Trying to figure out exposure for Stouffer TP120-31

Discussion in 'B&W: Film, Paper, Chemistry' started by qualsound, Dec 10, 2013.

  1. qualsound

    qualsound Member

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    Hello guys,

    I want to conduct the "elaborate & precise" filmtest described in WBM 2 written by Ralph Lambrecht.
    I tried the test in march, but the densities i measured were so low, that i came to the conclusion, that i had severely underexposed the Stouffer tablet trying to get a 1:1 copy of it.

    Now i want to test two different films again but i am stuck at how to correctly expose the stouffer.
    In march i used a bellows system on my medium format camera to get 1:1 copy of the stouffer. i taped the Stouffer to a window, relied on the average reading the built in viewfindermeter told me and exposed the films.

    Result = underexposed negatives.

    I then built a kind of lightbox, where i can fire a flash from behind the stouffer (which is taped to a 6x6 negative carrier) to expose it and place the camera with bellows in front of it to take a 1:1 copy of the stouffer.
    My question now is how can i figure out the right exposure using the "flash method". I have go a sekonic L-358 lightmeter that can measure flashlight...do i only have to take a incident meter reading towards the flash (when the stouffer hasn't been taped to the negative carrier) and take the f-stop and time shown?!?
    Logically i think that this won't work because for the actual exposure i have to tape the stouffer to the negative carrier and it will decrease the light that comes from the flash...

    Which bar number on a Stouffer TP120-31 approximately equals a transmission density of medium grey?

    Thank you for your help, as i am pretty confused... :-(

    Best regards,
    Christoph.
     
  2. Bill Burk

    Bill Burk Subscriber

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    You might find this thread has the information you need to set your flash correctly...

    My take on the whole thread is that while there are more expensive and accurate lab equipment available at an extremely high price... A good electronic flash and flash meter can provide high quality readings and consistency.

    http://www.apug.org/forums/forum43/110273-flash-sensitometer.html
     
  3. qualsound

    qualsound Member

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    Thank you Bill Burk for your answer!

    anyway, i am more confused now...because i haven't understood how to correctly meter and set the flash...also taking into consideration the inconsistency in the output of the flash,....i would use a sb800

    Having said that i am wondering if it is not easier to follow Ralph's advice, getting a TP 4x5 with 21 bars/31bars, taping it flat to a window and masking it to reduce flare.
    I could then use an extension tube (36mm) on my medium format camera with an 80 mm lens (set to infinity) so that the 4x5 stouffer fits onto a 6x6 negative (finally adding 1 stop to exposure because of the extension).
    So far so good...

    But how do i meter the correct exposure then/and where (meaning which bar number) on the stouffer?

    I have got a Pentax Digital Spotmeter but i am not sure if i can meter so narrow, meaning 1 single bar on the tablet, because after asking at Stouffer's they told me that: The step size of the TP4x5-21 is 9.525mm and the TP4x5-31 is 6.4mm.

    Any advice?!

    Thank you for your help!

    Best regards, Christoph.
     
  4. Muihlinn

    Muihlinn Member

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    IIRC in 21 bars step wedges each step is half stop. AFAIK all the info related to the wedges were in their website.

    hope this somewhat helps.
     
  5. qualsound

    qualsound Member

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    Thanks for your answer Muihlinn! I know that the 21 step wedge increases in 0.15 densities (1/2 stop), and the 31 in 0.10 (1/3 stop).

    what i still don't know is which bar (number) on the step wedge (either 21 or 31) represents medium gray...so that i could take a spotmeter reading off of that to determine expousre...

    Any suggestions?
     
  6. Muihlinn

    Muihlinn Member

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    you can place medium gray whenever you want as long as you don't want a full scale representation. For a #2 paper print , IIRC, it should be about 0.60
     
  7. Bill Burk

    Bill Burk Subscriber

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    I much prefer a contact exposed test strip because it eliminates flare. Ralphs instructions are thorough, you certainly may follow his plan.

    Consistency is important (like flash on manual mode). But it really "doesn't matter" about the exposure, just take a stab at it.... Look at what you got (from a quick test where you make a best guess of the proper exposure and develop)... and count the "steps" from what you got to where you would rather have it be... Then change your exposure by half stop per 0.15 step difference.

    Without being precise or even without considering meter calibration point (which could give you a real answer)... Just look at about the seventh step ligher than the darkest step.
     
    Last edited by a moderator: Dec 11, 2013
  8. wiedzmin

    wiedzmin Subscriber

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  9. Bill Burk

    Bill Burk Subscriber

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    Hi wiedzmin,

    Both posts give good advice.

    The reason you don't need a definitive amount of exposure for a step wedge on film...

    An ideal exposure would give you the entire toe of the film curve. Worst case you overexpose by several stops and even the darkest step wedge patch makes a density over 0.10 on your test film.

    Then you only need to count out how many steps you wish you had in the toe... and give that many steps less exposure when you make the next test.

    You are exposing a test pattern that covers 10 f/stops worth of range. If you overexpose or underexpose by several f/stops, you still will get a test pattern on film that will tell you how far off you are in terms you can easily convert to f/stops.

    Being off by 1/3 stop (which might be noticeable if you were shooting transparency film in camera), has negligible impact on a film test. To be off, you have to be several f/stops off. And the necessary correction is easy to quantify.

    Now for Consistency sake, it would be nice to be within 1/3 stop of repeatability... but as far as the landing point, you do not need accurate placement of the test exposure.
     
  10. qualsound

    qualsound Member

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    wiedzmin,

    in Ralphs post http://www.apug.org/forums/viewpost.php?p=1119941 does he refer to a stouffer with 21 or 31 steps, by measuring the exposure off of step number 11? either way step number 11 on a 21 stouffer should be the same as step number 16 on 31 stouffer...or am i wrong?
    the density of step 16 on a 31 stouffer is around 1.5....

    what Muihlinn said it should be at a density of 0.6 so that would be somewhere between step number 6 and 7 on a 31 stouffer...?!? so which step number is the one to measure the exposure of off?

    Thank you Bill Burk for your answer...i have attached my (failed) test results from march for 120 rollfilm hp5+ and fp4+...could you please write your take on the results...i appreciate your knowledge and insight into the topic very much...as i am lacking there.... ;-)

    Thank you and best regards,...Christoph.
     

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  11. wiedzmin

    wiedzmin Subscriber

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    Bill,
    thank you for the explanation. It is much more clear now.


    Christoph,
    I think he refers to 21 steps stouffer because OP says that he uses 21 step http://www.apug.org/forums/forum44/85932-stouffer-step-exposure.html

    mine 21 tablet step 11 = 1.54
    31 tablet step 16 = 1.57

    I think you are right, they should be close
     
  12. wiedzmin

    wiedzmin Subscriber

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    Christoph,

    Can you verify that effective film speed -summary tab in FilmTestEvaluation_HP5.xls looks like on attached jpg?

    stouffer-tp120-31-filmtestevaluation_hp5-summary.jpg
     
  13. qualsound

    qualsound Member

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    wiedzim,...yes it is the same as the jpg isn't it a screenshot from my excel sheet? ;-)
     
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  15. wiedzmin

    wiedzmin Subscriber

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    thank you, yes it is from your spreadsheet, I was not sure if my excel 2010 displays charts correctly.

    Did you exposure hp5+ as 400 iso?
     
  16. qualsound

    qualsound Member

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    yes, i exposed hp5 at iso 400 because as Ralph states in WBM 2 "Assume the box speed to be correct and determine the right exposure with an average reading, or use a spotmeter for the medium gray bars. You can use the manufacturer’s recommended film speed, since the actual exposure is not critical as long as it is within 1 stop."

    But what/which number on a stouffer are the medium gray bars?
     
  17. wiedzmin

    wiedzmin Subscriber

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    yes.. I had the same question. Very specific intrusion and suddenly in few places I was stuck.

    I'm wondering why effective film speed is so low in your test. Underexposure? Massive Dev Chart suggests 13 minutes as standard http://www.digitaltruth.com/devchart.php?Film=HP5&Developer=ID-11&mdc=Search&TempUnits=C based on your test N dev time is slightly more that 8 minutes.
    Maybe in that case dev times should be 5.5 8 11 16 22 minutes ?
     
  18. qualsound

    qualsound Member

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    yes i think i underexposed the film quite a bit. that's why i am asking on how to determine a "correct" exposure for the stouffer.
    i am hoping that Bill Burk can somehow explain the test results...and point out where i was stuck...so that the next test will work out...and i could get usable results...

    wiedzmin are you stuck in the same places i am?
     
  19. wiedzmin

    wiedzmin Subscriber

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    I was more "lucky" with exposure but I would like to establish consistent and easy to repeat procedure. I also tried BTZS Plotter program, but I'm getting different results than using method from WBM 2
     
  20. Bill Burk

    Bill Burk Subscriber

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    Hi Christoph,

    Your tests look GREAT, I don't see anything but careful attention to detail!

    These look like camera tests, which includes flare. That messes with the toe, it never really gives you a low Base+Fog. So take the "effective film speed" results with caution.

    But AVERAGE GRADIENT for the development time is very good information that doesn't depend very much on the toe. You can use that information to make good development times decisions.
     
  21. Bill Burk

    Bill Burk Subscriber

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    So the main point ... you don't get the film speed from the lightmeter setting. When you do film tests like this...

    You get the film speed from the box. The rated speed of fresh film in a standard developer, is very close to the "speed" of the curve that has an 0.62 average gradient.* So take that as "Zero" - your other speeds are relative to that box speed.

    When you develop less, you get less speed. Develop more and get more speed.

    *I see Ralph suggests 0.57 - close enough for you to use his numbers for practical purposes.
     
  22. Bill Burk

    Bill Burk Subscriber

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    Maybe one f/stop underexposed, but still you have enough curve family information to make processing decisions.

    Again the only thing you miss out on, is a few more steps of information that you could have had in the highlight densities on the right side of the graphs.

    And the underexposure does not in any way impact the speed of your film that is revealed by the curve family.
     
  23. wiedzmin

    wiedzmin Subscriber

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    Bill,

    but why for HP5 effective film speed is so low ~10 for 400 iso film (summary tab)?

    Tomasz
     
  24. qualsound

    qualsound Member

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    Hi Bill Burk!

    Thank you for your explanation! I read Ralphs instructions on that excel sheet once again...and came to the conclusion, that i missed one IMPORTANT point!...to change the EI --> 2/3 less speed in this field,...so 400 --> 250 for HP5 and 125 --> 80 for FP4. Then i had to change the logH min until the curve, the red and black line intersected...! i think i can use these results now... Bill what is your opinion on the filmspeed and development times provided by the excel sheets for fp4 and hp5 (i attached them once again after changing the parameters i described above).

    Thank you for your help! Best regards, christoph.


    P.S.: So tomasz, stating that from above is the answer to your question "but why for HP5 effective film speed is so low ~10 for 400 iso film (summary tab)?"
    Have you got a similar problem...so maybe that is the solution...! ;-)
     

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  25. qualsound

    qualsound Member

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    what i am able to read out of that excel sheets is:

    FP4
    N+3: iso 125 ??? @ ???
    N+2: iso 125 @ 16'40"
    N+1: iso 100 @ 11'30"
    N: iso 80 @ 8'
    N-1: iso 50 @ 5'30"
    N-2: iso 40 @ min 5'
    N-3: iso 40 @ min 4'20"

    HP5
    N+3: iso 400 ??? @ ???
    N+2: iso 400 ??? @ 15'50"
    N+1: iso 320 @ 12'50"
    N: iso 250 @ 8'50"
    N-1: iso 200 @ 7'
    N-2: iso 160 @ min 5'50"
    N-3: iso 125 @ min 5'10"

    Bill Burk your opinion on my conclusion is very much appreciated! Thank you for your help!

    Best regards, Christoph.
     
    Last edited by a moderator: Dec 12, 2013
  26. wiedzmin

    wiedzmin Subscriber

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    Christoph,

    yes I think that was it. Are you going to perform real film speed test based on WBM 2?