Paper exposure by 1/3 f-stops with Stouffer Step Wedge

[Oliver Roch]

Dear Darkroom Friends,

this is my first post here at apug as i normally post at aphog.de where i didn't get any answer for my problem this time.

I recently read about f-stop based paper exposure and gave it a try for myself.

Here's my experiment:

I contact printed a Stouffer step wedge T3110 in 6.4, 12.7 and 25.4 seconds on a grade 3 paper. The paper is an old brovira baryt, so maybe it has already gone softer.

I took the exposure times based on an 1/3 f-stop timing table:

6.4 (Base exposure) (First contact print)
7.2 (+1/3 stops)
8 (+2/3 stops)
9 (+1 stops)
10.1 (+1 1/3 stops)
11.4 (+1 2/3 stops)
12.7 (+2 Blenden) (Second contact print)
14.3 (+2 1/3 stops)
16 (+2 2/3 stops)
18 (+3 stops)
20.2 (+3 1/3 stops)
22.7 (+3 2/3 stops)
25.4 (+4 stops) (Third contact print)

Now i visually compared the prints. I took step 7 at an exposure at 6.4 seconds as my reference gray. At an exposure of 12.7 seconds, i found the same gray on step 11. The last exposure (25.4 seconds) gave me the same gray on step 15.

So every 'overexposure' by 2 stops moved my reference gray 4 steps on the step wedge. I thought a density increase on a 1/3 stop step wedge is 0.1 (as stated by the manufacturer). How is it possible that my density increased only 0.4 by an overexposure of 2 stops. Shouldn't be 2 stops (6 * 1/3 stop) increase my density by 0.6?

I measured the step wedge on the base plate of my enlarger using a Kunze MP104. Every step was indeed increasing the density by 0.1

Where am i missing the point? I just don't get it.

Hoping for your help,
Oliver

P.S.: Sorry for my english. It's quite hard to explain such an technical problem.

 

[richard ide]

I do not know either. I have used 21 step wedges for many years and in my experience when you double exposure there is a variation of 3 steps.
As a simple test; make an exposure and then another but double the time and compare results. Keep it simple by trying 10 seconds and then 20.

 

[RalphLambrecht]

Oliver

Ich kenne mich mit f/stop Timing und den Produkten von Stouffer gut aus, aber leider habe ich Dein Problem nicht verstanden. Was funktioniert nicht und was hast Du erwartet das nicht eingetroffen ist?

Übrigens, denk dran: APUG hat auch ein deutsches Forum.

 

[palec]

To me the timing table has wrong values, between 6.4 and 12.7 is 1 stop difference not 2. Same applies to 12.7 and 25.4 values (it's double time, so 1 stop).

 

[Nicholas Lindan]

As Palec noted, your 1/3 stop times are wrong.

Base times, at one stop intervals, are 1, 2, 4, 8, 16, 32 ... seconds.

At 1/2 stop intervals they are the familiar: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 .... seconds

At 1/3 stop intervals they are 1, 1.26, 1.59, 2, 2.52, 3.17, 4, 5.04, 6.35, 8, 10.1, 12.7, 16, 20.2, 25.4, 32 ... seconds

The times you have are for 1/6 stop intervals. Note that a 1 stop difference in time doubles the number, but a 1 stop difference in f-stop multiplies it by 1.4 (the square root of 2). The use of aperture diameter for indicating a lens' exposure rather than aperture area is just another of Man's Really Dumb Ideas®.

However, this leaves you with the problem that step 7 @ 6.4 seconds == step 11 @ 12.7 seconds. Step 7 should equal step 10 when exposure time is doubled.

 

[ic-racer]

Just multiply by 1.26 (two to the one-third power) to increase exposure by one-third of a stop or divide by 1.26 to decrease by one-third stop.

 

[RalphLambrecht]

If you prefer an f/stop timing table over calculations:

Dead Link Removed

 

[Oliver Roch]

@Richard: This is exactly the variation in steps i expected.

@Palec & Nicholas: Thank you for your hint. I think i mixed up aperture based stops and time based stops. So you are right, i doubled the exposoure time between my contact prints.

@Ralph: I know about the german forums at apug.org. The english forum just felt more active compared to the few posts over months in the german one. I already have your f/stop timing table (can't wait to preorder your new book though). It is the main reason (next to a wallner report with f-stop based timewheel) why i started experimenting in this type of exposure. My idea was to find out how many stops i need to move from one step on the wedge to another. With this information i thought i could improve my process like in the following:
1. Take a sheet of gradation 2 paper.
2. Find out the correct exposure time for the highlights.
3. Select a matching gradation for the shadows.
4. Based on the gathered information from my experience i can now re-adjust the exposure time for the highlights.
Am i overengineering my process?

Best regards,
Oliver

 

[RalphLambrecht]

Oliver

You are not overengineering your process. Steps 1 and 2 are fine. Step 3 works with a rule of thumb (1/3 stop = 1/2 grade). However, step 4 does not work without another test. Your Stouffer is designed to have 1/3 stop increments in exposure, but your paper has a non-linear response to linear log increments. In other words, double the exposure will not give you double the paper reflection density.

This is explained on my site here:

Dead Link Removed

Scroll down to 'Paper Exposure Compensation' and use the attached Excel file.

Hope this helps.