Superia XTRA 400
Unicolor kit, about 12 days since mixing

  2016 April 9    Film   Developing

Introduction


Many people ask, "How many rolls can you develop with a C-41 kit?" 

That's a very good question, since you probably don't want the developer to quit halfway through a roll of film.

With HC-110 in black & white developing, we knew exactly how much is used up per roll.  We don't know that with the C-41 kit.  Therefore, we'll have to try making an educated guess.




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In This Article


20 Rolls?

30 Rolls?

40 Rolls?

The Equation

Conclusion




20 Rolls?


The recommended capacity of a Unicolor 1-liter kit is 12 rolls of 135-24.  A few different sources say 20 rolls max. 

If that's true and you've already developed 12 rolls, that would mean you could still develop 8 more rolls.  The developer solution would be at 8/20th of its original strength, or 40%.  Again that's probably not the exact amount;  we don't know for sure how much developer there was, or how much gets used up per roll.  But let's go with these numbers.

40% of original strength means the developing time for the 13th roll would be 1/0.4, or 2.5 times the original.  So, instead of 3 minutes at 102 Fahrenheit, the correct time would be 7.5 minutes.

However, I noticed with the Unicolor C-41 kit that it's about as good at 10-12 rolls as it is at 1 roll.  Same developing time! 

That means the absolute capacity of the solution has to be considerably more than 20 rolls. 

Some say they've gotten more than 25 rolls out of a Unicolor kit, with acceptable results. 

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30 Rolls?


Because some people have said "at least 25 rolls", let's guess 30 as the absolute capacity.  How's that for precision?

After 12 rolls the solution would still have capacity for 18 rolls out of 30, or 60% remaining.  The correct dev time for the 13th roll would therefore be 1/0.6, or 1.67 times normal.  So, instead of 3.5 minutes at 102 F, it would be 5.8 minutes.

I didn't run the developer for 5.8 minutes on the 13th roll, though.  I ran it for about 4 minutes and the negs looked great.

That's only 1.07 to 1.14 times the normal dev time.  That means the solution might still be at 1/1.14 = about 88% of full capacity. That means 12 rolls was only 12% of the total capacity.  12 is 12% of 100, which would mean the absolute capacity were 100 rolls.

Somehow I doubt it's really 100 rolls, though.  Probably it's much lower, and the negs are actually under-done with a dev time of 4 minutes at this point.  Even if it were 100 in concept, I'm sure oxidation lowers the number.  So, maybe it's not 100 rolls, but how about...

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40 Rolls!


Somewhere I read that a person got 40 rolls out of a one-liter kit.

I have no idea if the quality was still there at 30-40 rolls.  As long as it was still developing film, that's all we need for the calculations.

So, let's set the absolute capacity as 40 rolls.  By "absolute capacity" we don't care if the rolls start to have "effects".  We just care that there's enough developer to complete a roll of film, even if the dev time needs to be twenty minutes or something.  I think 40 rolls' absolute capacity is a reasonable first-approximation, taking into account auto-depletion due to O2 and CO2.

After the 12th roll, there would be 28 rolls of capacity left.  That's 28/40, or 70% capacity.  The correct dev time for the 13th roll would be 1/0.7, or 1.43 times normal. 

So, the original 3.5 minute dev time would become 5 minutes.

I went ahead and used that 5-minute dev time for roll #14.  (Technically it should be 5 minutes and 23 seconds, but 5 minutes is close enough.)  The picture at the top of this page is from roll #14, dev time 5 minutes.  Good contrast with no color shifts, no mottling or banding, and no uneven tones. 

The next roll after that was developed with a 5-minute 36 second dev time.  That was according to the calculations I worked out.  Again, great results.  The next roll after that: 5 minutes, 50 seconds.  No discernible decrease in quality.

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Equation


Now we've got a simple algebraic way to figure out the dev times for your Unicolor C-41 kit.  It may not be perfect, but it seems to work.  It may even err on the side of overdevelopment, which is the better way to go with C-41 if you're not sure.  More about that later.

The math reduces to a simple equation:

t = 140 / (40-n), where "n" is the nth roll that you're going to be developing.  If you're doing two rolls at once, just develop them both as "n", and it's close enough.

t  is the developing time you use in place of the recommended 3.5 minutes.  The temperature should be 102 degrees F.

Failing to extend the dev times could explain why some people get blue images.  When you under-develop or under-expose C-41, the shadows (especially) become blue.  I've noticed that many times.  I think it might be that the yellow dye layer (you know, the top one) gets more development relative to the other two layers, which take longer for molecules to diffuse into them.  When you color-invert yellow, you get blue.  So, if your pictures have too much blue in them and the shadows are too grainy (and blue), it could be from not extending the dev times to compensate for depletion.  This would be similar to running the chems at too low a temperature.

Remember, this equation is for the FPP / Unicolor C-41 kit in one-liter size.  The 2-liter kit should theoretically be able to develop twice as many rolls. 


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Conclusion


If you just want to develop eight to twelve rolls in your C-41 kit, you might not even need to extend the times.  However, the kits can be stretched well beyond that number of rolls. 

Good technique should yield good negatives to 20 rolls or more. 

For the first seven or eight rolls, no change in dev time should be necessary.  Beyond that, I'd start using a conversion factor, based on an absolute capacity of 40 rolls for a one-liter kit  By the tenth roll, the time would be about 4 1/2 minutes.  (I was using 3.75 to 4 minutes by pure guesstimation, before I ever sat down and figured any of this out.)

Where n is the number of the roll you're developing, the correct time would be 140 divided by (40-n), to give a time in minutes.  So, on the 18th roll, you'd need 140/22, or about 6.3 minutes. 

Again, you don't really need to mess with this if you're developing 8 rolls or fewer, but it's more important the more rolls you do.  At 20 rolls it would be about 7 minutes;  at 25 rolls, you'd need to use 9.33 minutes.  That's quite a difference from 3.5 minutes.

You'll have to experiment and see how it works for you.  If anything, color shifting is more likely to occur when you don't extend the times correctly.

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