Confusing messages from the community

Discussion in 'Black and White' started by chris_cartwright, Feb 6, 2011.

  1. Hi
    I'm trying to get my head around the pros and cons of scanning B&W film.
    Reading zone-system articles (e.g. this one http://dpanswers.com/content/tech_zonesystem.php) indicates that B&W neg film is able to reproduce a very high dynamic range, EV 14+, which is higher than most digital cameras (except artificiall HDR techniques) and much higher than colour slide (EV 5) and print film (EV 10). Thus the problem for scanning them is similar to that for printing them - how to get as much of this range as possible into the scan with a scanner having a more limited range (EV 10)
    Reading scanner articles about scanning B&W negs (e.g. this one http://robertdfeinman.com/tips/tip11.html ) says that the problem with scanning B&W negatives is their low density range (around 2) compared to colour slide film (around 3.5). My understanding is this equates to EV 6.5 for B&W neg and EV 12 for colour slide film. Thus the problem with scanning them is that the whole negative is scanned into a small subset of the scanner's density range.
    Does not compute. Can someone explain which is correct to my small brain please?
    Many thanks.
    Chris, UK.
     
  2. I believe that you are confusing sensiometry and densiometry. Check out the characteristic curve for a given BW film (Ilford HP4 here on page 4). What you call the dynamic range of the film is on the X axis, and the density range of the developed negative is on the Y.
     
  3. You are, perhaps, confusing the subject brightness range (SBR) with the film's density range. The two are independent; you can't use them interchangeably.
    Given an SBR of, say, five stops, and exposing three films (tranny, color negative, B&W negative). The tranny might give you a density range of 3.6, the color negative a density range of 2.8, and the B&W a density range of 0.7. A typical "consummer" flatbed scanner can perhaps handle a density range of 3.0. So when it scans the tranny, it clips the highlights. OTHO it handles the color negative just fine, and only uses a part of it's digital range for the B&W.
     
  4. Hi Matthew
    Many thanks for your response. I think I understand. Let me play it back and see if I'm right.
    The film's dynamic range is a measure of what range of exposure it is able to capture through meaningful variations in density. In other words the range of X over which it has a linear (or at least non-flat) characteristic curve. In the example you link, this is about 1.0 - 3.8 or a range of 2.8.
    The density range is the corresponding Y axis range i.e. 0.2 - 1.8 or a range of 1.4.
    Both of these numbers are (I believe) base 10 logs, so this film would have a dynamic range of 10**2.8 or 630 or EV 10.5 and a density range of 10**1.4 or 62 or EV 7
    A typical scanner has a sensitivity range of around 3 equals 10**3 or 1000 or EV 11. The appropriate measure to compare with this is the film DENSITY range not DYNAMIC range, hence the negative has a mush SMALLER density range than that of the scanner. The consequence of this is that it is easy to capture the full range of tones in a scan of a negative. The challenge is to get enough distinction between them as they are compressed into a subset of the scanner's full potential. This points to the need for a scanner with higher bit depth for negs vs trans since it needs to be able to distinguish much smaller variations of tone in the compressed area.
    Did I get it?
    Thanks, Chris
     
  5. Thanks Bruce also. Our posts crossed.
    I think you're reinforcing the same point. Basically the characteristic curve of a B&W neg is long and shallow, whereas for a colour tranny it's short and steep. So max information transmission from scene to scan should in theory be B&W film plus high bitdepth scanner (using a subset of its range). Though if scanner bitdepth is the limitation, a colour neg film might be a better compromise?
    Cheers, Chris.
     
  6. B&W neg film is able to reproduce a very high dynamic range, EV 14+ ... the problem with scanning B&W negatives is their low density range (around 2) compared to colour slide film (around 3.5). ... Thus the problem with scanning them is that the whole negative is scanned into a small subset of the scanner's density range.Does not compute.​
    There's a missing piece here. The standard operating procedure (at least it is if you're reading about the zone system) is to shoot/develop with a desired density range in mind. That is, the developed film's density excursion is customizable to fit the scene brightness dynamic range. Your specifics of your particular scanner dictates the magnitude that "perfect" density span.
    For example, a misty, foggy scene may range only two brightness stops. Maximizing digitized precision means amplifying that two stops, with respect to the density range presented to the scanner. You might underexpose by two or three stops at time of exposure, then soak the film in the developer for 20% or 30% longer.
    A high contrast scene, say under direct noon sun with important detail in dark shadow, will span more than 10 stops. The goal here is to compress that wide scene dynamics into detectable, meaningful film density changes. In this case, you might overexpose by one or two stops, then decrease development time by 20%.
     
  7. "That is, the developed film's density excursion is customizable to fit the scene brightness dynamic range. Your specifics of your particular scanner dictates the magnitude that "perfect" density span."
    Robert said it better than I can. The first thing you notice when scanning BW negatives is not the limitations of your film, but the limitations of your scanner. Scanners don't do well with shadow detail.
     
  8. Thanks Robert
    I'm learning all the time here. Please bear with me...
    I'm not sure I quite follow your argument.
    As I understand it (probably wrongly), a good B&W film has a dynamic range of EV 14+ (in other words 14 stops). However the end product - paper (wet or dry) - can only deliver around 8 or 9. The reason you expose for the shadows and underdevelop to hold the highlights in 'wet' zone system is so that the dynamic range of the negative is held back to 8 or 9 to match the paper, so you can print straight without burning etc.
    But if a scanner can cope with the full dynamic range of the neg (since it is represented by a density range of only about 2 or 2.5), then for the high dynamic range scene, why would you not just expose for the shadows again, but develop normally (since the neg can cope with the rull EV10 range in the scene, as can the scanner)? I can see why you might still want to 'stretch' the low contrast scene to maximise the scanner's ability to grade the tones.
    Thanks in advance, Chris.
     
  9. In short, I believe the truth is somewhere in between those numbers.
     
  10. ... a good B&W film has a dynamic range of EV 14+ (in other words 14 stops). ...
    But if a scanner can cope with the full dynamic range of the neg (since it is represented by a density range of only about 2 or 2.5), then for the high dynamic range scene, why would you not just expose for the shadows again, but develop normally (since the neg can cope with the rull EV10 range in the scene, as can the scanner)?​
    The assertion that B&W film can record 14+ is the problem. It's true in the limit but forcing the media this much isn't generally desirable.
    Say you get here with stand development. Yes, the film records a total of lots of stops, but it does so by flattening the highlights. The decreased tonal separation means selectively less digitized numerical precision the brighter that section of the scene. To the extent that compensation is done in post, the result tends toward exaggerated grain in the highlights.
    This by the way isn't necessarily bad; perhaps it's exactly the right aesthetics for a particular frame. As a matter of practicality, however, it won't be desirable across every frame of the roll.
    ... for the high dynamic range scene, why would you not just expose for the shadows again, but develop normally (since the neg can cope with the rull EV10 range in the scene, as can the scanner)?​
    You could, and you would. Recording a usable 10 stops doesn't take most film out of its design envelope. Trying for 14+ stops is something else.
     
  11. Thank you all for your insightful contributions. I have a much clearer understanding of the issues now.
    All the best, Chris.
     
  12. There is one misconception in here - scanners don't scan with a fixed exposure / gain. At least not mid range scanners like Nikon Coolscan. The exposure and analog gain is changed depending on how dense the negative is. So you get to use the full range of the scanners bit depth regardless of the negative density.
     
  13. Thanks Pete.
    Interesting. So I guess the remaining question is - do you need to push-process a low-contrast scene to use more of the film's gamut, as was suggested or can you just leave the scanner to expand the available dynamic range? Presumably since the scanner is well within it's capacity it will capture any tonal graduation that is there. The issue must be whether you get more detail in the first place by pushing the neg or does this just serve to distort the tonal relations in the scene needlessly? Seems like the latter to me.
    Cheers, Chris.
     
  14. I'm wondering who ever said that a properly conceived continuous tone negative had 10 f stops of latitude. A frontally lighted sunny landscape (or other subject) has an 8:1 lighting ratio. A grey scale has 6 2/3rd f stops (actually that is 6 f stops but we give it a +/- 1/3 stop for pure black and pure white), with an 8:1 you get a bit less than 9 stops. A negative designed to print full scale with a world standard #2 paper (Density Scale 1.00) will have a GBar of .50 give or take .05. If you have more contrast as possibly side or back lighting or high altitude, then you increase development and decrease development (provided you know how-and it really won't be found in Zone system), on the other hand if you have a lower contrast then you decrease exposure and increase develoment. The only way you can get 10 stops under normal lighting, is to make a flat negative.
    That ought to anger lots of people, however, this is science.
    Lynn
     

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