GRADUATED NEUTRAL DENSITY FILTERS have varying density throughout the filter and the hardness of the transition from light to dark depends on the type of the filter. ZERO filters currently produce four different types of graduated ND filters including Ultra Soft, Soft, Medium and Hard ND Grad filters. All these types come in a variety of strengths ranging from ND4 to ND16 with an exception of the Soft ND Grad Filters which range up to ND32.
Before i discuss using graduated filters, I would like to briefly address a question whether to use them at all. Many photographers consider using graduated filters to be a "destructive workflow" and instead, they prefer to bracket their exposures and blend them in post-processing. It is true that if you use a graduated or any other filter when taking an image, it can not be undone. Knowing how to use filters to your advantage therefore also means knowing when not to use them. This however, is something that every photographer has to decide for himself based on his own criteria because bracketing has its own disadvantages, especially when it comes to photographing moving objects or extra time spent on post-processing.
The obvious advantage of using graduated filters is capturing all the necessary information in a single frame. This goes hand in hand with other benefits such as less data, faster workflow both in the field and in post-processing or ability to shoot handheld. Of course it is possible to take bracketed exposures without a tripod as well, however this introduces further problems into post-processing because the images do not align perfectly. Even in situations when bracketing is necessary, you might still want to use graduated filter. These techniques do not exclude each other and every photographer might benefit from combining these techniques in some situations. Using a filter might give you much better starting point for blending exposures by reducing the areas of an image which require blending. Personally, I like combining both these techniques especially if there are moving objects in my frame or if it helps me to avoid blowing up highlights in the sky of my base exposure.
Optical resins had gone a long way and the quality of current materials is comparable to the qualities of optical glass. When using filters, every photographer wants the filters to be "invisible" in the final images and graduated ZERO Filters can do just that if they are used correctly. Even if you stack several filters (full and/or graduated), the quality of the resulting image - its colour, contrast and sharpness - will not be affected. While optical resins have somewhat worse reputation compared to optical glass, it is practically impossible to spot any difference in image quality. In fact, resin filters have some advantages compared to glass filters. First of all, they are lightweight, but more importantly, they are not as brittle as glass so they will not break easily. They might scratch easier than glass but because ZERO Filters are coloured by bulk tinting, light scratches will not show in photographs.
Those who are new to filters might also ask themselves why to use square graduated filters instead of circular ones. While square filters are larger and therefore less convenient when fitted on a camera, they also have several important advantages. First of all, it is possible to adjust the position of the filters according to the scene. It is also easier to use several filters at a time without running a risk of vignetting when shooting wide. All in all, manipulation is easier and the same filters can be used on lenses with different thread diameters providing that you have the right adaptor rings for your lenses.
The characteristics of individual filters - the hardness of the transition and the strength of the filter - make them suitable for different scenarios and the range of graduated ZERO Filters covers all your needs in virtually every situation.
Grad filters are most commonly used for photographing landscapes and the general rule is that the straighter the horizon, the harder the filter. So if you are photographing a seascape or any other landscape with a straight horizon, you might want to use a hard grad filter. On the contrary, you might prefer a rather soft grad filter if you are photographing a mountain scene with jagged peaks or a skyline of a modern city with tall skyscrapers. Of course you might also choose to combine graduated filters to darken different parts of an image using graduated filters of different strengths and with different transitions to ensure the best possible results of the final image. If necessary, you could even use one graduated filter to darken the sky at the top of the frame and a second filter "upside down" do darken the foreground. How you combine filters really depends only on your creative vision.
There is no reason to limit the use of graduated filters only for shooting landscapes. Personally, I particularly like using ultra soft grad filter when shooting in the streets. Usually, there is considerably more light on the buildings rising high above the street than at the ground level, where all the action is taking place. Darkening the top of the frame helps to balance the image and bring more focus to the subject of the image. While using graduated filters when shooting in the streets might not be necessary on cloudy days with relatively flat light, it might still help to manage the deep shadows at the ground level of the street. In situations when direct sunlight is hitting tops of the buildings, the dynamic range of light is often unmanageable and use of graduated filters might be a very convenient way to preserve all necessary information in the highlights.
It does not really matter whether you shoot landscapes, architecture, street photography or anything else. Graduated filters can be used in any situation where the levels of light considerably differ in different parts of the image. They can be also used horizontally or even "upside down" which i found very helpful when I photographed architectural details of Slovak traditional cottages. It really is up to your preferred workflow, creativity and vision, when and how you use graduated filters.