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  1. #1
    Michel Hardy-Vallée's Avatar
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    Measuring the grey scale

    I was reading yet another article on how to calibrate for the Zone system, how to ensure that a half-step of exposure on paper would correspond to a full Zone change, yadda yadda....

    When suddenly it dawned on me: What's the unit for the tonal scale? It's all fine and dandy to want to standardize, but perhaps we should start by having a way to mean the same things when we say grey, light grey, dark grey, etc? Between what our eyes accept as full black and full white, how do we split the interval so that tones appear more or less equidistant?

    Let's say we take sound for comparison.

    Between 440Hz and 880Hz I have a full octave. I could play whatever frequency if I want (that's what synthesizers are good for), but I limit myself to a particular set of frequencies. That's a scale.

    On a scale, each note is separated from each other by a given criteria: ratio of pitch, multiple from a given pitch, etc. As far as I understand musical theory, that's called temperament. It's what ensures aural distinguishability between notes.

    Now, if we translate that to the visual arts: on a sheet of white paper, I can dilute India ink with water to obtain a continuum of greys between black and white. Perhaps it would be useful to me, as a painter, not to have an infinity of steps between black and white, but rather a more handy set of grey tones. Let's say I want ten, labeled G1, G2, ... G10. How will I ensure that they are sufficiently distinct from each other?

    What measuring tool can I use to measure grey (the equivalent of frequency in Hz for sound), and what intervals between these measures should I respect so that these greys are perceptually "equidistant" from each others (the equivalent of temperament) ?

    I know that's what Munsell and others have done on color system, but I'm not interested in the whole chromatic wheel: I'm just interested in the ten steps it employs to represent value. How can I measure them practically?

    The logic behind all that abstract thinking is simply this: once a good grey scale is established, one works backwards to figure out the technical means to achieve it (paper development, film development, film exposure, in that order) and to move and down (what technical adjustments are needed to go from G1 to G2).

    But the fundamtental question is: How to establish a good grey scale in the first place?
    Last edited by Michel Hardy-Vallée; 09-09-2011 at 11:50 AM. Click to view previous post history.
    Using film since before it was hip.


    "One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. p. 11

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  2. #2

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    Gray tonal scales would correspond more to amplitude = volume, to spin off from your analogy.

    However, practically speaking, doesn't one measure the density of a range of tones on paper, to establish what White is, what deepest black is, what the first noticeable gray tone just below white (call it W-) and the first just noticeable tone above black (call it B+)? Each step would one JND (Just Noticeable Difference) from the next. It is said that humans can distinguish about 200 steps on prints (glossy paper).

    One can also divide the range from W- to B+ into ten zones. The exposure steps that would achieve that on different papers and different grades would be different.

    Is your question not a technical one rather than an aesthetic one?

  3. #3
    jp498's Avatar
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    I understand your analogy between music note frequencies and B&W photography, but the part about being a painter and having steps of gray I don't know how to apply that to photography. Photography went down a separate path from painting 80-100 years ago.

    I've read lots of things about the zone system too, but it's not something I practice; I don't use a spotmeter to check the levels in a scene, etc...

    Things are made distinct by the total lighting setup rather than the reflectance of individual objects, and of course how colors are rendered by the B&W sensitivity. If you need more distinction between objects of medium brightness, you can use contrast to do that, and can reduce or increase scene contrast in the highlights or shadows so contrast is consistent through the range. Use a reflector or flash to boost the shadows, and a light diffuser to tame highlights.

    If it's something you can't control the light in, like an outdoor landscape, weather, film choice and time of day choice are important. For example, when photographing rough granite (like the rockland breakwater) that has lots of similar tones and I want differentiation, I can shoot when the light is low and surface character creates more shadows. The low light angle emphasising the surface creates differentiation. Similarly, shooting rock when wet increases material differentiation.

    I also use different film and or developer for different tone needs, in much the way you wouldn't be likely to use a harpsichord sound for low bass rhythm. TMY2 does a good job for film where I need to cover a wide range of lighting in a scene as it has a big "scale" it can handle. Pyrocat HD does a nice job with midtone and darker differentiation. PMK does a good compensating job for me where highlight detail is important and the rest of the scene has normal natural differention. For scenes where the range of lighting in a scene is narrower, I think fomapan 100 in pyrocat-hd does a good job.

    In addition to the film choice based on sensitivity, filters do well. Skin texture for example can have different tone differention based on filter choice. red/orange to reduce blemishes, fomapan100/txp to increase freckles, etc...

  4. #4
    Michel Hardy-Vallée's Avatar
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    Quote Originally Posted by Monito View Post
    One can also divide the range from W- to B+ into ten zones. The exposure steps that would achieve that on different papers and different grades would be different.

    Is your question not a technical one rather than an aesthetic one?
    "Dividing the range from B to W into 10 zones" is what I'm talking about, but my issue (theoretical, I know) is HOW to do it!

    As far as I can see, you can either:

    a) eyeball it, and judge that a given grey is middle, that another is between white and middle, that another is between black and middle, and so on. The problem here will be that of one's eyes accuracy.

    b) slavishly follow a sequence of exposures and use the resulting tones. For example, expose a sheet of paper to a step wedge corresponding to 0.3 increases in density. The problem here is that tones are not linearly spaced on a paper characteristic curve, so that a given pair of successive tones are separated by, let's say 0.6 density, while another pair of successive tones are separated by 0.9 density.

    c) use a reference grey scale (a Munsell chart or a Ross scale) and work to replicate the tones on the photographic medium. The problem here is that the medium used to make the reference scale may not have the same reflective property as a given photographic paper, so it's approximative at best.

    My ideal method would be:

    1) Find maximal black and white on a sheet of photographic paper
    2) Do a first approximation of the scale by using either of the three methods above
    3) Measure the intervals between successive tones
    4) Normalize the intervals so that tones are equally distinct from each others

    You end up with a series of greys G1...G10 that is matched to a series of exposure time t1...t10 (or densities d1...d10, depending on how you work), so that t1->G1 and so on.

    It's really step 3) and 4) in the above method that I'm trying to figure out. Step 3) is definitely a technical problem (use a light meter? use a reflection densitometer? use a method that involves both the measurable and the perceptual aspects?).

    But I think step 4) really is an aesthetic problem. Again, if I look at music, there are more than one way to space your notes, and not all temperaments equally space notes.

    The real goal here is tonal control in printing: how can you establish a reference palette when you are printing, so that you can make the minute adjustments in dodging/burning/etc to help you with achieving a particular tone.
    Using film since before it was hip.


    "One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. p. 11

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  5. #5
    Bill Burk's Avatar
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    The tool to measure the print reflectance is called a reflection densitometer. A similar tool, transmission densitometer, is used to read the negatives.

    I'm always interested in discussions about visually evenly spaced scales, though 0.3 steps are common because they are practical (not because they are visually correct).

  6. #6
    georg16nik's Avatar
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    Michael, lets assume an equal-tempered chromatic scale, the one mostly used in the west. The twelfth root of two is an algebraic irrational number, it represents the frequency ratio of a semitone in equal temperament.
    So, you have the frequency ratio of a semitone = 106%
    These days, they use pitch shifters in recording studios.
    Back in the days, reel-to-reel magnetic tape recorders typically had a pitch adjustments of up to ±6%..
    To step even further, the 6♭ and the 6♯ scales are not identical.. (even though they are on the piano keyboard) the ♭ scales are one Pythagorean comma lower.

    In parts of the world, where they wanted to be as close to the real thing as possible, they tied their scales to harmonic series, so the intervals are complementary values of the harmonic overtones series and so on.

  7. #7
    jp498's Avatar
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    Musical tone consistency is also important so people can do music together. You want some consistent voices, or instruments working together. Two guitar players with one out of tune is going to be very noticeable, as would be one person singing the wrong notes in a duet's chorus part.

    However, go into a museum gallery of photography. You'll find grays all over the place. You might even find different tints of mattboard and framing material between photos. Nothing is super consistent, just like the subject matter. If it looks good and relevant, it gets hung on the wall. It doesn't have to be accurate in any way.

  8. #8
    georg16nik's Avatar
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    Quote Originally Posted by jp498 View Post
    Musical tone consistency is also important so people can do music together. You want some consistent voices, or instruments working together. Two guitar players with one out of tune is going to be very noticeable, as would be one person singing the wrong notes in a duet's chorus part.

    However, go into a museum gallery of photography. You'll find grays all over the place. You might even find different tints of mattboard and framing material between photos. Nothing is super consistent, just like the subject matter. If it looks good and relevant, it gets hung on the wall. It doesn't have to be accurate in any way.
    Generally - You are right
    In music it is a little bit more esoteric and multilayer like, so is photography and rest of it.. probably thats one of the charming faces of it.



 

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