angeloks
Well-known
Hi,
I just got myself an Heliopan orange filter for my CV 50mm f2.5. It's written on its side '4x -2' and now I was wondering what is the proper exposure correction factor? Anyone could clear this for me?
I just got myself an Heliopan orange filter for my CV 50mm f2.5. It's written on its side '4x -2' and now I was wondering what is the proper exposure correction factor? Anyone could clear this for me?
payasam
a.k.a. Mukul Dube
Orange filters are on the deep side. If 4x means two stops, that sounds about right.
richard_l
Well-known
4x means that the filter reduces the light reaching the film by a factor of 4. I.e., 1/4 of the light passes through the filter. Stopping down 1 stop cuts the light by 1/2. Therefore stopping down 2 stops cuts the light by 1/2 of 1/2, or 1/4. So a 4x filter cuts the light by 1/4, or 2 stops. To compensate, open up 2 stops.
Similarly, an 8x filter cuts the light by 3 stops. (1/2 of 1/2 of 1/2 is 1/8.) In general, for a filter factor of Nx, the equivalent number of stops is equal to the base 2 logarithm of N.
Richard
Similarly, an 8x filter cuts the light by 3 stops. (1/2 of 1/2 of 1/2 is 1/8.) In general, for a filter factor of Nx, the equivalent number of stops is equal to the base 2 logarithm of N.
Richard
angeloks
Well-known
That's what I was thinking. But I still wonder why there is the x4 mark and the -2...
Thanks for your help!
Thanks for your help!
payasam
a.k.a. Mukul Dube
I can only guess that they're two ways of saying the same thing: 4x is the factor and -2 is the number of stops compensation needed.
markinlondon
Elmar user
payasam said:
Heliopan seem to go in for this double labelling. My UV filters have "1x -0" on them and my yellows have "3x -1.5". A bit confusing at first.