Cs41 and developer reuse time calculator

Ilford's powder developer fact sheet (August 2004) says a liter of ID-11 or Microphen stock develops 10 rolls of 135-36 or 120 when you reuse it, and Perceptol 4. After the first film, add 10% of the standard time for each film that went through before. The tenth roll in ID-11 gets +90%, which matches the N+90% in Ilford's table, so the steps are fixed and don't compound.

With more stock the steps come less often: in 5 liters, Ilford adds the 10% after every batch of 5 films, so rolls 11–15 get +20%. The method assumes the 250–300 mL used for one film is poured back into the stock bottle and mixed with the rest before the next film.

The same sheet lists the downsides. Ilford doesn't recommend reusing the 1+1 and 1+3 dilutions, or push processing in reused developer. It says reuse lowers image quality slightly and the time correction is an approximation, because a roll of bright beach scenes uses up more developer than a roll of night shots. When consistency matters more than cost, Ilford recommends one-shot developing.

CineStill's Cs41 instructions (rev. 9/26) tell you to combine the used developer with fresh developer and add 2% to the developer time for each roll previously processed when you mix a quart (946 mL). That is a fixed step on the fresh time: after 4 rolls the next ones get +8%, after 8 rolls +16%, and a 24th roll would get +46%. A pint uses 4% per roll and a gallon 0.5%. The same sheet lists 8 rolls of 135-36 or 120 per quart and 4 per pint, says to use the rule for the developer only, and says the blix time stays the same. It also says mixing a weakened developer solution is not recommended for push processing and is less effective at lower processing temperatures, and that results are best when the chemicals are used once. CineStill's example steps (+8% for the next four rolls after four, +16% after that) are worded loosely, so check them against the PDF. CineStill's product page gives a larger figure: the kit is reusable for up to 24 rolls per 1000 mL, or 12 per 500 mL, at +2% per roll. The 8 and 4 are the instructions' fresh-solution chart; 24 and 12 are the most the product page says the kit can do with reuse. The picks above fill in either one. Rolls and times for each.

Other C-41 kits use different numbers, and some give a table instead of a percentage. Use the rule printed in your kit's instructions; the examples here only fill in the fields.

CineStill's Cs41 liquid instructions list the fresh-solution capacity: a quart (32 oz) processes 8 rolls of 135-36 or 120, and a pint (16 oz) processes 4. A gallon takes 32. The product page counts differently, up to 24 rolls per 1000 mL and 12 per 500 mL, so the "rolls left" line follows whichever capacity you enter.

The reuse step depends on the volume you remix to. The same sheet says to add 4% per roll previously processed for a pint, 2% for a quart and 0.5% for a gallon, after combining the used developer with unused developer to make up the full volume. That may be why forum answers range from +2% to +4%: the figures belong to different volumes. The "Cs41, 1 pint" pick above fills that in.

Some sheets give only a capacity. Kodak's HC-110 sheet (J-24) rates dilution B in a tank without replenishment at 5 sheets of 8x10 (or equivalent) per liter, and its replenishment section counts a 135-36 roll or a 120 roll as one 8x10 sheet. It gives no time increase per roll, so for that case enter the capacity and 0%. For one-shot use, the dilution calculator mixes it.