Exposure

Discussion in 'Black and White' started by brizzybunny, Oct 8, 2020.

  1. It occurs to me that the terminology that's being used has become conflated, leading to a bit of confusion. Here's a clarification from the Foreword of the 1964 edition of The Negative:
    As a result, my understanding is that relevant terms are: 1) Exposure Zones (scene referred, always a fixed difference of one stop between Zones); 2) Density Values (translation of the exposure Zones to negative densities; determined by the film's characteristics, exposure, and processing); 3) Print Values (translation of negative densities to print densities). The progression is: Exposure Zones (scene) --> Density Values (film) --> Print Values (presentation).

    These terminology changes were formalized in print nearly 60 years ago!
     
    Last edited: Oct 19, 2020
  2. James G. Dainis

    James G. Dainis Moderator

    "It's not correct that each zone represents a certain, fixed difference in camera exposure. You can expand and compres the luminance range to fit a desired zone difference. "

    To re-quote Ansel Adams - "We define a one stop exposure change as a change of one zone on the exposure scale, "

    Zone 7 is a two stop exposure change over zone 5. If one wants to expand a difference after the exposure has been captured, then one give increased development to move that zone 7 to zone 8.
     
    rodeo_joe|1 likes this.
  3. My take:

    You have a scale with the different zones. Think on a printed scale, from black to white with different gray levels, to have it graphically on mind.

    You are in front of the scene, say in front of a huge wall, and you take a meter reading of it. If you expose the film with that reading, you are exposing for Zone V in the scale.
    But if you want this wall in the next, brighter step on the scale, you have to expose at one stop wider than the meter says... so you are then exposing for Zone VI.
    There is certainly "... a one stop exposure change as a change of one zone on the exposure scale".

    But, a one zone difference (that is, looking at the scale, from that zone VI to Zone V), doesn`t mean the exposure has been of one zone difference... you may have exposed for say, a Zone V, and gotten any subject in the scene (e.g. a shadow) in the zone VII, not VI.
    After the proper compressive development, this zone VII goes to the zone VI... so there you have that one zone difference, without exposure changes.

    The thing is to understand (as Roy said above), that we have a "scale Zone" (the scale) that refers individually to the three steps (metering or luminance, negative density and print value).
     
    Last edited: Oct 20, 2020
  4. Obviously I wanted to mean a brighter area, not a shadow.
     
  5. brizzybunny has posted 1 post on photo.net!
    On the same day a they registered. I don’t think they really care what you think.
     
  6. While what people write is meant to be in answer to some question, it is usually discussion that can apply to others.

    Looking back, I think most of what was written is meant for general readers, and not specifically brizzybunny.

    Some questions are very specific, but everyone in photography needs to know about exposure.
     
  7. Yes. The question we each must answer is just how much we want to know about it. :rolleyes:

    Not speaking to you on this, but at a certain point this discussion began to seem about exposure and also about didacticism and ego. YMMV.
     
  8. Somehow I'm reminded of the invention of fly fishing. Old geezers with nothing better to do one winter decided to invent a very difficult and complex way to catch trout when a simple worm on a hook will do the job much more efficiently.
     
  9. Thanks to everyone who posted. The feedback is greatly appreciated.
     
  10. Which either makes Zone 8 wrong by half-a-stop, or Zone 5 needs to be re-defined as 12.5% reflectivity.

    If Zones, in the subject, have a flexible spacing(!) then how on earth is an exposure ever to be calculated? You might as well just stick a wet finger in the air to guage your exposure if that's the case.

    "Today, my Zone VII is going to be 3 stops different from Zone VI, since I didn't like the look of it when it was only half-a-stop different yesterday."

    That's not a system.
     
    Last edited: Oct 24, 2020
  11. James G. Dainis

    James G. Dainis Moderator

    So if I change my exposure from 1/125 sec@ f/16 to 1/125 sec@ f/5.6 I am only opening up 2-1/2 stops not the 3 stops that I always thought it was?
     
  12. One of the many disadvantages of Roman numerals is the inability to express fractional values. Should something that falls on Zone 7.5 be rounded up to Zone VIII or down to Zone VII?

    Somewhere, Adams specifically addresses fresh white paint and he notes that it is somewhat shiny, the degree depending on the paint's glossiness; therefore it is not an entirely diffuse reflector, which allows it to fall higher than Zone 7.5, depending on the angle of the light. The same holds for snow, being comprised of tiny ice crystals.
     
  13. Of course it is.
    You open three stops to get three zones above on the print. Two and a half stops should be at a middle point between zone VII and Zone VIII.

    You read the scene, say, you want the grey card at Zone V. Then look at the e.g., snow, white paint or whatever. Take a reflected metering of the snow; as you say you find it falls 2 and a half stops over in your meter.
    Now think; do you want it with the brightness of the zone VII in your printed scale Zone, or maybe on the Zone VIII? (... or maybe in Zone X !!!). So you`ll need to expose and develop the film accordingly, with normal, expansive or compressive development, depending in your own tests and materials, to get a printable negative with the snow on the right zone.
    As you say, the snow may be not three stops over (in your meter) -as Adams table says-, it may be (actually) 2 and a half... it does`t matter. Adams said that snow, to be realistically rendered, *should* be rendered in zone VIII (the printed scale) to look right -on the print-.
    In Adams words, the table gives "t... he approximate values for various types of subjects... " etc., and also "... to emphasize, again, however, that one of the great advantages of the zone system is that it does not require a literal rendering, and we are entirely free to depart from the description in the chart as our visualization demands".

    Kodak grey card reflectivity looks to be 18% in an on axis reading, but it is reduced by a 30% (approximate) at a 45º angle (that is, near 12% reflectivity).
    Kodak`s literature recommend to use their grey cards at that angle. I have the original instructions who came with mine somewhere.
    Most meters seem to follow the ANSI standard, which is near 12% (a slight error margin is permisible).
     
    Last edited: Oct 24, 2020
  14. Any grey card that reads differently off-axis isn't a very good grey card.
    A reflectance standard should closely approximate to a Lambertian surface, i.e. a surface with equal reflectance at all angles.

    You can wriggle and squirm about manipulating the development to place Zones on the print all you like.
    The fact remains that objects 'white with detail' are only 2.5 stops more reflective than 18%.

    If a half-stop error in exposure measurement is irrelevant, then why make such a song-and-dance about it; as in the Zone system?
     
  15. So, I understand your argument is, that when looking at the final print, the snow would`t be three times more reflective than the neutral grey?
     
  16. No. You obviously haven't understood the argument at all.

    How the film is developed and printed is totally beside the point. Adams clearly describes real life objects and ascribes Zone values to them. That those Zonal values are an impossibility if the Zones are to be one stop apart is the crux of the matter.

    The appendix of 'The Negative' shows a number of H&D curves, upon which are clearly marked Zone divisions along the X-axis (log exposure) at one stop intervals. The slope of the film curve has absolutely no relevance to, nor influence on, the spacing of those divisions.
    There you go!
    What does 'three times more reflective' mean?
    That would be a reflectance of 54% then, would it?

    Get your head around the maths properly, and you might see the light!
     
    Last edited: Oct 25, 2020
  17. Right; to be more precise, that description is ascribed to the Zones as a visualization aid, that is, to visualize how this real life objects *should* appear on the print.

    But it doesn't mean that in the scene there must be one stop metered differences between the Zones (or real life objects) mentioned in that description.
    When you meter a scene, real life objects show their own luminance... whatever it is. It is not the purpose of Adams` table "Description of Zones" to coincide with that meter readings.

    About the grey card; don't know who made the first 18% reflectance "grey card"... I'd not be surprised if it was Kodak. I`d say it makes more sense a 45ª reading than an on-axis reading, so the card is fine.
    Anyway, a real Lambertian type card could be overkill for photographic purposes. The cost of manufacturing and/or materials could make it not worth it. Cheap cardboard ones work.

    So the only debate I see here is about the so called 18% reflectance "neutral grey" card, or the spreaded idea of the mentioned reflectance as a reference.
     
    Last edited: Oct 25, 2020
  18. Forgot to mention that the relationship between net densities and Zones in the appendix are also related to the print, not to the scene. He describes the correspondence between a given density and the Zone value on the print.
     
  19. Joe! Perhaps the reflectance of the subjects aren't that much different but part of the subject is lighted differently. The subjects are not lighted evenly so the part in bright sun can be many stops brighter than the part in shade although their reflectance and not that much different?
     
  20. The entire point of the zone system is to see the print when watching the scene. You meter parts of the scene, decide how they should appear in the print, and base your expsoure and processing on that. You cannot separate zone assignments from that all encompassing proces, and discuss luminance differences in the scene in isolation without leaving the zone system. Adams describes steps of that system. which should not be taken in isolation. How the film is exposed and processed is not entirely beyond the point. It is the main point of the zone system.

    I repeat: how many zones there are between two parts of a scene is not just what your meter tells you (in stops), but how many you decide there must be (within the limits of what film, processing and printing allows).
     
    Last edited: Oct 25, 2020

Share This Page