Cinematography Mailing List - CML




RGB No.’s On Short End Recans

Published : 9th February 2005

I have a few cans of short ends in my fridge and never understood how to interpret the red, green and blue values given by the place that sold me the stock. For example, one can of 5218 has R:19, G:58, and B:89.

What exactly does this tell me? Should I compensate exposure based on this info?


Nick Anthony
Aspiring DP, SF.

class="style9">>For example, one can of 5218 has R:19, G:58, and B:89.

Difficult to give a simple and generic answer on this. It all depends…

Ageing of stock is most easily measured by the increase in d-min (the unexposed film density). A typical emulsion when new might be around R0.18,G0.54, B0.88 or thereabouts, but it's not exactly the same for all stocks (Kodak and Fuji are different for starters). Aged stock also tends towards grainier shadows.

It's mostly d-mins and shadows that are affected, thereby reducing the contrast. The effect on your images depends on the exposure, and on the contrast range of the shots: daylight shots with flat lighting and no shadows will be safer than night work or high contrast scenes.

If the dmin values the lab quotes are more than -say - .10 above the d-min of similar new stock, you would be well advised to overexpose by half a stop or so. Much more than that, dump the stock.

Ask the lab (or stock supplier) how far off normal the figures are. Don't ask them to rule the stock "in" or "out". It's not their decision, and without knowing the subject matter, your exposures or your expectations, they can't be responsible. If the risk is between dumping (at extra cost to the production) stock that would have been just OK, or going ahead and getting un-useable images, it's your call. If in doubt shoot a picture test rather than relying on a set of numbers.

Dominic Case
Atlab Australia

As Dominic note, the "d-min" varies somewhat with the particular type of stock and the lab's processing conditions. When a lab runs a "clip" test, the densities are usually with reference to a sample of fresh stock of the same type run in the same process.

Some labs can also expose and process full sensitometric exposures, giving the complete characteristic curves, allowing a complete evaluation of not only fog level, but any change in speed or contrast due to aged film or improper keeping.

Although fog level can help predict any build up of graininess, running a short camera test (e.g., a MacBeth Color Checker) and viewing the results is the best way to see any build up of grain due to age or improper storage.

John Pytlak
EI Customer Technical Services
Eastman Kodak Company
Rochester, NY 14650-1922 USA

class="style9">>Some labs can also expose and process full sensitometric exposures, >giving the complete characteristic curves, allowing a complete >evaluation of not only fog level, but any change in speed or contrast due >to aged film or improper keeping.

A dip/clip/snip (!) test will either say the stock is quite OK, or totally useless, or somewhere in the grey area. It's only the grey area that's problematical.

A few labs have their own sensitometers to do these tests: however, doing them is one thing, interpreting the results is another. (Both by the lab technician and by the cinematographer). I could do a senso test and report the gamma of each layer as compared with a reference batch of the stock (if I had one). Also some abstruse computations of various esoteric speed measurements. Then what?

A visual test is often easier to interpret. If it's 35mm, a still camera is
all you need.

Dominic Case
Atlab Australia

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