Many filter types are available in a range of "grades" of differing strengths. This allows the extent of the effect to be tailored to suit various situations. The grade-numbering systems may vary with manufacturer, but genrally, the higher the number, the stronger the effect. Unless otherwise stated by the manufacturer, there is no mathematical relationship between the numbers and the strengths. A grade 4 is not twice the strength of a grade 2. A grade 1 plus a grade 4 doesn't add up to a grade 5.
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