I am looking to get some ND filters for landscape photography but am totally confused on what is best to get. I have a budget of around £100 but am firstly confused whether I would be better off with screw-ons or square filter system. Apparently square is better to avoid vignetting? I have been looking at Hi Tech but there seems to be very mixed reviews of these, most mentioning that they create quite bad colour casts.
I enjoy DIY projects.
A good quality square filter holder can be used with your home-grown versions as well as both round and square store-bought filters. You can use gelatin, glass, or plastic. That way you can also buy filters intended for other equipment so long as they are larger in diameter than your lens. You don't even have to cover the whole lens for some effects that begin or end at the horizon or other straight-line in your shot.
Try building a tiny glass "tank" to fit inside the filter holder that you can fill with food dyes mixed with water (or alcohol on winter days). The dyes can be dropped into the tank and used before and after becoming evenly mixed for a myriad of unique effects.
A small removable liquid filter tank is made with two squares of glass sealed with silicone sealer and thickness spacer of plastic tubing in the shape of a "U." Leave the top of the tank open to fill with an eye-dropper and empty by removing the tank and spilling out the liquid.
Hand-held shots can be ruled-out and a good tripod is necessary for this technique; but, you should be using one with a shutter release for good landscapes anyway.
The formula for re-creating certain hues is counting 1 drop = 1/20 millilitre. When you know the volume of the tank, you can recreate your favourite colours to-go. Keep notes
Is this a hassle? YES
Is this fun? YES
Will you regret this? NEVER
The square filters are generally of better quality, and have tons of variety, and are more versatile.
But since it seems you're just starting out with them, perhaps some easier to use and cheap screw-on filters would be good to start out with. Sets of NDs and even ND graduated filters can be had inexpensively, and produce some darn good results. And are just sorta useful to have in the camera bag.
Landscapes may benefit from the use of neutral density and circular polarising filters. Neutral density filters are used to equally modify the different wavelengths of light. The purpose of these filters is to help the photographer to extend the exposure time. A common use is to freeze a waterfall... The shutter speed may be fast enough to render the waterfall as static in appearance. Extending the exposure time to a matter of seconds will blur the water beautifully and lend an air of grace and magic to the scene.
Polarising filters are excellent for looking past specular reflections and seeing the scene directly behind them. Glossy car paint, window or water reflections can be penetrated by a polarising filter. Doubled polarising filters can be used as a variable neutral density filter although using a neutral density and a polarising filter tends to produce useful results. When a polarising filter is used at a 90 degree angle to the sun, it can help to give blue skies more colour.
The only other filters which are commonly used are known as contrast filters. These lighten their own colour and darken their complimentary colour. The commonly available colours are light and dark yellow, orange, green (light and dark) red and blue. They are usually only applied at the taking stage of capturing a monochrome image.
Other filters tend to be the graduated and half colours and diffraction gratings which can produce star patterns around specular highlights such as street lamps.
Lots of cheap plastic (CR39 optical resin) filters are available. Often they can be purchased in complete sets; in sizes that have various adapters which permit attachment to 35mm and medium format lenses. Superb quality filters can be purchased in circular brass mounts by manufacturers such as Heliopan, Schneider Optics B+W MRC and Kasemann edge sealed. All use Schott glass flats and are the very best quality made. Cheap filters will affect the image and will make you wish that you had spent far more money. Any item you put in front of the lens has the potential to spoil the image.
Square/rectangular is better. Round is generally cheaper.
The biggest advantage of square/rectangular filters is with graduated ND filters. They are usually rectangular and can be slid up or down on the holder to move the line between the darker and lighter sides of the frame. This allows more compositional freedom. With a screw-on graduated Neutral Density filter you're stuck with the position of the graduation in the filter and may have to compromise your composition of a scene. Either that or you have to shoot wider and then crop to move the graduated line away from the middle of the frame and give up resolution in the process. Maximum resolution is often a major concern for landscape photographers.