Many times you will see a thread where someone is asking what ND filters are, and how they are used. Here's a beginner's guide to explain what they are and what the different types do.
Question: What is a Neutral Density Filter?
A Neutral Density Filter is simply a piece of glass or resin that restricts the amount of light that makes it into your camera. When you're shooting in bright sunlight, it's impossible to get a 2 minute exposure without the use of a Neutral Density filter.
Question: Why and when would I use a Neutral Density Filter?
Once again, a Neutral Density Filter is just a filter that restricts the amount of light entering your camera. There are filters that restrict the amount of light in a certain part of the frame, all the way to solid Neutral Density filters that simply reduce the amount of light by a set amount of stops.
Question: What is a Graduated Neutral Density Filter?
A GND is a filter that is clear on half of the filter and goes from Light to Dark to the top of the filter. In the middle of the filter, the amount of filtering increases from 0 stops to 1 - 3 stops usually, towards the top of the filter.
The most common use for GND filters is when one half of the frame is extremely bright and you wish to reduce the amount of light from that part of the frame so that you can get detail on the other part.
Let's say you're taking a shot where the sun is towards the top of your frame. Without the use of a GND, if just take the shot where you meter for the bright sky, then chances are that your foreground will be underexposed. If you meter for the foreground, then chances are that your clouds and whatnot will be overexposed.
Same scenario with a GND, you put the filter in place to where the darkest part of the filter is towards the top of the frame (where the sun is), and that will reduce the amount of light coming in from that part of the frame and will properly expose the entire shot.
Question: What is a Reverse Graduated Neutral Density Filter?
Pretty much the same as above, except here, the darkest part of the filter is right in the middle of the filter and then gets lighter towards the top of the frame. Most common use for this filter is shooting a sunset when the sun is right on the horizon. You will be reducing the amount of light coming in from the horizon (the sun) and letting the light in from the other parts of the scene.
Question: What is the difference between a Soft and Hard Edge Graduated Neutral Density Filter?
A Soft Edge filter gradually changes from light to dark from the middle to the top. I will use a 4x6" filter as an example. This filter is 6 inches high. This filter will have the bottom 3 inches of the filter as clear, or 0 stop. Then at the horizon line, it will gradually get darker towards the top. So on a three stop filter, from the horizon line up, the filter will gradually change from 0 stops to 3 stops of light filtering.
A Hard Edge filter will have half the glass clear, the other half as the stated stop level. So a 4x6" 3 Stop Hard GND will have 3 inches of clear and 3 inches of 3 stop filtering.
Question: What is a solid Neutral Density Filter.
This filter is a filter that reduces the amount of light entering your camera, throughout the whole frame. Let's use an example of where you are shooting a waterfall and would like the exposure to be longer so that the water looks "blurred". If you're shooting in bright sunlight, chances are that you may only be able to get the exposure time to fractions of a second. If you want to have the shutter stay open for longer, you must reduce the amount of light entering the camera.
So, let's say that you're shooting a waterfall, and you're metering that the proper exposure for the scene is 1/30. While 1/30 is pretty slow for exposure time, chances are that it will not give you the silky smooth appearance to the water that you're looking for.
In this case, there are ND filters that you can either screw in, or plate filters that fit into a filter holder, that will reduce the amount of light coming in. The two most commonly used solid ND filters come in 3 and 10 stop varieties.
So in the scenario above, if 1/30th of a second is still too fast, you can put a 3 stop filter in front of your lens, and you will now have an exposure time of 1/4 of a second. If that is still too fast to get you the exposure time that you're looking for, you can put a 10 stop in front of the lens and that will change the exposure time from 1/30th of a second to now a 30 second exposure.
Question: What are the differences between using a screw in filter versus a plate filter with holder of some sort?
Some photographers have lenses that come in all sizes. Personally, I use 3 different size lenses when I use filters the most. My 21mm lens has an 82mm thread size. My 35mm lens has a 72mm thread size and my 70-200 has a 77mm filter size.
Instead of my buying three different screw in filters, I have 3 adapter rings that all allow my filter holder to snap on to them. This way, I can change lens, and have my Polarizing and Neutral Density Filters ready to go on to the next lens.
Also, with screw in filters, where the density is is where it stays. If you're talking about a solid ND filter, this isn't a big deal as you're reducing the amount of light coming in through the whole scene. If you now are talking about a Graduated Neutral Density filter, then you're stuck with the graduation as it is. You cannot move the screw in filter. If you have a plate filter, you can move the filter up and down to meet your needs for the scene
Question: For plate filters, what are the different sizes like P-Size, Z-Size, etc.?
Much like Kleenex is a brand name of a tissue, and folks ask for a Kleenex, P-Size, etc. are a naming convention of filters made by Cokin. Most manufacturers use mm of width of the filter as a sizing convention, but some folks use the P-Size, etc. when talking about filters.
The main sizes of plate filters are 67mm wide (Cokin A), 84mm wide (Cokin P), 97mm wide (Cokin Z and also popularly in inches of 4x6") and 130mm wide (Cokin X-Pro).
The main reason one usually moves up from an 84mm to 97mm wide filter is because of the use of Ultra Wide lenses, and in trying to reduce the chance of causing shading (vignetting) in a shot. As an example, I use 4x6 filters because my 21mm lens is 82mm wide. Once you put a filter adapter ring and holder on to the lens, an 84mm size filter would cause some shading. Because the 4x6 filters are bigger, so is the holder, which moves further away from the barrel of the lens and does not cause any shading.
Personally, I also tend to hand hold filters in front of my lens at times. The bigger piece of glass gives me more real estate to hold the filter with without having to worry about my fingertips getting into the shot.
Question: What are the most common brands for these filters, and why is there such a large price difference between them?
For plate filters, the 4 most common companies that make these filters are Cokin, HiTech, Lee and Singh-Ray. There are other companies that make these filters (Tiffen), etc., but these are the main ones talked about on POTN. Cokin and HiTech are usually cheaper than Lee and Singh-Ray. Unfortunately, like most of the time with photography gear, you usually get what you pay for. There have been multiple experiences from users, including myself, where the Cokin and HiTech filters put a color cast on the image. The cast isn't drastic, but it's just another thing to work on in post. I use Lee and Singh-Ray and have been very happy with the results of the images SOOC. That being said, HiTech is a VERY popular brand, and are very affordable. If you're just trying out GNDs for the first time, this will usually a brand that gets recommended first.
For circular solid neutral density filters, the most common recommendations are B+W, Heliopan or Singh-Ray. All three have very positive reviews, but I cannot really speak much to them as I do not tend to use these much
Question: What if I want a solid ND filter that changes from light to dark?
These are called Variable Circular Neutral Density Filters. It used to be that only Singh-Ray made these with their popular Vari-ND filter. This filter changes from 2 to 8 stops of light filtering, simply by turning the ring. Another cheaper option is a variable filter made by a company called Fader.
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