What are NEUTRAL density (ND) filters?

Neutral density filters are in one, odd way similar to close-up lenses, in that they come in different strengths (in the case of close-up lenses it's "DIOPTERS", while neutral density filters are rated in "DENSITY") -- AND, their different strength designations vary from one manufacturer to another.  In short, whether we are talking about neutral density filters or close-up lenses, people can become needlessly confused. For example, one neutral density filter maker might label a filter as "ND2", while another might label the exact same filter as "ND100".  Likewise, an "ND2" filter from one company might be completely different from another company's "ND2" filter.  Is an NDX2 filter the same as an ND2X filter?  Your guess is as good as mine!  

Here is a sample of the many ND designations that you may see -- but all mean the same thing:


Light loss
in f-stops

Density

NDX

XND

Density

Light
Intensity

Fraction
of Light

-0.5f

0.15

NDX1.5

1.5XND

ND.15

75%

3/4

-1.0f

0.30

NDX2

2XND

ND.3

50%

1/2

-1.5f

0.45

NDX3

3XND

ND.45

37.5%

3/8

-2.0f

0.60

NDX4

4XND

ND.6

25%

1/4

-2.5f

0.75

NDX6

6XND

ND.75

18.75%

3/16

-3.0f

0.90

NDX8

8XND

ND.9

12.5%

1/8

To top it off, there are different styles of neutral density filters.  They are used for different situations and purposes:


black

SOLID

GRADUATED

ROUND

SQUARE or
RECTANGULAR

CENTER,
CENTRAL,
or CENTERED

black

And let's not forget that neutral density filters, of whatever type, style and density, come in different SIZES -- from very small to very large!

Here is a link to a well-written article from B&H PHOTO that has lots of the basic information about neutral density filters -- so I won't repeat it here:

https://www.bhphotovideo.com/explora/photography/hands-on-review/a-guide-to-neutral-density-filters

Another good summary -- an explanation of most of these crazy, confusing numbers & designations -- can be found at:

https://shuttermuse.com/neutral-density-filter-numbers-names/

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