JaZ99 @JaZ99wro

Monday, February 1, 2016

Kodak Ektachrome 6121

There is something special about low contrast duplicating film. Its dynamic range is huge, but it means the picture must be postprocessed to look nice. Second problem is that it is tungsten film, which requires correction filter to shoot in daylight. CDUII - Fujifilm duplicating film - requires 85A filter. I've tried my first batch of 6121 with 85B filter, but I don't think it is a good match. Each frame required different color grading, so next time I will use the 85A filter. My test suggests that 6121 is ISO 50 film -- ISO 32 with filter factor included.
Below I show how direct scan looks like (with exposure matched to the finished picture below).

My other 6121 pictures on Flickr.
My CDUII pictures on Flickr.

Sunday, January 31, 2016

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.

Friday, January 15, 2016

Black Glacier

Shot on Provia 100F with GF670

Wednesday, January 13, 2016

Green mountain

Shot on Provia 100F with GF670

Tuesday, January 12, 2016

Axlarfoss

Shot with Fujifilm GF670 on Astia 100F

Monday, January 11, 2016

Glacier view

Shot with Fujifilm GF670 on Provia 100F

Metamerism and Scanners

Recently I've encountered a problem scanning Astia on my new OpticFilm 120 scanner. The colors were off and hard to correct. After some research it turned out that the problem was caused by the metameric failure.

The solution is fairly simple: I had to create the ICC profile for each film type I use. To do that, I ordered IT8.7 targets and created profiles using Argyll CMS software.

Below you can see the IT8.7 targets with different ICC profiles applied. Color changes are most pronounced on OpticFilm 120 scanner which uses LED light source. The color changes are less pronounced on Epson scanner which uses CCFL light source.

Plustek OpticFilm 120