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1. Originally Posted by Bruce Osgood
Maybe I got it now....

If the filter factor is 3 and the correct number of stops is 1.74, then the factors' Sq Rt equals the number of stops: Sq Rt 3 = 1.74

By jove, I think I've got it!
No, it's not the square root except in the case of filter factor 4, which is a 2 stop correction.
It's actually 2^s = ff, where s=stop correction and ff= filter factor. In other words, two to the power of the stop correction = the filter factor.
2 to the power of 2 = 4
And 2 to the power of 3 = 8 (2 x 2 x 2). (8 is the filter factor, 3 is the stop corrrection)
It's best to find a table with the equivilants, or make one yourself with a calculator that will do roots. Finding 2 to the power of ? = 5 (for instance) in your head ain't easy. Or 2f/2f's method will get you close.

2. Originally Posted by Michael W
I would give 3 stops more than the meter reading.
Does the filter have the filter factor engraved on the ring? If it says "x8" then that confirms 3 stops.

I actually dont know I have yet to purchase it

3. I couldn't find the two-stop recommendation on the Tiffen site within a few minutes looking.

x8 (three stops) used to be printed on the slip of paper you got with your typical filter.
-If you want to keep it simple you can use that factor and be fine.

Adrian mentioned the instructions and chart from Gordon Hutchings.
-In the specific example of the 25 Red filter, AFTER reading through the filter, you open up an ADDITIONAL two stops.

Hutchings method sounds like a good way to go.

4. Here's a table of filter factors and the corresponding f/stop correction:

Fact..Stops
1.0...0.0
1.5...0.6
2.0...1.0
2.5...1.3
3.0...1.6
3.5...1.8
4.0...2.0
4.5...2.2
5.0...2.3
5.5...2.5
6.0...2.6
6.5...2.7
7.0...2.8
7.5...2.9
8.0...3.0
8.5...3.1
9.0...3.2

This derives from the relationship between factor and stops thus: factor = 2^(stops).

So the factor for 1 stop = 2^1 = 2; the factor for 2 stops = 2^2 = 4; the factor for 3 stops = 2^3 = 8; the factor for 4 stops = 2^4 = 16.

Going the other way, stops = log(factor)/log(2).

A filter factor of 3 corresponds to a correction of about 1 2/3 stops (actually 1.585).

B+W shows a filter factor of 5 for their light red (25) filter, which is about 2 1/3 stops (2.322).

- Leigh

5. According to this article on wikipedia, 25A has a factor of 5, which is slightly over 2 stops http://en.m.wikipedia.org/wiki/Filter_factor , i have hoya and opening up roughly 2 stops worked fine for me.

6. Originally Posted by Helinophoto
According to this article on wikipedia, 25A has a factor of 5, which is slightly over 2 stops http://en.m.wikipedia.org/wiki/Filter_factor , i have hoya and opening up roughly 2 stops worked fine for me.
awesome this was great ! Thanks everyone for the input

7. Originally Posted by Helinophoto
According to this article on wikipedia, 25A has a factor of 5, which is slightly over 2 stops http://en.m.wikipedia.org/wiki/Filter_factor , i have hoya and opening up roughly 2 stops worked fine for me.
And according to this one (http://en.wikipedia.org/wiki/Wratten_number) 25A has a filter factor of 8. Thus my comments to check the recommendations of each filter manufacturer.

8. Another point to consider is that monochrome contrast filters will not have the same factor with all films.

It varies with the spectral sensitivity of the individual film. That's particularly true at the red end of the spectrum.

- Leigh

9. As stated it's very dependant on film and film speed/developer, the latitude of the film will also take care of minor differences between makes of filter I tend to use +1 stop for yellow, +2 for orange and +3 for red. Seems to work okay.

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