Velvia 50 ISO sensitivity

My Ektachrome E100G stash is running low, so it is time to switch to the next best thing, which is Velvia 50 (RVP50). Since I shoot only 2 or 3 rolls before, I do not have much experience shooting this breed of Velvia. That's why I've researched online for tips. I came across several statements, that this film is indeed ISO 40 film. I thought I need to look into it.

 So, how the ISO speed is determined? It is actually quite simple. First step is to chart the characteristic curve. Fujifilm already done that, of course, and published it in the Velvia 50 Datasheet.

 

Next step is determining the speed point. The procedure is outlined in ISO 2240:2003 document. The speed point for chrome film is in the middle between two points: the point on curve which density is 0.2D greater than minimal density (lower red line above), and the point on curve which density is 2.0D greater that the first point (upper red line above). I've read from the characteristic curve, that the speed point has log H (x axis) value of Hm=-0.68.

The last step is to put the log Hm of the speed point into the equation: S=10/Hm, so: S=10/10^-0.68=47.9

It is worth to note that for every Fuji and Kodak E-6 curve I've checked, the speed point lays on curve at 1.0D density.

Here it is, the Velvia 50 is indeed ISO 50 film. But take a closer look into the central portion of the curve:

 

The curves for each base color don't overlap at the speed point location, there is a slight speed difference between red and green curve. Let's check it out:

Sred=10/10^-0.65=44.7

Sgreen=10/10^-0.7=50.1

 Assuming the datasheet is accurate, it may be a reason why some people use ISO 40 setting, especially when there is a lot of red color in the picture.

But there is more.

There is another crucial piece of equipment needed to expose the film. The lighmeter. This device is calibrated according to another ISO document: 2720:1974. Why is this important? Because the lightmeter manufacturer can choose certain calibration constants according to their own statistical tests. Full explanation is outlined in an excellent paper: Exposure Metering by Jeff Conrad, and I urge you to read it.

Jeff's paper contains the equation (equation 18) explaining relation between speed point exposure (Hm) and lightmeter exposure (Hg) for the chrome film:

Hg=bKHm/10, where K is lighmeter calibration constant, and b is constant accounting for light loss in the lens (b=0.728).

Each manufacturer uses different K value for its meters. Here is the list I've searched online:

Sekonic: K=12.5

Gossen: K=11.37

Minolta, Pentax, Polaris: K=14.0

Lets calculate Hg/Hm ratio for each manufacturer:

Sekonic: Hg=Hm*0.728*12.5/10 => Hg=0.91*Hm

Gossen: Hg=0.83*Hm

Minolta: Hg=1.02*Hm

Now, lets calculate film speed again, using Hg instead of Hm:

Sekonic: 10/(0.91*10^-0.68)=53

Gossen: 58

Minolta: 47

So, Sekonic and Minolta users are almost spot on. Gossen users, like me, underexpose chromes slightly, because the meter thinks the film is more sensitive than it is. The difference is log2(0.83)=-0.27 EV.