Getting development to gamma 0.6, making your own print calibration negative

[whojammyflip]

Hi all, this is me trying to share an idea I have just stumbled upon regarding calibrating development times, potentially calibrating VC paper grades, and also investigating ISO settings for your film/developer/meter combination. I was trying to calibrate VC paper using a contact print of a thin step wedge, and found that I was getting a lot of flare mucking up the prints of the wedge.

I realised that to get around the flare issue, I could just create a negative of the step wedge, creating a high density area around the edges of the step wedge by placing it on the lightbox and photographing it. Then I thought the problem here is that I will no longer have 0.5 steps, but 0.5xgamma of my development. I realised that this aspect might be pretty useful, as I don't know whether my processing is way off. So I set about making a negative of a step wedge, and hey presto, I could a) know if my development sucks, b) calibrate my enlarger c) know what speed to rate my film (provided there are no significant spectral differences between light box and daylight), all in one shot!

I use a thin (1cm wide) Stouffer calibration step wedge bought from RH in the UK, which covers a density range of 10 stops or 1:10^3 ish. I also have a Darkroom Automation enlarging meter, which means that I can get a direct measure of density of light from the enlarger lens on the baseboard. I am using a lightbox I picked up on Ebay originally sold by Calumet. I meter off the lightbox using a Gossen Lunasix F with the dome moved aside so it is reading a reflection reading (not incident).

Steps:
1. Fire up your lightbox, lay your 20 x0.5 step Stouffer step wedge on top, frame it up with the camera and probably a macro lens so that the wedge fills the frame
2. Overexpose the step wedge by 5 stops, which should land the middle of the step wedge at Zone 5
3. Develop, making sure that temperature and agitation are noted down exactly
4. Put negative into enlarger and measure readings of light intensity for each step of step wedge (you could try to use a Ilford EM10 for this with different aperture settings on the enlarging lens)
5. You should see a decrease in brightness of 0.3 between each step in the step wedge, for a step wedge with 0.5 stop wedges (this is the gamma 0.3/0.5=0.6)
6. Use your new step wedge with custom steps (somewhere in the region of 0.3) to find your ideal enlarger contrast settings with your paper, for a given range of steps (ie, you could try to get steps 5-15, which is an actual 5 stop range in real life scene) to land perfectly in the range of paper base white to darkest black

Notes:
a. by overexposing the shot by 5 stops relative to the light meter reading, the middle of the negative should be back at zone 5, and the lightest part of the step wedge should be zone 0
b. through overexposing by 5 stops on the lightbox, the area of the negative around the step wedge blocks a lot of light from the enlarger and effectively forms a perfect mask on your step wedge, any bleeding of light between different steps being the remaining issue, but replicating what will happen during printing anyway
c. if you find that you do not get a tone forming on the negative at the first step, it means that you need to adjust your ISO ratings
d. obviously, this is limited to the range and accuracy of densities on your Stouffer step wedge
e. credit is due to the Anchell book, Variable Contrast Printing Manual, but this differs significantly in that it includes the additional step of photographing the step wedge first on the lightbox, I wouldn't have done this had I had a full frame step wedge, but then I would miss out on the calibration of negative development procedure and ISO settings. I'd still recommend picking this book up for understanding the VC paper calibration process.

Hope someone finds this interesting.

 

[RPC]

I have never used a Stouffer step wedge but when you say it is 20x.5 do you mean each step is .5 density units apart or .5 stop? If .5 stop apart, then each step would be .15 density units (.3 density units for one stop times .5 or one-half) apart. That is what my old Kodak step wedge is. Thus your negative steps would be .15 times the gamma of the film. If .6, then .15 x .6 = .09. If each Stouffer step measures .5 density units apart then you would be correct.

 

[whojammyflip]

I think there is a misunderstanding potentially because of the number 0.3 appearing, and most people thinking in log10, as in 10^0.3~2. However, here I am talking in stops, and 0.3 stops for a 0.5 stop step, but this is a different 0.3, its 2^-0.3, or 0.8, so that each increase in density in the negative of the step wedge would result in 80% of the transmission of the previous step, if developed to a gamma of 0.6.

For clarity:

i) My Stouffer step wedge is in 0.5 stops, ie, the log base 2 density change is 0.5, log base 10 is 0.15, from one step to the next. The entire range on the Stouffer step wedge with 20 steps is 10 stops, ie a density change of 1:2^10, or 1:10^3.0103~10^3, as pointed out in the beginning of my passage with the 1:1000 density change.
ii) The negative of the Stouffer step wedge, if developed to a gamma of 0.6 would result in the conversion of a 0.5 stop change to a 0.3 stop change per step of the step wedge.

But the real advantage of this is that you can tailor your printing and processing all the way through with this method, from initial scene to final print, and to boot, have an idea of whether your film speed rating needs adjustment.

What can be measured can be improved, etc, etc

 

[Stephen Benskin]

You could always do the testing by the traditional sensitometric method. Contact the step tablet.

 

[whojammyflip]

a. that wont help with film development
b. see first paragraph...I had originally been making contact prints of the step wedge, and realised that contact prints do not replicate the same flare characteristics of printing, whilst introducing other issues of their own, such as light bleeding and bouncing around the step wedge

 

[Kirk Keyes]

You CAN contact a stepwedge against film.

 

[whojammyflip]

This is for 35mm film and the step wedge is 10cm long. I could not then print from the negative.

 

[Jonathan R]

According to the Stouffer web site, the model TP 35-21 step wedge is already the dimensions you want (35 mm). It would allow you to calibrate your printing process, but not your development, as you say.

 

[Kirk Keyes]

You can cut a 4x5 step wedge in half and contact it against 35 mm film.