RMS Value. High or Lower number means less grain?

Discussion in 'Film and Processing' started by final cut cafe, Feb 25, 2004.

  1. 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).
     
  2. The lower the number, the less graininess...
     
  3. 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.
     
  4. 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.
     
  5. jbq

    jbq

    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.
     
  6. 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.
     
  7. 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.
     
  8. jbq

    jbq

    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.
     
  9. 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.
     
  10. jbq

    jbq

    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).
     
  11. Hi Jean & Andrew (& Charles, etc.),
    It looks like you are both right, but looking at different things (like the blind men feeling different parts of the elephant). The RMS number, itself, is not logarithmic, but the RMS scale is.
    First, here's the "official" definition of RMS Granularity: "Standard deviation for random-density fluctuations for a particular film."
    http://library.thinkquest.org/10015/cgi-bin/glossary.cgi?m=RMS+granularity
    No logarithms there. But now read this simplified definition, which talks about the scale:
    "The RMS granularity rating indicates the film's relative graininess. Each successive RMS number represents a doubling of the graininess. For example, a film with an RMS 5 granularity rating is twice as grainy as a film with an RMS 4 rating. Note: RMS granularity ratings of print films and slide films are not directly comparable. As a rule of thumb, you should multiply a print film's RMS number by 2.5 to approximate its graininess compared to a slide film's RMS rating."
    http://www.photographic.com/buyer'sguides/98/
    So the scale is indeed logarithmic, because equal distances or steps on the scale represent equal ratios of increase. http://www.google.ca/search?q=define:Logarithmic+scale
    The RMS number is the same as a standard deviation, if you know statistics. It's a "mean" or average describing how fat or thin a normal curve is. It is useful because it gives a number to the dispersion or spread of a group of data -- in this case, the fluctuations in the density of the film. (Don't worry about the 'root' and the 'square' part...it is just a little math trick, because if you add up all the numbers that fall below the middle of the curve -- i.e., negative numbers -- and then add up ones that fall above -- i.e., positive numbers -- you would end up with zero if the curve is normal. So you just square the numbers, add them up (sum of squares), and then un-do the all that squaring with a square root. Easy! Now you have a number to divide and get your mean or average...but I digress).
    Anyway, I just got my very first SLR camera last week, and am having a difficult time learning all the different terms. After all I've read, I still don't know how to tell which film will give me the best *resolution* with the least *noise* (other than experience).
    cheers,
    Chris
     
  12. Slide film with RMS 8 is less grainy than negative film with RMS 5
    (duh) due to higher contrast in the slide film.
     

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