Film control strip interpretation
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Date

Red

Green

Blue

YB

/

/

 
Dmax B

/

/

 
Retained silver

/

/

Dmax B-YB

Contrast Spread

CRCB

CGCB

1

Contrast

DHR-DLR

DHG-DLG

DHB-DLB

DH      
DL (Speed)      
Dmin ( Fog )      

Exercise: After measuring your control strip, fill up the empty space in the above table. Calculate the retained silver contrast and contrast spread as in the formula. See the example below.

Date

Red

Green

Blue

YB

/

/

2.70

Dmax B

/

/

2.98

Retained silver

/

/

0.18

Contrast Spread

0.861.05=0.82

1.011.05=0.96

1

Contrast

1.30-0.44=0.86

1.77-0.76=1.01

2.03-0.98=1.05

DH

1.30

1.77

2.03

DL (Speed)

0.44

0.76

0.98

Dmin ( Fog )

0.32

0.63

0.72

For best quality, the contrast for blue colour should be limited from 0.91 to 1.09. At below 1 you will get a lower contrast than the real object, and above 1, you will get a higher contrast. The real density ratio (DHB DLB) in the control strip is 10. We use logarithm in photography. Log of 10 is 1.

We do not worry too much with the green colour because it will normally correct as long as red and blue colours are acceptable.

On assuming contrast spread of blue colour is 1. The contrast spread of red colour in this example is 0.86.

Contrast spread means the degree of deviation of the three colours from each other. They should be as close as possible. However, it is acceptable even as low as 0.8 because we can always use printer colour balance adjustment to correct them.

The PH of the working solution affects the red colour. While the density control affects the blue colour. On realistic, adjusting either one of them would also have some effect on the other. Therefore, you should not be too concern about matching a perfect control curve.

By looking at the contrast of blue colour:

If it is below 1, you will get a lower contrast than the original. Use density booster to bring up the contrast.

If it is above 1.1, you will get a much higher contrast than the original. Use density reducer. Diluting the chemistry will affect the efficiency of your developer, therefore, it is not recommended.

By looking at the contrast of red colour:

If it is above normal, the PH is too high. You will get too much red colour on your negative. The print out will be too cyan colour. Use PH reducer to reduce the PH value.

If it is below normal the PH is too low. You will get too little red colour on your negative. The print out will be too red colour. Use PH booster to bring up the PH value.

If your Dmin reads higher than normal, it is because of fogged or outdated control strip or over developing. If it is caused by over developing, use antifog to suppress the fog.

By looking at the retained silver.

Retained silver tells the silver left over after bleach. It affects only blue colour. If it reads higher than normal, you may have trouble with your bleach. Use bleach booster to correct it.

On plotting the control curve, you normally require to compare the data with a known reference. The curve you obtained is a relative value. What we have discussed in here are absolute values.

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