THE NEUTRAL DENSITY FILTERS!This type of filters are made either of glass or resin. You can find them in circular shape like this one that screw onto the front of your lens: Or you can find them square /rectangle, and they require a filter holder that clips onto a ring that you screw onto the front of your lens: I will do a review of the kit that I use which is of the brand Formatt Hitech, but they are another brand like Cokin, Lee… Neutral Density Filters, aka ND Filters, are not meant to alter the colours in your exposure and their sole purpose is to reduce the brightness level in your scene and therefore in your resulting image. However, note that some ND Filters can differ in quality and some, like Cokin filters, I have read on many reviews do impact more or less the White Balance. ND Filters are defined by the number of Light Stop (ref. The ISO) they reduce: -1, -2, -3… -10. Whether you use circular or the filter holder type, you can stack up your ND Filter to increase the reduction level. However be aware this could result in some Vignetting with some lenses and by reducing the brightness level your AutoFocus feature might have some trouble finding contrast and set the focus at all (just like with the usage of 10 stop ND Filter). In regards to high stop ND Filters, above 9, when using a filter holder you are bound to have rather long exposures and this can lead to some light piercing through from the side and behind the filter itself. Personally, I own a circular Hoya 9 stop ND Filter, and once it is screwed correctly onto the front of the lens, no unwanted light beam will ever get through. However, this also means I can only use this Filter on the lens that has the same front end diameter, in my case 54mm.
1st example of a situation where an ND Filter is requiredIf you were to capture a moving car and you do not wish to freeze the action, but on the contrary, you want to inject some movement as seen in Creative Shutter: Implying Motion. You will want a slow Shutter speed and a wide Aperture to increase the phenomenon. As a result, you may end up with an overexposed image. Since your driving factor is Creativity, you won’t want to alter the Aperture or the Shutter speed. Therefore the only solution is to use an ND Filter.
2nd example of a situation where an ND Filter is requiredIf you want to shoot some portraiture in a very bright location or at the time of the day where the sun is the highest. Shooting portraiture will lead you to use a wide Aperture (ref. Creative Aperture: DOF). Your camera will choose the Shutter speed and to compensate for the important amount of light coming through the lens, it will try to set a very fast Shutter speed. On most DSLR if not all, the fastest Shutter speed you can get is 1/8000sec. While this is hugely quick, in a certain condition like in the desert or at midday, it might not be fast enough. You can use an ND Filter to reduce the brightness level of the scene which will make your 1/8000sec speed fast enough or even too fast depending on the strength of your filter (number of a stop).
Landscape Photography and ND FiltersHave you ever noticed that when shooting on day time, the sky will always be brighter than the ground? This obvious fact brings its load of challenges when Light Metering a scene. As a matter of fact, if you meter using the sky, you are bound to get a reading that will slow down (darken) your exposure and subsequently darken even more the foreground making it overexposed. On the other hand if you meter from the foreground, you will get it properly exposed but your sky will end up being blown out ( totally white). You may wonder how an ND Filter is going to help since it will affect equally the sky and the foreground, so you are bound to end up with the same result. You would be right of course. Nonetheless, there is a special ND Filter that is called the ND Graduated Filter, aka ND Grad. As graduated implies, the density part (the dark part) of the filter goes half way, living the rest absolutely clear and therefore not affecting the exposure.
Surprisingly you can find ND Grad Filters as circular type which I absolutely do not recommend since you cannot adjust where the graduation finishes.
Regarding the graduation, you can find two kinds: Soft or Hard. This defines how the graduation occurs from the dark to the clear part of the filter. I own Soft ND Grad since it is more versatile. The Hard ones are great if you have a straight horizon line, like the ocean. But living in Scotland, I have amazing hilly landscapes. The drawback with Soft ND Grad is that it is more difficult to find where the graduation takes place.
Below are shots I took after recording the episode and you can see the impact of using an ND Grad Filter: