B&W Color Balance Under the Forest Canopy

[Vaughn]

Dan,

I have noticed that TMax 100, when photographing scenes with blue sky in them, is like using a conventional film with a yellow filter...which is about as dark as I personally like skies. An orange or red filter too way overboard for my liking of skies.

SFX is sensitive to some UV if I remember right -- if so, then one is getting some exposure from light one does not even see.

Vaughn

[dancqu]

The unseasonable wet cold weather here in Western Oregon
kept me out of the woods. Did at last make it out and to a falls
with cavernous overhanging back cut. Awesome. Threatening.
One million tons of black basalt overhead.

Five filter changes over 12 exposures, all same scene.
Processed in my 8-80 version of D-23. Dilution 1:7, 500ml,
16minutes. All came out splendidly. A good appraisal must
await printing. Dan

[dslater]

Originally Posted by dancqu

Balanced as with color films are balanced for indoor
or outdoor use; ie taken to mean tungsten or daylight.
Unless I've forgotten the terminology?

Apart from all that I'm now struggling to understand
why precisely filter factors are necessary. Any clues?
I think it may have to do with an averaging of negative
densities across the spectrum. Filtering out some portion
of the spectrum does not affect film sensitivity yet most
filters require an increase in exposure. Dan

Dan,
I believe that filters reduce the amount of light passing through them across the whole spectrum, they just reduce the complementary color more than the rest of the spectrum.
Dan

[CBG]

Originally Posted by dancqu

Apart from all that I'm now struggling to understand
why precisely filter factors are necessary. Any clues?
I think it may have to do with an averaging of negative
densities across the spectrum. Filtering out some portion
of the spectrum does not affect film sensitivity yet most
filters require an increase in exposure. Dan

Dancqu - It's not a loss of sensitivity on the part of the film. It's that less total light gets to the film. It's probably not very helpful to think of it in terms of some averaging. It's just a loss, and unless you want to reduce total exposure you must compensate for the loss.

A filter factor is designed to get you back in the ballpark when using an external - handheld - meter or the like, in fairly normal conditions. It tells you roughly what fraction of total light outside the camera is getting to the film. A 2X factor says half the light is absorbed by the filter.

Filter factors can be a conundrum. One you start thinking about them a lot, life can get more complex than you probably want.

Why?

1. Because many of us have in camera meters - life gets confusing. Were an in-camera meter truly matched in response to the film, a factor would be un-needed for most general uses. Best to simple meter externally when using strong filters.

2. Because not all subjects are "normal". If you photograph a bright red object in a bright red room - with only one blue detail you just want to turn black by using a #29, your filter factor may screw you up. The dominant red image will pass largely unimpeded, much as if no filter were present. The blue item will go black. If you bump up the exposure several stops, you will probably overexpose.

Best,

C