Rating Tri-X?? Im confused with the packaging..

[greg_8mm]

Ok here's one thing that I don't really get, on the tri-x package it says:

DAYLIGHT (Without filter) -> EI200
TUNGSTEN (Without filter) -> EI160

But why would these values be different if you're not using any internal filter anyways? I thought the different ASA values between daylight and tungsten were to compensate for the use of the internal filter. Tri-X is a B&W stock, so you don't really need a filter, and that's what the package says.. But why would you rate the stock differently inside or outside?

[MIKI-814]

The reason is that B&W film (at least Kodak's) are a bit less sensitive to tungsten lights than they are to daylight due to UV rays etc. So in this case it has nothing to do with the filters/Kelvin degrees/colour balance etc.

[greg_8mm]

That doesn't make sense to me, i mean, photons are photons right? What you're saying is that a lightmeter can not sense the difference in intensity between tungsten and daylight whereas kodak's tri-x film can?

edit: i overlooked the uv-rays part. I didn't know uv radiation could affect film drastically enough for one to have to compensate for it..

[vidwerk]

I think I remember reading somewhere that B&W is more sensitive to the bluer spectrum(daylight), hence the higher asa.

vidwerk.

[MIKI-814]

Thinking of this, I think now that the UV-rays theory makes no much sense... Lots of people use a UV filter to protect the lens.

[alex-rus]

Any panchromatic b&w stock (like a Kodak Tri-X) has various ASA values for tungsten and daylight. The majority of automatic cameras will be automatically expose Tri-X as 160ASA ...

[greg_8mm]

Thinking of this, I think now that the UV-rays theory makes no much sense... Lots of people use a UV filter to protect the lens.

Yeah that was my first reaction too..
But it doesn't necessarily need to be UV radiation which influences the film, it could be other wavelengths of the electromagnetic spectrum. Although hard to imagine, it must be that those wavelengths have an influence on the film stock, but not on the light meter...

So if the 'problem' is the light meter, why aren't they designed to pick up the same wavelengths as film stock does?

[Jim Carlile]

You're complicating it-- tungsten light is composed of different wavelengths. The light is actually a different color-- which effects emulsions in different ways. B/W film records colors in a non-uniform way.

In the old days, film was orthochromatic, and before that it was even less sensitive to the broad spectrum, so different methods were used to get a good exposure, usually through processing tweaks.

Light meters measure through intensity, not color temperature.

[richard p. t.]

So if the 'problem' is the light meter, why aren't they designed to pick up the same wavelengths as film stock does?[/quote]

Its not a light meter sensitivity issue at all. The issue is entirely to do with the fact that the film itself has a different sensitivity to the different colour temp light sources. Modern film is panchromatic. The manufacturer has made every effort to make the film sensitive to not just blue light (like the old orthochromatic films were) but to a broad range of light frequencies. This is not a perfect science however and there is still a disproportinal sensitivity to different parts of the spectrum. The particular ratio of sensitivity to different parts of the spectrum that the manufacturers aim at is of course in part a technological issue and in part an aesthetic issue. All you need to know however is that the film has a differing sensitivity (asa) rating to different colour temperature lights.
rt

[audadvnc]

Let's step back a little bit, say 150 years. In the days of wet plate collodion photography, all film emulsion was blue sensitive - it would only react to blue, violet, or ultraviolet light. When you use cyanotype emulsion you get a feel for what the original photographers had to deal with: long, long exposures, and no semblance of even color sensitivity across the spectrum.

Long about 1890, chemists using analine dyes were able to extend film light sensitivity into the green portion of the spectrum, and produced orthochromatic film stocks.

Not until the 1920's were films produced that had much red-sensitivity at all; these panchromatic films were eagerly embraced by the movie studios, who wanted to get away from noisy blue arc lamps and standardize on silent running incandescent lamps for their sound motion studio productions.

Even panchromatic films are not even in their spectral response; tri-x, for instance, is still more sensitive to the blue end of the spectrum. Even infrared films have sensitivity in the blue end of the spectrum; that's why you have to use deep red filters in order to see the infrared effect in action.

[Jim Carlile]

To make it even more complicated, some light meters are sensitive to the whole spectrum, which means you can rely upon their settings and then rate the film at the original higher ASA.

All Kodak means by the lower tungsten ASA is that a conventional f/stop measurement might not take into consideration the color temperature sensitivity, and thus will cause a slight underexposure. So, the lower ASA means that the exposure should be tweaked upwards a little with old-fashioned meters. It's all a matter of trial and error, and the difference is really very slight, like 1/4 f/stop at the worst.