Neutral Density Graduated Filters
Quite often the correct exposure for a background in a scene is not the best one for the foreground or viceversa. The most common problem is that the bright sky is reproduced perfectly while the landscape is underexposed; in fact pitch black most of the times.
These are the occasions where the color and neutral density (ND) graduated (grad) filters can make the difference between a bad image, a good image and a better one.
As per these sunset examples, the problem can be solved by placing a "grad" filter, neutral density or colored, in front of the lens.
The brightness of the sky is then reduced without affecting the correct exposure of the landscape.
With no filter
The best way I’ve found to select which filter to use is to spot meter foreground and background, to determine the number of f/stops difference.
The image at right was made for illustration purposes with a 0.6 grad, knowing it required a 0.9 one.
With 0.6, 2 f-stops grad ND filter
Once the filter needed is chosen and placed, you can safely switch to Matrix Metering.
Apertures of f/11 or f/16 are usually preferred to have the best DOF, without the risk of diffraction and possible chromatic aberration when the lens is fully closed down to f/22, in fact reducing sharpness.
With 0.9, 3 f-stops grad ND filter
There are times when two filters are required, combined, like in the sunrise image at right, which needed both a 3 and a 2 f-stops graduated density filters, the second one placed diagonally.
Hard edged transition filters are good for shots where there is a very well defined, straight line transition, like in sunsets or sunrises over water as in the examples in this page; or when you are using a telephoto lens which will soften a hard filter transition. For all other scenes soft edged filters are preferred since they allow for positioning errors. Just remember that the ND value is not reached at the soft edge but further "up".
COLOR GRADUATED FILTERS
The French photographer Jean Coquin, invented the concept of creative filter photography, meaning "creating colors and effects through filtering."
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