Trying to figure out exposure for Stouffer TP120-31
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... :-(
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.
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.
Thank you for your help!
Best regards, Christoph.
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.
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).
Originally Posted by Muihlinn
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...
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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
I much prefer a contact exposed test strip because it eliminates flare. Ralphs instructions are thorough, you certainly may follow his plan.
Originally Posted by qualsound
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 Bill Burk; 12-11-2013 at 10:38 AM. Click to view previous post history.
I'm also trying to find definitive answer to that question. To be able to repeat the process and get consistent results.
Bill do you think that following methods will work? I'm referring to the establishing correct exposure only.
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.
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.