Which filter factor to use?

[PViapiano]

I have a B+W light red filter equivalent to a Wratten 25. The B+W info for the filter says the filter factor is 5, while the 320TXP data sheet says to use 8.

Which factor do you regularly use? The stated factor from the filter manufacturer or the film manufacturer?

Thanks all!

Paul

[Nick_3536]

The filter maker knows thier filter. Which B+W do you have? I bet it's really closer to a 23 then a 25.

[PViapiano]

It listed like this in their catalog:

B+W Light Red Filter 090 (29)

[Nick_3536]

http://www.schneideroptics.com/ecommerce/CatalogItemDetail.aspx?CID=601&IID=3544

That's the B&W 91. It's got a filter factor of 8. I'm thinking the 90 is closer to a #23 filter.

[Jonathan Brewer]

I believe an Orange 21 carries a filter factor of x4(2 stops)............a light red 23 has a filter factor of x5(about 2 1/3 stops)................red 25 is x8(3 stops), usually the manufacterer of the filter has tested the spectral transmission of these filters, they can perform differently w/a particular film, but the importance of the filter factor is the amount of light the filter absorbs before it gets to the film.

A case in point would be Rollei infrared film that has come out recently, particularly the 120 rollfilm which I've done test on, both Kodak Hie and Rollei infrared have the same rated film speed(assuming no filtration), the Rollei film doesn't have the same sensitivity to infrared as the Kodak film, so slapping on an 89b, I'm starting with approx. a 3 stop increase in exposure over what I'd use for the Kodak Hie w/the 89b in order to get some decent results.

[Brian Ellis]

The difference between 5 and 8 is 2/3 of a stop. That's not enough to worry about, variations in the colors within various scenes will produce bigger variations than that. Filter factors are rough guides at best.

[PViapiano]

I mistyped the Wratten number for my light red filter...it's a 25 not a 29.

Thanks to all...I'm going to use the filter factor given by the manufacturer, but I also realized I could just test it with my light meter!