Now that you know the types of GND filters, how do you use them? There are 2 ways, namely by using a filter holder and the manual method by hand.
There are several types of filter holders that are adapted to filter threads ranging from 52mm to 82mm. Of the many filters that can be installed, there are from 1 to 3 filters.
The advantage of using a filter holder is that it is easier, once set, you just have to focus on shooting. But there is a drawback, if you use a filter holder it means there is a distance between the lens and the filter, if you use a wide lens, the filter's shadow can enter the photo, resulting in vignettes.
The second way is to hold the filter in front of the lens with your hand. This method is more practical because we don't need to install and disassemble the filter holder and can place the filter right in front of the lens so that it can reduce the occurrence of vignettes. But there are drawbacks, namely that it is more troublesome because we have to hold the filter while taking photos, and the filter is prone to getting dirty from being held around. After holding the filter in front of the lens, then do metering.
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It is recommended to use Shutter Priority mode (Tv on Canon and S on Nikon). To read which type of filter you can use, you can estimate the difference between the light meter used on the sky and the ground. For example, for the sky, use a shutter speed of 1/250, while for the ground, use 1/30. There is a difference of 3 stops (250>125>69>30) so you can use the GND 0.9 filter.
You can also vary the use of the GND filter, by tilting the filter/diagonally if the horizon line is like a hill/mountain that is tilted. Apart from that, you can also combine 2 GND filters to get more than 3 stops of exposure.