Confused about film speeds

[frusciante]

Hello everybody,

I am getting a bit confused and I want to clear thing up a bit.

When a film is marketed with different EI for Daylight and Tungsten, is this because the film has different sensibility to Daylight and Tungsten light?

OR is it because it is assumed that a color correcting filter is going to block some light?

For example the classic 160T/100D film, is:

1) an 160 film both under Daylight and under Tungsten but under daylight is marketed as 100D due to the necessary filter?

-or-

2) an 160 film under Tungsten but an 100 film under daylight because it is physically less sensitive to the color temperature of daylight?

[bolextech]

Its answer #1 for most color films.

Answer #2 applies to certain black and white films such as 7278 that are deemed to be less sensitive in tungsten light as opposed to daylight.

Cheers,
Jean-Louis

[frusciante]

Thank you Jean-Louis,

So this explains why Kodak Tri-X black/white reversal, type 7266 is marketed with EI daylight 200 [without filter] and tungsten 160 [without filter].

On the other hand Ektachrome 100D is marketed as:

Daylight (5500K): 100
Tungsten (3200K): 25 (with 80A filter)

*since 80A has a film factor of 4*

Thanks again, problem solved