filter factors: measuring them using a lightmeter?

[Havoc]

I'm slowly collecting b&w filters. Right now I have the basic stuff sorted with a dark green, orange, yellow-green and 2 kinds of yellow. But those filter factors keep me mystified. Added to the problem is that they came without notice and the markings don't correspond with what I find on the net when I google.

So I tought I could perhaps measure them with a light meter. Woudl this be a good enough procedure:
- I wait for a day with nice even sunshine
- I put my Sekonic in incident mode and make a kind of tube in black paper with just the bulb poking in
- I measure the light as it is
- put on the filter and take another reading
- difference is the filter factor

Would this be a way to work and give usable results?

[Michael Clark]

Kind of, I do it once in a while, but its best to use the filter factor that came with the filter, the reason being is the meter color sensitivity may not match the color spectrum the film does.

[Maris Rusis]

Use film to determine filter factors. Shooting a series of stepped exposures through the filter in question will find the negative you prefer and your careful notes will reveal the exposure compensation, alias filter factor, required.

Also take a meter reading through the filter of the test subject. The difference between the filter factor suggested by the meter and the real filter factor discovered by the film is the meter's "fudge factor". For example my Sekonic L-758D meter suggests 2 stops for a #25 red filter but typical panchromatic film requires 3 stops. Hence the red filter fudge factor for this meter is "add one stop".

An additional confounding factor is the subject matter. For example a sunny-day front lit landscape through a #25 red filter needs the usual 3 stops extra BUT if it is a back-lit landscape with important shadow detail illuminated by blue light from an open sky then 4 stops extra is a better decision.

The rabbit hole goes deeper. Filter factors vary according to the quality of the illumination: daylight versus tungsten versus fluorescent versus led .... testing, testing.

[Two23]

I test them first on a digital camera, then write the result on the rim with a permanent marker.


Kent in SD

[Pfsor]

But those filter factors keep me mystified.

Why?

[Corran]

Check to see if the filter factors are given on your film's datasheet. Then use that. Metering through the filter gives erroneous exposure with many filters due to the difference in metering different colors. When it doubt, give a little more exposure than you think, and consider the light temperature when shooting through certain filters at certain times of the day.

[Light Guru]

If you have an iPhone check out the app Reciprocity Timer.


Sent from my iPhone using Tapatalk

[Doremus Scudder]

I'm slowly collecting b&w filters. Right now I have the basic stuff sorted with a dark green, orange, yellow-green and 2 kinds of yellow. But those filter factors keep me mystified. Added to the problem is that they came without notice and the markings don't correspond with what I find on the net when I google...

You've got a lot of good advice so far on testing. However, if you are having trouble identifying the filters you have and finding the appropriate filter factors, then ask here: Unfortunately, there are several systems of filter identification from different manufacturers, which can be confusing. Also, some filters have factors listed as true factors while others indicate how many stops to compensate with.

What brands and what designations do you have? We can likely come up with the right factors for whatever filters you have.

On the topic of filter factors in general: Filter factors given by manufacturers are average values based on "normal" scenes and "average" daylight. Once you get a good idea of how filters work, you'll likely be modifying the factors for specific situations. Maris' example of adding another stop of exposure for a #25 filter when important shadows are lit with blue skylight is a perfect example. I use "fudge factors" like Maris described and meter through my filters. Others calculate exposure and then apply the filter factor. Both methods work just fine. There's a lot of info here and on Photrio about filters and factors. Have fun researching.

Best,

Doremus

[tgtaylor]

About a week or so back I tested my Schneider 4'X4" UV-410 (Haze I) glass filter by taking a reading off a grey cark with a Sekonic L-758DR meter perpendicular to the grey card and then placing the filter on top of the card and taking a reading. The meter recorded a 1/10 stop factor for the filter which is rounded off to 0 factor.

Thomas

[Havoc]

Why?

Well, for instance I never can get it is a factor of 1.5 means you have to multiply the stop by 1.5 which is what a "factor" is, something you multiply another number by or if it means adding another 1.5 stops. But from what I read you have to add 2/3 stops which makes hardly sense in relation to 1.5. A filter with a factor of 1.5 transmits only 67% of the light which is about 2/3's, it is a 1/factor relation, not a multiplication.

But back to what I got as filters:

- Izumar yellow K1
I find it is like a Wratten #6 but no factor

- Kinor darker yellow Y2
Hoya puts a Y2 (alternate Y) with factor 2.

- Prinz yellow-green YG
According to the Hoya site a yellow-green is X0 (YG) factor 2.5. But the photo at the same page shows a G(X0) for this filter!

- Prinz dark green X1
Find it as a Wratten #58 "green" factor 4. But on the wikipedia entry for Wratten filters a 58 is "tricolor green" with alternate designation "B". Hoya also puts X1 as green with factor 4.

- Prinz orange G
This is the alternate designation for a Wratten #15 "deep yellow" factor 1.6. But it is clearly orange. And to my eye it is a lot darker than the YG which has a factor 2.5.

Do you now understand why I have no idea where to start with those filters?

Ok, from what I got here I'll just "measure" them with the lightmeter and use that as a sensible point to start. Noting the exposure, maybe take 2 shots, one with and one without and see after develpment what comes out.