Kodak Characteristic Curve - Log H Ref equals middle gray?

Oliver Roch · Aug 28, 2010

Can anyone please explain me the value of Log H Ref given in most of Kodak Tech. Pub. Characteritic Curves? E.g. Ektar 100 E-4046 curve gives a Log H Ref of -0.84. Is it middle gray when shooting at box speed? Isn't middle gray normally placed at log 0?
Compared to E-4040 Log H Ref -1.14 the Portra 160 VC seems to handle an enormous overexposure of nearly 7 stops.

markbarendt · Jul 8, 2012

Old thread old unanswered thread, but I was searching for the same answer this morning.

Found this: http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

Page 6 seems to have the answer.

Log H seems to be defined as "Exposure", I'm going to make the assumption that this essetially means "the camera setting". That seems to fit the curve, leaving room for about 3-4 stops down to the toe.

Portra 160 http://www.kodak.com/global/en/professional/support/techPubs/e4051/e4051.pdf

Portra 400 http://www.kodak.com/global/en/professional/support/techPubs/e4050/e4050.pdf

I checked the TMax tech pub and it lacks the reference point.

http://www.kodak.com/global/en/professional/support/techPubs/f4016/f4016.pdf

So a couple questions crossed my mind.

First question, asked three ways: How is zero defined? What does zero match in the real world? Is zero a reference for white in a print?

Second: Why does the B&W curve lack the reference?

Open to other thoughts here too.

markbarendt · Jul 8, 2012

Oh and am I right in my definition of Log H?

Bill Burk · Jul 8, 2012

It's 10 to the power of 0 = 1. Given as lux-seconds.

Log H is clearly explained on Page 6 of the workbook you linked... Yes it is "time-light" that you control when you change f/stop and shutter speed.

Bill Burk · Jul 8, 2012

I didn't answer the part about Log H Ref...

markbarendt · Jul 8, 2012

So we have "exposure" understood but:

"We" are still confused on what "0" represents.

"We" are still confused about why "0" is at the thick (variable) end of the negative.

Bill Burk · Jul 8, 2012

Zero is 10 to the power of 0, which is 1, a whole lux second. So -1 and -2 are like tenths and hundredths of a second. More like the exposure that would be used on film...

Photo Engineer · Jul 8, 2012

Hah, Bill, beat me to it while I was looking at the refs to refresh my memory. Good thing I refreshed the screen first.

PE

markbarendt · Jul 8, 2012

Cool, I get it.

So, in layman's terms, zero essentially defines a standard amount of illumination. A whole lux second.

That explains the longer curve left of zero for Portra 400, vs 160, and the difference in offset from 0 to Log H.

Thanks

Bill Burk · Jul 9, 2012

Hey Mark,

You're pretty much right...

Just in case it hasn't sunk in yet. The scale of the x-axis is logarithmic. So it goes from left to right like 10 to the minus 3, 10 to the minus 2, 10 to the minus 1, 10 to the zero power, 10 to the first power. This works out as 0.001, 0.01, 0.1, 1, 10.

markbarendt · Jul 9, 2012

Thanks Bill.

My confusion wasn't actually with the scale. I simply didn't understand what the definition of the anchor point itself was; what it meant, why Kodak was starting the scale in the highlights.

You said "pretty much".

Is there something wrong with the definition of zero as 1 lux second of exposure?

Is there something wrong with thinking of Log H as the intended camera setting, and by inference, the equivalent of an incident meter reading?

ic-racer · Jul 9, 2012

Anyone??

My guess is that the reference exposure indicated at the top of each H&D curve is the exposure which will give the appropriate gray card density each film. Or the exposure that a "KODAK Gray Card (gray side) receiving same illumination as subject" produces on the film when the film is correctly exposed.

Example: Ekatar 100, an exposure to the film of Log H -0.84 should produce a density of 0.77 to 0.87 (correct gray card density)
Example: Elite 200, an exposure to the film of Log H -1.14 should produce a density of 0.8 to 1.0 (correct gray card density)

markbarendt · Jul 9, 2012

Kodak defines Log H as "exposure", not as a reference point.

Kodak's grey card instructions include using one of several offsets to find the actual "exposure" so I'd say the grey card value itself isn't the target.

Photo Engineer · Jul 9, 2012

The target is a step wedge. The exposure is a set value that will be equivalent to what results in the lux values shown on the X- Axis. If exposure time varied, then you would have reciprocity enter into the equation.

PE

Bill Burk · Jul 9, 2012

markbarendt said:
You said "pretty much".

Is there something wrong with the definition of zero as 1 lux second of exposure?

That part is exactly right. I just wanted to emphasize that the horizontal units aren't linear.

markbarendt · Jul 9, 2012

Photo Engineer said:
The target is a step wedge. The exposure is a set value that will be equivalent to what results in the lux values shown on the X- Axis. If exposure time varied, then you would have reciprocity enter into the equation.

PE

As a practical matter, in normal daylight conditions, will an incident meter used normally give us Log H/exposure?

Photo Engineer · Jul 9, 2012

Mark;

IDK really. I know that the light used in the exposure of the examples made by EK is adjusted for color temperature for either daylight or tungsten. Meters generally are calibrated for either illuminant (AFAIK) but it depends on the scales on the meter as to what units you can derive from them. It will take some degree of calculation to get Log H / exposure from the meter. Going from a curve back to the meter reading might be easier.

I've usually found that the conversion between the EK curves and reality are useless. The curves can be internally compared and the "real curves" can be compared. In this instance, "real curves" = pictures or a step wedge exposed in-camera. i have used this method in film work as well as the "backward calculation" that I mentioned in the last paragraph.

PE

markbarendt · Jul 9, 2012

Thanks PE.

ic-racer · Jul 9, 2012

markbarendt said:
Kodak defines Log H as "exposure", not as a reference point.

Kodak's grey card instructions include using one of several offsets to find the actual "exposure" so I'd say the grey card value itself isn't the target.

Kodak does not define "Log H Ref"

I'm pretty sure "Log H Ref" to be an exposure reference point. Specifically, the exposure produced by a gray card when the film is exposed per manufacturers instructions. EDIT: I thought it might be speed point but see below for ISO 5800.

markbarendt · Jul 9, 2012

ic-racer said:
Kodak does not define "Log H Ref"

I'm pretty sure "Log H Ref" to be an exposure reference point. Specifically, the exposure produced by a gray card when the film is exposed per manufacturers instructions. In short it is the 'speed point' as color negative films are not rated the way we are so accustomed to with B&W films. ISO speed for color reversal film is determined from the middle rather than the threshold of the curve.

http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

See page 6

Also see PE's answer above about it being tied to step wedge and specific time.

ic-racer · Jul 9, 2012

markbarendt said:
http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

See page 6

Also see PE's answer above about it being tied to step wedge and specific time.

Nothing about sensitivity of color negative films on page 6. Color negative films are dealt with on page 14 but as you can see it is incomplete.

Photo Engineer · Jul 9, 2012

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

ic-racer · Jul 9, 2012

Just to follow up on PE's post, here is ISO 5800 (Speed of Color Neg.): http://bzwxw.com/soft/UploadSoft/new4/ISO--5800-1987.pdf

Photo Engineer · Jul 9, 2012

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

Bill Burk · Jul 10, 2012

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.

Kodak Characteristic Curve - Log H Ref equals middle gray?

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