How Do They Calculate Development Time

Hi all,

 

I'm new to the forum, but I've been reading posts for some time. I've been recently getting into the chemistry behind film photography but I'm struggling to find useful articles for how film manufacturers actually calculate a development time for a given film in a given developer.

 

Obviously, you could establish a rough estimate through trial and error. I've seen a way of estimating by:

• Dipping a portion of film in the developer (specifically dip, not keep submerged)
  
• Count the time (in seconds) that it takes for the dipped side to turn the same colour as the undipped portion
  
• Given these time, divide it by 3 and that is the time (in minutes) that this film should be developed in the given developer at the given temperature
  

This is obviously reminiscent of the fixer trick. It also only gives an estimate (and I've only seen it mentioned a handful of times, surely if it was that effective it'd have more renown?). Then of course there is the zone system and "by inspection".

 

Any help you guys can give would be greatly appreciated.

Why not just follow the film manufacturers developing instructions ? They're all online or inside film boxes.

Here is one useful source of info LINK Massive Dev Chart Film Development, Film Developing Database

if you Google™ film development times -- there's a lot

 

e.g.,

Massive Dev Chart Film Development, Film Developing Database

Thanks for the recommendations. The Massive Dev Chart is a very valuable resource for sure- one I've used a few times. However, what I'm really looking for is the method by which the manufacturers arrive at these recommended times and temperatures. It's of interest to me as I'm currently trying combinations of homemade developers as a side project and I'd like to see if there's a way to get an optimal time, outside of simple trial and error.

Find things on sensitometry like https://www.kodak.com/uploadedfiles/motion/US_plugins_acrobat_en_motion_newsletters_filmEss_06_Characteristics_of_Film.pdf

Film development times are not calculated. Instead, they are determined through a process of testing. OP referred to this as 'trial and error', but it's actually a fairly rigorous process.

but I'm struggling to find useful articles for how film manufacturers actually calculate a development time for a given film in a given developer.

The short answer is: They don't. Calculate the time, that is. It's found empirically in the first instance, by exposing film to a known series of exposures, and finding the development time that gives appropriate densities.

 

Calculation could be done, but different developing agents have differing 'induction' times - the time it takes for the faintest image density to appear. This induction time is modified by other developing agents (modern developers rarely use just one agent) and by other chemicals in the formula. Developing agents also have different temperature coefficients.

 

Programming all this into a calculation would most likely take far longer than just dunking a batch of test strips into the developer formula, and pulling them out at varying lengths of time.

• Dipping a portion of film in the developer (specifically dip, not keep submerged)
  

• Count the time (in seconds) that it takes for the dipped side to turn the same colour as the undipped portion
  

• Given these time, divide it by 3 and that is the time (in minutes) that this film should be developed in the given developer at the given temperature
  

That doesn't sound right at all.

You need to find a time that gives a good density - around 2.5D - to a fully-fogged piece of film, and after fixing.

I'm currently trying combinations of homemade developers as a side project and I'd like to see if there's a way to get an optimal time, outside of simple trial and error.

That way madness lies.

It's interesting, but don't expect to come up with a magic formula that's better than any commercial product.

 

Personally, I found it more interesting to see what readily-available household or pharmacy products could be turned into a reliable developer formula - discounting instant coffee.

 

For example: Paracetamol (Acetaminophen) tablets are easily converted to Para-aminophenol, which is the active agent in Rodinal. And Vitamin C powder (ascorbic acid) can be substituted for hydroquinone in almost any formula that uses it.

 

However, most commercial formulae make use of the phenomenon of 'super additivity' where the developing power of two chemicals together is far greater than their individual strengths. Metol and hydroquinone, or phenidone and hydroquinone are common super-additive pairs, as is phenidone and an ascorbate salt.

 

That's probably enough to be getting on with!

Edited June 1, 2020 by rodeo_joe|1

Film manufacturers have nearly 200 years of practice making and developing film. Films are blends of three salts of silver listed in order from least sensitive to most sensitive. A. Silver iodine B. Silver chlorine C. silver bromine. These are silver combined with a halogen (Swedish for salt maker). Silver halogens are mixed with gelatin. The result is called an emulsion. Innumerable recipes yield films that react differently to light and other radiant energies. We are talking, the color of the light and the ISO of the resulting film.

 

 

Test films are made using a precisely calibrated exposing device called a sensitometer. Generally these films are exposed to create a pattern of 21 rectangular areas. The first is unexposed film followed by a series of spots with increasing exposure. Each step is incremented ½ stop (50% increases). The last step is premeditated to be D-max (maximum blackening). Many test strips will be fabricated; each will be developed in standardized developer formulas.

 

 

 

The processed test strips are then measured using a precisely calibrated instrument known as a densitometer. This instrument shines a light of known strength, and this beam is caused to transverse each step of the test film. The amount of light that exits each step is measured as to its brightness. This value is recorded and converted to a mathematical value in logarithmic notation.

 

 

All 21 steps on the test film will be thus read and then plotted on special graph paper ruled in a logarithmic pattern. Logarithmic notation is used because otherwise the graph of the 21 steps would be huge. This method compresses the resulting graph to a manageable size -- plus the shape of the graph using this notation has well studied.

 

 

The result is ½ of a bell shaped pattern. From the graphed pattern, the ISO speed of the film can be deduced as well as the film’s scale and contrast. This is a tried and true method that has been immortalized by the ISO (International Standards Organization Geneva Switzerland).Below is H & D graph, named in honor of Charles Driffield and Ferdinand Hurter who in 1890 published this method.

Edited June 1, 2020 by alan_marcus|2

Once it is determined for one temperature, there is a way to calculate times for other temperatures.

 

Otherwise, changing the development time changes the resulting contrast (gamma).

 

Some data sheets will have graphs for different times, so you can see that the numbers

aren't fixed, but selected for an optimal contrast. (EI value changes, too.)

 

A little is that people like faster films. Decreasing the time tends to decrease

the effective film speed (EI), so there is some tendency not to select times with

low EI values.

 

Note that it is mostly different for color films. In that case, the films are designed

to match the development time, which is chosen for each process.

And so much of that work was done with not a spreadsheet or calculator in sight! Somewhere I've got a nice original match-the-dots Kodak densitometer- need to find it just to have a look at it again. Something the OP might try is the classic "ring-around". That can get exposure and development nailed down quite well. Surprised at how few references Google picked up but go to the very bottom of this page- Johnston's Not Much of a System System

I also have one of those Kodak densitometer laying around somewhere. I know I used it years ago when learning the zone system. Damned if I can remember how it works now.