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# Thread: Kodak Characteristic Curve - Log H Ref equals middle gray?

1. Basically B&W = Color Neg = Color Reversal as far as densitometry and speed measurement are concerned. A 400 speed film must be 400 speed and etc. Neutral = Equal sensitivity to all regions of the spectrum under a given light with either Daylight or Tungsten balance (temperature in degrees K).

So, given a method, it applies everywhere. The curves look different but mean the same in "engineering" terms.

PE

2. Just to follow up on PE's post, here is ISO 5800 (Speed of Color Neg.): http://bzwxw.com/soft/UploadSoft/new...-5800-1987.pdf

3. And basically the same method is used for reversal films except that you work from Dmax and not Dmin.

We had several members of that committee working in our division as you might guess. And, our "workshops" were given by these people. We called the classes "Ding Dong School" and we had to take them to be "certified".

PE

4. I've been avoiding the word "Ref".

From the label on the Characteristic Curves...

Portra 160
Log H Ref: -1.051

Portra 400
Log H Ref: -1.44

I will guess the meaning of Log H Ref:

I agree with ic-racer, it appears to be a middle-gray value. It is about 1.26 to the right of the black and white speed point (In ZS terms, figure the speed point is Zone I, this puts the "Log H Ref:" about 4 stops higher, somewhere in the vicinity of Zone V). I am looking at a graph and I can't tell with sufficient precision whether that is the incident meter aim, or the 18% gray card.

I am thrown off by the older Portra 160 Log H Ref: -1.14 because everything else is pretty close to 1.26 from the speed point. I believe it might be an old typo - (maybe the old chart had a copy/paste error from a 200 speed film). If it's not a typo, then the old Portra 160 just barely missed being 200.

5. I don't disagree with the thought that the "Ref" falls in the mid-tones.

I found this, this morning. http://wiki.magwerks.com/wiki/images...T_SP250-37.pdf

It does appear to be a measure of exposure time, in relation to a standard illuminant, and in the Kodak cases the scene is a step wedge.

What I can't find is a way to convert it to an "EV" or lux value.

6. Originally Posted by markbarendt

What I can't find is a way to convert it to an "EV" or lux value.
The curves in the quoted Kodak PDF files look to have the X axis as the log of lux-seconds, so without knowing the exposure time of the sensitometer you can't get the lux value.

7. I can break down my system and give you a practical example of how much lux there could be.

For example maybe you are trying to adjust your enlarger to a certain brightness to make a test setup...

My sensitometer is basically an electronic flash aimed up at a piece of glass with a Stouffer scale taped to the top. There's an ND filter in between, this helps make the graphs fit on paper for the films I use most ( 400 to 32 speed film).

The sensitometer has 3 choices of exposure times: 10 to the minus 2, 10 to the minus 3 and 10 to the minus 4...

So that's 1/100th second, 1/1,000th second and 1/10,000th second.

At 1/100th second (the "shutter" speed I use to test film) my unit delivers 100 lux to the film under the step that lines up with the "0" on my graph x-axis.

I have three x-axis markings on my graphs. There is the lux-seconds log marking at top (mcs = lux-seconds), the corresponding film speed just under it, and at the very bottom is the density of the step wedge.

http://www.beefalobill.com/images/sensitometry-CP1.pdf

An important thing about my top scale: I calibrated it by a simple practical method that is so obvious it's almost stupid. One of my graphs of TMY-2 happened to hit the ASA target. So I lined up the 400 where that curve crossed 0.10 above B+F.

---
Back to the "Ref" - it seems geared towards color negative material. I think it is gray card 18%, because so much literature suggests shooting a gray card and measuring its density.

There isn't much emphasis on shooting gray cards in black and white.

I'll guess it's because color film is pretty much developed for a "standard" time for the best color balance. While black and white is developed for various times to increase/decrease contrast.

So the place you check for proper exposure for black and white is down in the shadows, while the point that you can easily check for proper exposure of color negatives can be in the middle gray.

ps I work for Kodak but the opinions and positions I take are not necessarily those of EKC. In other words the stuff I do at work has nothing to do with film, when it comes to film I am a hobbyist...

8. I'm still not buying the idea that H (exposure) = the grey card reading. The grey card, even with all it's accolades doesn't read "exposure" directly.

My impression is that log H is an expression of the "real" exposure not an arbitrary reference point.

The biggest reason is that any reference point in the scene can be used to find the camera setting, with equal quality; it is simply a matter of knowing what offset to apply.

A grey card for example, given it's known status in relation to exposure settings; is truly the equivalent of an incident meter reading when done properly. The only difference in practice is the need to do a bit of Maths, applying the proper offset.

9. You are making too much of a deal over this guys!!!!!

Put it in a camera and expose it!

PE

10. Originally Posted by markbarendt
I'm still not buying the idea that H (exposure) = the grey card reading. The grey card, even with all it's accolades doesn't read "exposure" directly.
I think it's the gray card landing!

According to the instructions, you don't read the gray card - you use an incident meter.

Then you set your densitometer onto the negative image of the gray card picture you took... after the film is developed.

If it's too dense or thin according to the guidelines you adjust your EI accordingly - and that puts all the mystery of the 18% to rest.

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