why do film specs start in the negative

Hi

I often wonder why the -ve value is taken for the log exposure. For example in the spec sheet of my film I see:

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why is the chart starting from -4.0 and going to something over 0?

thanks

Because the logarithm of a number smaller than 1 is negative. -2 log exposure means 0.01 lux-seconds, -1 is 0.1 lux-seconds, 0 is 1 lux-second, etc. A log scale is used because the film responds logarithmically to exposure.

The horizontal axis values are from a logarithm... so log .001 = -3, log .01 = -2, etc.

SamWeiss said:

The horizontal axis values are from a logarithm... so log .001 = -3, log .01 = -2, etc.

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<returnUnrequiredSarcasm>w o w .... a what-a-rithum really .... how do you spell that </returnUnrequiredSarcasm>

you know, I noticed the Log in the scale ... but you know, you can get positive log values as well like log of 100 ... and well I've got a lux meter which reads values like 350 in my kitchen under just fluro lighting ..

Thinking about it I guess that its because the values are read at the film (and when combining shutter and aperture that what my light meter tells me is a a 1EV scene is some decimal of lux

thanks for real the helpful answer

Ok I was coincidentally thinking about this just today. The values of lux-s, or actually log(H), are measured at the film plane like you said. So whatever the actually luminance of your scene is, it's going to be a lot less once you are talking about actually light reaching your film. Where the density rises 0.1 above the film base + fog (fb+f) in terms of log(H) is determined by the ISO speed, by H=.8/ISO. Take the log of that, and that's where your film density = fb+f+0.1.

I was curious as to where the nominal metered value (of 18% gray) is. Turns out it's at fb+f+0.1 + 1.0. Or 3 1/3 stops up. So you basically get 3 stops below and 6-7 above, depending on the film.

I wrote some of this up quickly and attached a pdf of it if you're interested. Of course, it could all be wrong, but I think I got it right

Tim Gray said:

I was curious as to where the nominal metered value (of 18% gray) is.

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Tim,

No pun intended.
Not to be negative. Again no pun intended. Actually it was intended.
But do you know that the gray card is exactly the same color as the sky most days in Rochester New York?

Steve

Hahaha. That's funny if true. But no I've not heard that.