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RMS Value. High or Lower number means less grain?

Charles Angelis , Feb 25, 2004; 02:37 p.m.

I have noticed several films with RMS ratings of 10, 8 and 7. I understand it is a measurment of grain on the film. But what I'm not sure about is which direction is better on this scale? Does a 10 represent LEAST possible grain? Or does 1 represent LEAST possible grain? The slide films I was looking at to find RMS values were Fuji Sensia (RMS 10), Fuji Astia (RMS 7) and Fuji Provia (RMS 8).

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Hans Beckert , Feb 25, 2004; 03:01 p.m.

The lower the number, the less graininess...

Ellis Vener , Feb 25, 2004; 03:34 p.m.

Hans is right, the lower the number the smaller the grain. Fuji Astia 100F (this is the current version, not the original Astia) has the smoothest grain structure of any film I have worked with, and is also the "sharpest' film I have worked with. Sharpness is a perceptual thing, part of of it is grain, another part is resolution of detail, and another part of it is the balance between distinction between distinctly different tonalities and rendering of the gradation between similar tonalities.

R.T. Dowling , Feb 25, 2004; 04:11 p.m.

It is worth noting that fine grain does not always equal sharpness. Fuji Velvia 50, for instance, has an RMS rating of 9, compared to Provia 100F, which has a rating of 8. However, Velvia 50 is the sharper of the two films.

Jean-Baptiste Queru , Feb 25, 2004; 08:32 p.m.

RMS is purely a measurement of the average noise, not of grain size. The smaller the value, the less noise. It also has nothing to do with sharpness.

Hans Beckert , Feb 25, 2004; 10:44 p.m.

Ellis Vener , feb 25, 2004; 03:34 p.m. "Hans is right, the lower the number the smaller the grain. Fuji Astia 100F (this is the current version, not the original Astia) has the smoothest grain structure of any film I have worked with, and is also the "sharpest' film I have worked with."

Kodachrome films are sharper than E-6 films. Look at the graphs. Astia is fine-grained all right, but nowhere near as sharp as K64.

Andrew Somerset , Feb 25, 2004; 10:47 p.m.

Jean-Baptiste is right, but "grain" in a print is not so much a function of the size of the silver grains, as it is the variability in their sensitivity to light.

Granularity, which strictly speaking is what you measure with an RMS number, is a measure of that variability (i.e. noise) under controlled conditions. Each step on the scale represents a twofold decrease in granularity, so Provia is half as granular as Velvia.

Ultimately, grain is subjective -- how grainy the picture will look depends on the subject and exposure conditions.

Incidentally, the latest Fuji data sheets show an RMS granularity of 8 for Sensia.

Jean-Baptiste Queru , Feb 25, 2004; 11:18 p.m.

RMS isn't a logarithmic scale, it's a linear scale. RMS means "root mean square", i.e. it's the square root of the (linear) average of the square of the noise, also known as quadratic average. It's a way to measure how "far" "on average" a measurement is from the intended value, in a way that gives more weight to the measurements that are furthest.

e.g. if the target measurement was 1000 and you took 100 samples, with 50 samples returning 999 and 50 samples returning 1001, the RMS is 1. Similarly if you got 99 samples returning a perfect 1000 and on sample returning 1010, e.g. because of a bad pixel, you'd get the same RMS.

RMS is used a lot because it allows to accurately model white noise (pure uncorrelated noise).

There's no such thing as a negative RMS.

Of course, the amplitude of the noise doesn't tell about the quality of anything you much unless you know the amplitude of the signal.

Andrew Somerset , Feb 26, 2004; 12:03 a.m.

I'll take your word for that. Somewhere I htought I had a reference that says one step represents a twofold increase, but I'm probably wrong.

Jean-Baptiste Queru , Feb 26, 2004; 12:12 a.m.

Andrew: you might be confusing with a signal-to-noise ratio expressed in dB, where an increase of 6dB means that the amplitude means that the amplitude of the noise is doubled (i.e. one bit).

(by definition, 10dB means that the noise carries 10 times as much energy).


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