If there is a single accessory I see as essential (not just useful) it’s the ND Grad (Neutral Density Graduated) filter. This is the filter that is clear at the bottom but a dark grey colour at the top. There is a graduated area to gradually transition from one to the other. It’s call Neutral because it’s not supposed to have any impact on the colour of the image although some do. If by the way you want to know more about these filters and the options there is a tutorial on my Lenscraft website at http://www.lenscraft.co.uk/training/160.html.
The ND grad comes in various strengths and is used to darken a bright area of an image such as the sky, which might otherwise cause the other areas to become too dark. As I’m sure you can imagine this is very important to Landscape Photographers especially when shooting scenes with a high dynamic range such as sunsets. Without this filter you will typically end up with either a lovely sky and a black ground or a well exposed ground and a white sky.
Not using ND grad filters is probably the biggest mistakes newcomers to Landscape Photography make. Certainly you can take multiple exposures and blend them together but this is additional effort and time. If we are to keep our workflow lightweight as well as our equipment, it’s important to get it right in camera where possible and this is why the ND grad is so important to me.
There are a number of manufacturers of ND grads. Lee Filters are widely considered in the UK to be the best and used my most of the Pro Landscape Photographers. Whilst I too use Lee filters, I find they are expensive, quite bulky and heavy. Certainly where I am using a small sensor camera such as my GX1 or LX5, I don’t need the size or weight of the 100mm Lee system.
Recently I have started to use Hi-Tech filters which in the UK are marketed by Format Filters and they have performed very well indeed. These filters can be purchased in P size (85mm), are slightly thinner than Lee, certainly cheaper and the accessories to attach them to the lens are much lighter. By carrying a 0.3 and 0.6 filter wrapped in a lens cloth I have everything I need at a fraction of the weight and cost. Additionally, if I need a Neutral Density filter (rather than a graduate) to slow exposure I simply pull the filter down lower in the holder so only the dark area covers the lens.
All this keeps my equipment light and allows me to enjoy my photography much more. The image shown here was taken using a ND grad filter to balance the exposure for the sky with the rest of the image or I would have lost the light rays breaking through the clouds.