In the digital age certain filters that were previously ‘must haves like skylight or warming filters are no longer necessary because we now have the ability to color correct or color enhance using white balance. As a matter of fact, colored filters can be a liability in many cases since they can either lock you into a colorcast or when using settings like Auto White Balance, attempt to correct for the intentional colorcast you wanted the filter to apply. This shortens the number of filters that are needed but there are still several 'must have' filters that every serious landscape photographer has in their bag. When it comes landscape photography there are, in my opinion, only three types and a total of six different filters that I consider, the essentials. These include a Circular Polarizer or CPL, Neutral Density or ND, and Graduated Neutral Density or Grad-ND. As far as Grad-ND’s there are four types that should handle about 90% of your shooting conditions, the 2 and 3-stop soft, and 2 and 3 stop hard, thus totalling six essential filters I strongly recommend for landscape work. Swift River, White Mountains, NH Circular Polarizer: If you are starting out and can only afford one filter this is it. The CPL is the single most useful filter and I strongly recommend a high-quality multi-coated version. Its virtue is it only allows light vibrating in one direction to pass through thus removing glare and scattered light. It is best known for its ability to darken bright skies when aimed about 90 degrees to the sun making it useful to control the exposure range. It also removes glare from water surfaces allowing you to see below the surface, and removes glare from shiny objects like wet rocks, leaves, metal, windows and is one of the most useful filters for enhancing color for fall foliage, especially when the leaves are wet. The CPL has another virtue for landscape photographers; it also doubles as a ND filter since its blocks about 1.5 stops of light which comes in handy when shooting waterfalls to slow shutter speed to help achieve that silky look. It is easy to use, just turn until you get the desire effect. In some cases partial polarization is all that is needed else your skies or water surfaces can get too dark. Also, some say that a CPL should not be used on wide lenses since only part of the field-of-view will be at 90 degrees from the sun. That’s true but I frequently use mine with my wide lenses when my composition does not include the sky since partial polarization to remove selective glare or slowing down my shutter speed is all that I may need. Winter in Greenbrier, Great Smoky Mountains Neutral Density: The ND filter does one thing, it uniformly blocks light. The idea is to increase shutter speed allowing for special effects like silky water, soften cloud movement or any condition where longer shutter speed will change the resulting look to something more interesting. They come in different densities like 1 through about 9 stops. Singh-Ray offers a vari-ND filter which allows you to simply dial in from 2 to 9 stops of density as needed. The most common and the one I use and recommend is the three stop. The reason why I recommend a 3-stop as one of the essentials is that if you already own a CPL then you already have a 1-2 stop ND in your bag. When you get to higher densities like 6 or 9 stops you may encounter a problem, especially with f/5.6 lens, since it is too dark to see what you are doing. You need enough light to compose and focus and often when shooting waterfalls you are using soft low light since midday direct light is often too contrasty to allow for good results. So if you can only have one ND, a 3-stop is my recommendation. When buying a ND filter it is important to understand that different manufacturers use different terminology to describe density ratings. For example, a 1-stop ND blocks one stop of light allowing only half as much light through and may be referred to as a 2x or 0.3 filter. 2-stop ND which allows only ¼ the light to pass can be referred to as a 4x, or 0.6, and a 3-stop may be referred to as a 8x or 0.9. Sunset: Cades Cove, Great Smoky Mountains Grad-ND filters are very useful for landscape photography but in the digital age the least required of the three essentials. I say this since if you can achieve the same result in Photoshop by bracketing and blending two exposures but anytime you can get it right in the field you save yourself work in the digital darkroom. The virtue of this filter is to reduce the exposure of a bright sky to keep the exposure range within the limits of dynamic range of your camera. Without this filter your beautiful blue or orange sky could get blown out when you are exposed for the ground ruining an otherwise great shot. Grad-ND’s come in different designs, strengths and transitions. The designs include circular screw-in types, and both smaller P-size and larger Z-size filters (there are even smaller A-size and larger X-size but their use is extremely rare for DSLR bodies). Avoid the circular types since they are almost totally useless in practice since their application is limited to when the transition is in the center, which is rare for a most well-balanced compositions. The P-size, which is 84-85mm wide, and 85-120mm high, is a common choice. These can be hand held but more often are used in filter holders. The larger Z-size is 100mm wide and 125 – 150mm high. This size is ideal for handholding but also works with a Z-size holder. (Z-size is what I currently use now and love them). Grad-ND’s come in both hard and soft transitions, and varying strengths from 1 to 4 stops. The hard transition is useful for sharp transitions like ocean horizons or flat mountain ridges and the soft is for everything else. I consider four of these to be the essentials, 2 and 3 stop hard and soft, since this set will handle about 90-95% of all landscape situations. If you can only afford two I suggest a 2-soft and 3-hard since when you need a hard edge the light transition is typically strong. My least used is probably the 2-hard but it depends on what I am shooting. If the horizons you shoot are never flat then start out with 2 and 3 stop soft. As a guideline for which strength to use I suggest metering for both the sky and the ground and allow for about a 1 to 1.5 stop difference. So, if the difference is 3 stops then a 2-stop is probably the one you should try first. There are many other useful filters like 1 or 4 stop grad ND’s, reverse grads, warming or blue-gold polarizers, color enhancement filters… Each has its place but are not essential and easily substituted for Photoshop skills. However, Photoshop cannot remove glare like a CPL or slow movement like a ND can. Blending exposures can certainly replace grad-ND’s but the cost is time. There are exceptions like when I include the sun in the composition since filters, any filters, and the sun often are a recipe for flare. So these days when I shoot the sun I use the bracket and blend technique instead of filters.