.
  Both regular ND and graduated ND filters come in different strengths, made to increase exposure by a set number of f-stops. The strongest regular ND filter I've ever seen was one that would increase exposure by about 20 f-stops.
  Graduated ND filters also come in various strengths but most are one, two or three stop filters. Some of them are round, screw-in filters generally evenly split with clear glass and ND glass with an area of graduation between. The most useful are square or rectangular filters that can be used in a filter holder or hand held in front of the lens. You can move the filter around, up and down as necessary to fit it in just the right area of the composition.
  With regular ND filters, you can just meter through the lens and use autofocus unless it's a very strong filter. In that case, don't use autofocus and meter without the filter then set the shutter or aperture to compensate for the exposure.
  With graduated ND filters, I've always made a meter reading without the filter and manually focused. I've then move the filter into position and did not set any exposure compensation at all. The only tricky part about using graduated ND filters is getting the line between the ND portion of the filter and the clear portion of the filter at the right point. It becomes easier with practice.
  Hope I haven't confused you more than necessary.
  Hi Snegron,
  What you are really talking about are Graduated ND filters, not just regular ND filters. There are some that are round that can rotate. They are split down the middle. These are not nearly as usable and adjustable as those that are rectangular.
  Yes there are different gradations and there are also differences in the number of stops between the darkest area and the clear area. Most Grad ND filters are rectangular and are dark at the top and grade to clear on the bottom. They fit a rotating holder that allows the gradation to but positioned in the proper location for taking an image, particularly on transparency material. Singh Ray also makes a special reversed Grad ND filter that is darkest in the center that grades to clear at the top; the lower half of these filters are clear. The Grad ND filters are also frequently available in a hard or a soft gradation, These filters are usually available in .45 (1 stop), .6 (2 stop) and .9 (3 stop) values.
  Apertures for these filters are normally set at f8 or smaller apertures to be most effective. As to usage with autofocus cameras, I have had no experience to comment.
  Thanks for all the detailed info! I guess I was refering to a graduated ND filter. I am planning on experimenting with one soon. The lighting conditions here in Florida are of very high contrast. The days are extremely bright providing more of a contrast between the subject and the background. My guess is that there is usually a 3 to 4 stop difference in both areas. What would be an appropriate ND filter number for this? Would I have to invest in the square filter system, or would one of the rotating ND filters be a wiser choice?
  These filters are usually available in .45 (1 stop), .6 (2 stop) and .9 (3 stop) values.Click to expand...
  Just to be clear for those who are new to this kind of ND numbering. 0.30 ND = 1 stop, so the first example above, 0.45 ND, should be 1.5 stops.
  Thanks for catching that Lee. It didn't look quite right to me for the 1 stop. I should have remembered (I do not use the 1 stop) that .45 is the Center Filter for my WA large format lenses (1 1/2 stops).
  Thanks for all the detailed info! I guess I was refering to a graduated ND filter. I am planning on experimenting with one soon. The lighting conditions here in Florida are of very high contrast. The days are extremely bright providing more of a contrast between the subject and the background. My guess is that there is usually a 3 to 4 stop difference in both areas. What would be an appropriate ND filter number for this? Would I have to invest in the square filter system, or would one of the rotating ND filters be a wiser choice?Click to expand...
  Definitely get a rectangular Grad ND filter and an adapter ring to use with the filter (s). They are available in different sizes including the Cokin P size. But, stay away from the Cokin Graduated grey filters they are not known for being very neutral. If you had to get one Grad ND filter, I would suggest a 2 stop (.6) and probably one that has a soft instead of a hard edge gradation to start. They are easier to work with in many situations.
  I have and use a rectangular system (in my case Hitech) and a screw-in glass nd grad. I use the latter only with a rangefinder, where you can't easily position a sliding rectangle correctly and its positively advantageous to know that the grad line is always central. Other than that I use the rectangular grads all the time with SLRs. They are expensive to get into with the need for an adapter ring and holder - though if it fits your lenses you can use cheaper Cokin ironware and plastic P size holder- but they are far more versatile. Bear in mind though that all the most available rectangular nd grads are C39 optical resin, so they scratch fairly easily and they don't last a lifetime. I replace grads on average after 3-4 years, though I'm sure there's plenty who look after their gear better than I do.
  On the strength issue, you need to take into account that skies are most often lighter than a mid tone, lighter than the foreground and in particular usually lighter than their own reflections. Attempts to equalise the brightness of sky and foreground often look unnatural, so you wouldn't generally want to try and equalise the 3/4 stops difference you refer to, and would likely want a 2 stop grad - leaving the ground area 1 stop under a mid-tone and the sky a stop over; or a three stop grad for a mid-toned subject and the sky a stop over. I think its moot between hard and soft and indeed I carry a set of both. Partly this is a brand issue. Hitech soft are very soft indeed and their hard isn't very hard. Partly its a subject issue. Lots of flattish horizons and seascapes tends to mean a preference for hard edge; trees and mountains and variable skylines often mean you need a soft graduation. Finally there's a lens component. If you use a lot of long lenses then hard edge tend to be most useful since you may not be using enough of the filter to darken much of the sky to the full strength of the filter with a soft edge. You'll find it easier to see a hard edge ttl than soft, and so hard-edge filters are easier to position accurately, so whilst the consequences of messing up are greater, the probability is a lot less when you can see what you're doing.
  Thanks for the info! I hadn't considered focal lengths and the effects of a ND filter on a wide angle vs. telephoto lens. As far as the ND filter materials, are all ND filters ( all brands) made of resin instead of glass?
  Grad ND filters are probably used more frequently with wide angle and normal lenses than with telephoto lenses.
  Lee, Singh Ray, and HiTech and several less expensive brands including Cokin make less expensive Resin Filters and specifically Rectangular (or square) Grad ND Resin Filters. Heliopan used to make Resin filters, but I am not sure if they still do and B & W may also make some resin filters.
  Glass Rectangular Grad ND Filters are made by Tiffen and Formatt (the parent and same company that make HiTech). I am not sure as to what glass filters are made by Helipan and B & W Grad ND filters in rectangular and rotating circular versions. I have a few Heliopan Glass Grad ND but, I am not sure what they presently make.