Yet another filter exposure compensation thread.

[JeffD]

Filter exposure question

I have the following filters, followed by the generailly accepted exposure factor in stops:

orange #16 1 2/3 stop
red #25 3 stops
Green #11 2 stops
Yellow (k2 #8?) 1 stop

I know that meters are not normally to be trusted metering through filters, but, on a whim, I used my Pentax spotmeter to meter through these a light grey carpet. I tried my sloppy test with tungsten, and also using a bluish "daylight balanced" photo flood bulb (light sources didn't make much difference, I found).

Here is what the Pentax thinks the corrections should be:

#16 2/3 stop
#25 1 stop
#11 1 1/3 stop
Yellow K2 1/3 stop

Those are some pretty radical differences over the generally accepted exposure factor! In all cases I would have considerably underexposed my scene, using the meter.

The deep red- a whopping 2 stop difference over "the norm", the orange a full stop difference, and the green and yellow differ by 2/3 stop over the general recommendation

Are my results similar to anyone else who has compared the generally accepted exposure factors, with what their meter thinks, when metering through the filter?

Again, I am aware that I can't trust my meter, and know the reasons why, I am just surprised at my differences. Especially the red filter... wow two stops difference!

If I was really anal, I'd actually be exposing film, and doing exhaustive density tests on the results... I'm not that anal, though.

Comments?

 

[Lee Shively]

Not unusual, I'd say. Your findings may also change, depending on the dominant color of the subject matter.

 

[SteveH]

Hrmm. I would go by published numbers. Honestly, do you think that a #25 - just by looking at it - is 1 stop ?

 

[JeffD]

Hrmm. I would go by published numbers. Honestly, do you think that a #25 - just by looking at it - is 1 stop ?

Um, no, obviously not. 2, maybe. Generally I am happy overexposing, if I am going to err, rather than underexpose. So, adding three stops doesn't really bother me.

 

[glbeas]

Those differences are mostly caused by the difference between the Pentaxes spectral sensitivity and that of the film you would be using. Also know that if you use different films the corrections will be a little different, especially towards the red end of the spectrum. The old Panatomic would need 4 or 5 stops correction as opposed to 3 stops for Tri-X and maybe less for something like SFX which has an enhanced red response.

 

[JeffD]

Those differences are mostly caused by the difference between the Pentaxes spectral sensitivity and that of the film you would be using. Also know that if you use different films the corrections will be a little different, especially towards the red end of the spectrum. The old Panatomic would need 4 or 5 stops correction as opposed to 3 stops for Tri-X and maybe less for something like SFX which has an enhanced red response.

Thanks Gary!

I use mostly Tmax 400, which, I am told, is a little weak as to red sensitivity (can anyone confirm this?). Maybe I should add a little more exposure two the generally accepted 3 stop figure. I don't use that red filter a lot- next time I'll try and see if I am getting the expected density where I place my shadows, to see if I am exposing enough.

I guess, if anything, my little experiment showed me how wrong it is to try and meter through my filters!

 

[Lee L]

Thanks Gary!

I use mostly Tmax 400, which, I am told, is a little weak as to red sensitivity (can anyone confirm this?).

See: http://www.kodak.com/global/en/professional/support/techPubs/f4016/f002_0511ac.gif
There is a downward tend in sensitivity toward the red end of the spectrum.

The real answer about how to compensate for filtration has to include your meter's spectral sensitivity, the light source you're shooting under, the color of your subject, and where you want the metered subject to fall along the scale of black to white, so there's no single answer to your question. I'd suggest something like a few test exposures with a Gretag-MacBeth Color Checker and the film, filter, meter combination you're going to use. Testing, I know, but it's the only way to get a good answer for your purposes.

Lee