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Thread: After spot reading the scene and selecting a curve...now what?

  1. #1

    After spot reading the scene and selecting a curve...now what?

    I have run some film and paper tests via the methods suggested in BTZS and have my graphs but I still have a question.

    Lets say I spot meter the scene of trees, overcast sky and water with reflections; I get a range of 5 for SBR. This gets me to select a curve and thus an ASA. Now what?

    Do I re-meter and pick an EV-shutter/f-number combination from the middle? Do double the film speed and use an incident reading from an “open” shadow area?

    I know that I will just have to practice and look at the negatives in the long run but I would like some input on the proper theory. I have visited the BTZS site and find my that my pals here at photo.net much more understanding and understandable.

    Many thanks,

  2. #2

    Join Date
    Dec 1999
    Posts
    1,905

    After spot reading the scene and selecting a curve...now what?

    Here is my method. First I meter th darkest area I care about (where I want detail and texture). Then meter the brightest area I care about (where I also want detail and texture.) t is thespread between these two that matters to me. A 5 stop spread is normal for me - some people use a 4 stop spread as normal. If the sprea is more than 5 stops Ihave to shorten my development time. If the spread is less than 5 stops I may decide to increse my developmen time.

    You have to do your own testing. Even with your pre-programmed device you still have to test.

    steve simmons

  3. #3

    Join Date
    Sep 2001
    Posts
    40

    After spot reading the scene and selecting a curve...now what?

    John,

    In order usefully to apply BTZS data, you must keep in mind how the system works. Why are you trying to find the subject brightness range? Because you are trying to make an easily printable negative. A long SBR will require you to compress the scene tones on the negative, which you do by means of short developing times; a short SBR, like the five stop SBR of your example, will require you to expand scene tones on the negative, which you do by means of extended development. Now as you may recall, the time of development has an affect of film speed. That affect can be substantial or negligible, depending on your film-developer combination. The data from your BTZS testing will tell you how your film speed is affected by development.

    So: you meter your high and low values, from which you get your SBR. Now consult your BTZS data. Your SBR will be associated with a film curve with the average gradient, or Gbar, to which you will need to develop your negative. If you have done your tests correctly and correctly graphed the information, your BTZS data will tell you what film speed is appropriate for the required time of development. Short development times generally cause you to lose some film speed; long development times generally cause a slight gain in film speed. Remember also that these gains and losses are relative to your personally generated effective film speed, which is almost invariably 1/3 to a full stop slower than the manufacturer's published film speed.

    Once you know the film speed, you would plug this value into your meter and remeter the low value to generate your exposure data. Remember, you need to place the low value in the proper zone and adjust the exposure; if you expose at the meter setting, you will be placing your low value in zone five, and your negative would be a couple of stops overexposed.

    Do not switch from Zone metering to incident system metering in the middle of the process. Use one method or the other--not a combination of both--for each negative you expose.

    Remember: your subject brightness range determines the development time you need to make an easily printable negative; the development time you need determines the film speed you should use to get adequate shadow detail in the negative. BTZS is not a difficult system to use, IF you keep in mind how the system works and what you are trying to accomplish.

    A couple of additional points: the exposure method Steve Simmons describes is a good simple method, and one with which you may be more comfortable, but it is not the BTZS method you asked about.

    Finally, I note that you did not post your question on the BTZS cite. I think that is a mistake. Phil Davis monitors the cite and answers questions; and in my experience there is no kinder or more patient teacher than Phil, and no one more interested in helping people to work with his system.

  4. #4

    Join Date
    Dec 1997
    Location
    Baraboo, Wisconsin
    Posts
    7,697

    After spot reading the scene and selecting a curve...now what?

    If I understand your question, and assuming you're not using a Palm Pilot with Phil's program, and assuming you're using a spot meter, meter the scene at whatever ASA your tests have shown to be your film speed with normal development and determine your exposure on that basis. If the SBR of the scene and your desires for the print are such that you'll be using plus or minus development instead, which means the effective film speed will change, then after determining the exposure based on your normal film speed adjust the aperture or shutter speed on the camera to correspond to the change in the effective film speed that your tests have shown to be called for with plus or minus development.

    An example might help. Assume that your tests show that your effective film speed for T Max 100 is 64 with normal development, 80 when using plus one development, and 50 when using minus one development. Meter the scene and determine your exposure with the meter set at ASA 64. If the SBR and/or your desires for the print are such that you'll be using plus one development instead of normal then don't re-meter or do anything else with the meter, just stop the aperture down by a third of a stop before making the exposure, which is the equivalent of metering with the film speed set at 80 rather than 64. If the SBR and your desires for the print call for minus one development, then just open up the aperture by a third of a stop, which is the equivalent of metering with the film speed set at 50 rather than 64. Changing the aperture will have some effect on depth of field of course. A third of a stop isn't enough to matter in most cases but if the change is great enough to make a difference then you can change the shutter speed instead.

    I hope this is understandable and is responsive to your question. I used to use the Casio pocket computer loaded with Phil's program for every photograph I made, I attended two of Phil's workshops, and I spent untold hours poring over his book (which is IMHO longer and much more confusing than it needs to be). After using this system for some years I decided that with black and white negative film I didn't need the absolute precision or perfection that his system is designed to produce so I stopped using the computer and I now only use the portions of his methodology that produce my effective film speeds and development times. I've found that for me photography is much more enjoyable without the computer and without worrying about the minutiae of the BTZS system. But if you're using it and think that it's beneficial for you, that's fine, many people do.
    Brian Ellis
    Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
    a mile away and you'll have their shoes.

  5. #5

    After spot reading the scene and selecting a curve...now what?

    The problem I see right away is that you are using a spot meter instead of an incident meter. If you use spot metering there is a formula you need to use to correct "zone" placement and convert it to SBR. You really should post this question in the BTZS web site. http://www.btzs.org



    With all due respect to all that helped you here, doing spot metering without doing the appropriate correction will give you wrong results, and eventually will lead you to disappointment. The other side of Brian's experience is mine, as a pt/pd printer I need the best negative I can get, and using the BTZS even without the expodev program has given me close to perfect exposures every time.

  6. #6

    After spot reading the scene and selecting a curve...now what?

    John,

    You have to be careful when you talk about BTZS methods, because many people do not use the exact BTZS approach as described in the book. Like Brian, I use BTZS mostly for the testing and establishment of film curves, etc. Unlike Brian, I think the BTZS book is the best book available on a ZS approach that actually has enough information in it to enable a reader to go beyond the book, if they choose to. (Give a man a fish, and you'll feed him for a day, TEACH a man to fish...)

    I meter the shadow and highlight to determine the range, and then decide how I am going to develop the negative (n, n+, n-, or whatever). That determines the ASA to set the meter to. Then I PLACE THE SHADOWS where they should be (zone III or whatever, depending on the subject) on the meter. This is important; always place the shadows where you want them.

    The reason is that the highlight density will be heavily affected by the development (as is intended with the ZS), but the shadows will not, assuming you have already made the adjustment in film ASA for the development. The exposure determines the shadow density, and the development determines the highlight density. I'm sure you've heard the phrase "expose for the shadows, develop for the highlights"...

    My method is not a pure BTZS approach, because I use N-numbers, but it is reasonably close, and I could easily translate it into SBR and work that way, if I wanted to, I suppose.

    Making adjustmants, metering modifications, and zone placement can be quite bothersome with some meters. When I started doing real ZS work, I quickly realized that the spot meter I had (a Minolta F) didn't really have a great interface for this type of manipulation. It's a great color meter, but I find the Pentax Digital spot meter to be much better for adjusting the ASA and placing a reading at a specific zone. It makes the procedure I described above fast and intuitive; occurring in just a few seconds once I have decided on my shadow and highlight readings.

    ---Michael

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