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All filters block some light and most all should be compensated for.  Some filters tell you exactly how much to open up in f-stop terms... Example... +1/2 or +1.0 f-stop.  Others give you a "filter factor" and this must be converted to an f-stop value ...  Example... filter factor 1.3 indicates that you need to open-up 1/3 of a stop...  See conversion chart below...  First locate the indicated filter factor and then open up the corresponding f-stop amount...  A filter factor of 1.0 indicates a very small light loss, 5% or less, and does not need to be compensated for.
 
 
Filter Factor to F-stop Conversion
Filter Factor
f/stop
  
Filter Factor

f/stop
1.0
0
  
4.0
2
1.3
1/3
  
5.0
2 1/3
1.4
1/2
  
5.6
2 1/2
1.6
2/3
  
6.3
2 2/3
2.0
1
  
8.0
3
2.5
1 1/3
  
10.0
3 1/3
2.8
1 1/2
  
11.3
3 1/2
3.2
1 2/3
  
12.7
3 2/3
     
16.0
4
 
 "When a teacher is ready to learn, he will surround himself with students." ~ Frank J. Dispensa
             "Photography: the science is a journey, the art is a gift." ~ Frank J. Dispensa 
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