Speed point of color negative?

[steven_woody]

hi,

 

i'v already know the speed of a B&W negative is 0.8/H, where H is the

exposure in lux-seconds which yieds film density of 0.1 above the

density of film base plus fog. but, how about if consider color

negative? you see the problem, there are three dye for a color

negative. should to measure the overall dendity with all dyes add up?

and, is the magic number still 0.1?

 

thanks.

[ronald_moravec1]

It is done on the red layer. sorry I don`t remember specifics.

[helenbach]

ISO colour negative speeds are determined by taking the exposure for Status M 0.15 above fb+f in each of R, G and B then using

 

Hmean= (Hgreen x Hslowest)^0.5.

 

Best,

Helen

[helenbach]

Using a speed point at the toe is not the best way to determine practical speed in all cases. One of the other methods is aim density for an 18% grey card exposure. This pegs the midtones (which are often the most important tones) in the place recommended by the film manufacturer.

 

Kodak and Fuji both quote recommended densities for a grey card exposure in the red-sensitive layer for still colour negative film in their published literature. It is usually between 0.7 and 1.0. So in that sense red is an answer. Both companies will give you the recommended densities in all three layers if you give them a call, and in that case you get the speed in all three and then take a vote (eg if you get 320, 320, 250 use 320) and that's the preferable way to do this method, but the red-only way usually gets to within half a stop. (I use aim density it as my normal method for cinematography) Aim density speed isn't an ISO film speed, it's an EI.

 

Best,

Helen

[steven_woody]

hi, Bach

 

ok, i get how to computer the mean speed from speed of each color. but what the formula to get each speed of individual colors? say, 0.15 above b+g density on red needs exposure of x, what's the speed of red dye? is there a formula so as to that in B&W ( 0.8/exposure ) ?

 

thanks.

 

-

woody

[helenbach]

Hi Woody,

 

If you wanted to do that, then just use the exposure that gives Status M red (or whichever layer you want to measure the speed of) 0.15 above fb+f instead of using the ISO formula.

 

Best, Helen

[helenbach]

If you want the speed formula itself, it is S = root2/Hmean for the arithmetic speed.

 

Unlike B&W film speed, there is no fixed contrast (ie 0.8 deltaD for 1.3 deltaLogH) - it is up to the manufacturer to specify the process used. They should also specify the type of illumination used (eg tungsten, 'daylight' etc)

 

Best, Helen

[steven_woody]

thanks!