The first problem is taking long exposures photos for smearing waterfalls, cloud movement or flattening water surfaces. This requires use of slow shutter speed and if done during day light it can easily add too much light and burn the photo, even when your ISO is low and the aperture is very small.
The second problem is balancing between the stronger light coming from sky (especially when shooting directly towards the sun like in sunrise or sunset) and the weaker light coming from the ground. With no aid or using HDR the result will usually be a well exposed foreground and over exposed sky, or well exposed sky and underexposed foreground.
So what can you do? Use Filters!
The kind of filters that will help you resolve these problems are called ND filters. ND stands for "Neutral density". These filters basically act as sunglass and their job is to reduce the amount of light coming through the lens to your camera. There are two main kinds of ND filters and each one helps to solve one of the problems mentioned above.
Solid ND filters:
Using a solid ND filter enables you to reduce the general amount of light going through the lens into your camera, meaning you can use slower shutter speed than without the filter and do long exposures! These filters are evenly dark through their surface. They come in all kinds of darkness levels and can reduce from 1 stop of light to 10 stops of light going through your lens. A one stop filter means half the amount of light will go in through your lens than without the filter. A two stop filter means only a quarter the amount of light will go in through your lens than without the filter, and so on…Solid ND's can be round or square. Round ND filters are usually cheaper and can be mounted on your lens just like UV filters or a polarizer. Once you put them on the lens it can be impossible to see, compose or recompose and focus ,so you have to get everything done before mounting the filter. Otherwise you'll have to screw it out to fix the composition or focus and then screw it back on. Square ND's require a Filter system (explained below) and they are usually more expensive but their advantage is that you can easily slide then in or out of their holder and fix composition and focus.
Graduated ND filters:
These filters are usually square. Their top half part is dark (varying from 1 stop to 4 stops) and their bottom half part is transparent. When using this kind of filter on your lens you can reduce the amount of light coming from the sky and leave the same amount of light coming from the ground and balance the light between the two! As before with square filters, you need a filter system In order to use. Using a filter system also means you can slide it up or down and set the filter's dark/transparent dividing line according to where you chose to compose your skyline. You can find Graduated filters in which the darkest part is at the top and goes weaker as you get to the middle of the filter. These filters fit situations where the brightest part of the sky is at the top of your frame. You can also find Reverse Graduated filters in which the top part is a little dark and goes darker as you get to middle which fit situations where the brightest part of the sky is at the horizon – like sunset/sunrise.
General information about ND filters:
ND filters have one major problem – they can cause a color cast on your photos. Despite their name "Neutral density" these filters are not always neutral and the color cast happens because of unequal transmission of different light waves (which represent different colors). The color cast is often seen as blue-purple cast on the photo. Cheap filters can cause a hard cast which you won't even be able to remove in editing. Good filters can cause a cast which can be removed in editing or no cast at all.
Recommended ND filters:
1. Solid round filters: NiSi, Haida, Hoya pro1 digital, Hoya ND400, B+W.
2. Solid square filters: NiSi filters, Haida filters, Lee filters, Hitech filters.
3. Graduated square filters: NiSi filters, Haida filters, Lee filters, Singh-Ray filters, Hitech filters.
NIsi Vs haida Vs lee:
My personal favorite filters are NiSi and Haida filters. Nisi currently has top, uncompromising quality of sharpness, lowest vignetting and neutral color cast. NiSi also has the best filter holder I've used. Haida filters quality is one of the best in the market in terms of sharpness and very low color cast, moreover they're about half the price other known companies. Lee filters used to be the most well known and best brand before Nisi and Haida, but they are not as good and very expensive.
The filter system:
Filter systems are designed to allow use of square ND filters. Their advantage is the ability to use several filters at once and mainly to allow you to adjust the height/position of graduated filters.
The filter system is comprised of several parts:
1. Adapter ring- matches you lens diameter and screws
on your lens like a UV filter.
2. Filter Holder- a holder which connects to the adapter ring on one side and has rails in which square filters slide in on the other side.
3. Square filters – solid or graduated.
SQUARE ND FILTER SYSTEMS COMPARISON:
18mm (crop) or 24mm (FF)
10mm (crop) or 16mm (FF)
High quality imitations, fits ultra wide lenses
Doesn't fit ultra wide lenses
Original systems are expensive
* If you have an extreme wide angle lens like 14mm (Full frame) you will probably
need a filter system of 150mm width. These systems can usually be found for your specific lens in known filter companies like Haida, Lee and Nisi.