Exposure compensation with Heliopan filter

[angeloks]

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?

[payasam]

Orange filters are on the deep side. If 4x means two stops, that sounds about right.

[richard_l]

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. :eek:

Richard

[angeloks]

That's what I was thinking. But I still wonder why there is the x4 mark and the -2...

Thanks for your help!

[payasam]

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]

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.

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.