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Thread: BTZS, Zone System, Everyone Else... Film Speed ?

  1. #41

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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Well, not quite. Since the incident measurement can't know anything about the grey card used in the reflective measurement, it needs an absolute intrinsic reference value. If both measurements agree, it means that the reference for reflective and incident metering are the same. If the reflectance of the grey card is 18% then this is the common reference - at least for this meter. By the way, this is only one meter allowing for both types of measurements.

    Now, whether the 18% standard is appropriate to putting it on zone V is another question. This is much more subjective in practice and has little to do with the densitometry. At least for me it works fine.

  2. #42
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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Quote Originally Posted by Larry Gebhardt View Post
    Ken, I think you are missing that Peter measured the grey card with reflectance and also using the incidence meter got the same results. We know the incidence meter is supposed to give a middle grey exposure, so for his meter it also means that the reflectance meter is calibrated to give middle grey based on an 18% card (assuming his card is 18% grey as Kodak says).
    Thank you for the explanation. I see now.

    From what I've read, not all meters are calibrated for 18%. His two are, and that's helpful.

    That being said, the fact that something is customary, does not make it right, and the quotations I gave earlier expressed concern about the validity of the number.

    As the original poster, I am grateful and satisfied with the answers I got to my original question. Because I am also fortunate enough to be a moderator here, I will take this opportunity to close the thread.

  3. #43
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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    I re-opened this thread because I finally found the quote I was looking for:

    Beyond the Zone System, 4th Edition, "Metering for the Incident System" pp 134:

    "Notice that these film speeds seem exaggerated; they are, in fact, just double the normal speeds. As explained earlier, this is done deliberately to compensate for the 1-stop overexposure that normally results when the camera settings are based on the low-light incident reading."

    In other words, the recommended technique is to take an incident reading in the shadow area of the scene, but don't use it directly: underexpose by 1 stop, by using twice the film speed.

    What I don't understand is how the doubled film speed relates to the Effective Film Speed derived by densitometric testing. Do BTZS practitioners routinely determine an effective film speed with sensitometry, but then double the speed in the field in order to compensate for having metered the shadows ?

    I had Fred Newman perform a BTZS test for me - with HP5+ and D-23 - and the effective film speed for normal scenes was 250. I subsequently performed my own densitometric tests and got the same results. In practice (and given a scene of normal brightness range) would I meter the shadows, and use a film speed of 500 ?

  4. #44
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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Commercial films have the ISO speed printed prominently on the box. Conducting tests to see if their meters match your meters is tilting at windmills. I have better things to do.

    Here's a hint: If you are not getting enough shadow detail, expose more. Can I have a book deal now?
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  5. #45

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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Quote Originally Posted by Ken Lee View Post
    What I don't understand is how the doubled film speed relates to the Effective Film Speed derived by densitometric testing. Do BTZS practitioners routinely determine an effective film speed with sensitometry, but then double the speed in the field in order to compensate for having metered the shadows ?
    Ken,

    You can determine a personal speed point reference if you like, but I always run my tests with Winplotter with the SPR set to 2.4, which as I recall is referenced to Delta 100 in Xtol. Use of the SPR of 2.4 gives you a useful anchor by which to evaluate all film/developer combinations. So in essence I determine a standard EFS with sensitometery, then adjust in the field for the method of metering. I am pretty sure Fred would have used an SPR of 2.4.

    You might be interested in this article by Phil Davis. http://www.btzs.org/Articles/PSP.htm

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  6. #46

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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Quote Originally Posted by Ken Lee View Post
    "Notice that these film speeds seem exaggerated; they are, in fact, just double the normal speeds. As explained earlier, this is done deliberately to compensate for the 1-stop overexposure that normally results when the camera settings are based on the low-light incident reading."

    In other words, the recommended technique is to take an incident reading in the shadow area of the scene, but don't use it directly: underexpose by 1 stop, by using twice the film speed.
    The quote seems like plain English, exactly as you summed. Take the film speed results from Fred Newman's sensitometry, which are reliably accurate film speeds. Since you chose to use the shadow incident reading, set the meter EI at double the real speed to keep from overexposing by one stop.

    You wouldn't use double the film speed results for other kinds of metering. For simple examples, in Zone System you still might use about half film speed, for averaging meters you would still use the whole film speed.

  7. #47
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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Quote Originally Posted by Brian Ellis View Post
    There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
    Just my own 2 cents, but--------exactly, my ZS tests using TMX with both d-76 1:1 and xtol 1:1 proved to yield an EI of exactly the box speed---ZS makes no claim what the real speed should be, only that the effective speed is determined by testing. It is true, though, as has widely been found, that the tested speeds are usually 2/3 to 1 stop less the box speed, but there is no gaurantee.

  8. #48

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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    Quote Originally Posted by Ken Lee View Post
    What I don't understand is how the doubled film speed relates to the Effective Film Speed derived by densitometric testing. Do BTZS practitioners routinely determine an effective film speed with sensitometry, but then double the speed in the field in order to compensate for having metered the shadows ?
    I guess my first question would be what does BTZS calibrate to? Do they peg normally to "open shade" or what?

    Once the "peg" is known then the offset they are suggesting may make more sense.

    For example, if "normal" zone system and spot metering techniques were applied and you had decided you always want to place the shadow point say 2-stops down, then instead of always doing the math in your head you could simply reset the spot meter's ISO setting.

    So for example if you were shooting film that tested at 125 you could reset the meter to 500, spot meter your shadow point, and the meter would read the camera setting directly.

    The film rating didn't change to 500 we just applied an offset to force the meter to give us the camera setting.
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  9. #49

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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    It's been too many years since I used the BTZS system to comment on the accuracy of the statements related to it. But as for the "traditional" zone system, I'm not aware that it gets around anything by reducing box film speed in half. That's probably a better practice for someone who's unwilling to test to determine their film speed than blindly using the box speed but ideally one tests to determine a film speed when using the zone system.

    In many cases it may not matter as michael slade suggests. Sometimes two mistakes can cancel each other out (e.g. underexposing and overdeveloping). And other times the range in the scene is well within either exposure. But it does or can make a difference in a situation where the shadows are important and the box speed would give insufficient density/detail to the shadow areas.
    Brian Ellis
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  10. #50
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    Re: BTZS, Zone System, Everyone Else... Film Speed ?

    What Bill said in post 46 makes sense to me, and matches my testing. I just wanted to make sure I wasn't missing something.

    When I performed BTZS testing with step-wedges and a calibrated sensitometer, I got the same results as Fred Newman. Using a spot meter and the Zone System, I use that film-speed as-is. When metering BTZS style (in the open shadows with an incident meter) I derive the same camera settings by subsequently increasing exposure by one stop - equivalent to Phil Davis' recommended short-cut of doubling film speed.

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