Use the GND filter which allows you to achieve the best balance in light between the lightest and darkest in your scene. If you are unsure of what strength would give you the best result for your image, give yourself some time to experiment with different filters at any given scene. These are some basic recommendations which you can modify to suit your own style:
GND4 | 2 Stop
2 stop GNDs are the ‘gentlest’ filters and should generally be used when shooting away from the light.  They are best used when the light is diffused such is in cloudy or misty conditions when the sky is part of a composition.
The choice between a hard, medium and a soft edged GND has been discussed in the previous section. When shooting into the light, a 2 stop GND of any sort will generally not block enough light from the sky relative to the foreground to allow capture of a scene’s dynamic range in one exposure.
GND8 | 3 Stop
3 stop GNDs are the most commonly used GNDs and are a good starting point for beginners. They will give similar if not more dramatic results when used with scenes described for a 2 stop GND. This strength of GND is best utilised for conditions where the sun is breaking through gaps in the cloud giving vibrant colours and light but little direct light.
Once again the choice between a hard, medium and a soft-edged GND has been discussed in the section above.
GND16 | 4 Stop
4 stop GNDs (and stronger) can be used when light conditions are very harsh such as when skies are predominantly clear with only a few clouds in the sky or shooting into the brightest part of the sky with as Reverse GND.
A soft edged GND works very well shooting away from the sun but may also be strong enough in some situations shooting into the sun. When using a Reverse GND of this strength only shoot into the direct light or brighter concentrations of light at sunrise/sunset.
Be aware that the foreground may appear unnaturally bright relative to the sky. This can either be corrected in post-processing, or with a second soft edged GND placed upside down in the filter holder. Avoid using a hard-edged GND of this strength when shooting away from the light as the sky will nearly always be severely underexposed.
