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

1. 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?

2. 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)

3. 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.

4. 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

5. Originally Posted by markbarendt
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.

6. Originally Posted by Photo Engineer
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?

7. 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

8. Thanks PE.

9. Originally Posted by markbarendt
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.

10. Originally Posted by ic-racer
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/uploa...y_workbook.pdf

See page 6

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

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