After reading about this developer on the web I decided it would be worth trying, especially for roll films.  When shooting large format sheet film I can develop each negative individually to control the brightness range on the film.  That isn't generally the case with roll films.  A single roll may contain images with brightness ranges very different from each other.  How do you process such a roll to achieve acceptable results for all the images?

According to the web review I read one answer might be Diafine.

So what is Diafine?  Diafine is a fine grain, full (or greater) film speed developer.  It has some great features.

• It provides almost a full stop of film speed increase versus the manufacturer's rated speed.  (This can be debated but suffice it to say that if you normally shoot 100Tmax (for instance) at 100 you can achieve comparable results shooting it at 160.)
• It can be used at any temperature between 70 degrees F and 85 degrees F.
• Development time makes no difference provided the film gets at least three minutes in each bath.
• Films of different speeds can be developed together in the same tank at the same time.
• The stuff seems to last forever and can be continually re-used.
• It is a compensating developer.  This means that shadows develop fully with excellent detail and highlights won't be blocked up.

How does Diafine achieve all this?  In part by being a two bath developer.  A Diafine kit makes two solutions.  Solution A is the developing agent and solution B is the accelerant.  When the film is processed it is first placed in Solution A.  The developer is absorbed into the film's emulsion layer.  After the recommended time of three minutes solution A is poured back into the bottle.  Since it doesn't have any accelerant in it it doesn't weaken over time.  If you leave the film in the solution longer than three minutes no harm is done.  All the developer that the emulsion can hold is absorbed within three minutes, additional time has no effect.

Next solution B is poured in also for at least three minutes.  Solution B contains the accelerant and causes the developer to begin working.  Since the amount of developer available is only what was absorbed into the emulsion the development runs to completion, unlike other developers where development is stopped at some pre-determined point.  After three minutes or more solution B is poured back into the bottle.  Since development has run to completion there is no unused developer left to react with the accelerant in the bottle (though the accelerant may be slightly weakened in practice this has no effect at all.

As with solution A if the film is left in for more than three minutes no harm is done.  Development runs to completion in three minutes so longer times in the second solution don't result in over development.

No stop bath need be used, again because development to completion, so fixer can be poured in immediately after solution B is poured out.  Fixing and washing are done normally.  If you re-use your fixer you can use a water rinse between solution B and the fix to slightly extend the life of the fixer.

One of the great things about two bath development is that the shadows will develop with great detail and the highlights won't blow out.  Highlight areas which received the most light require more development.  The since the only developer available is what is contained in the emulsion (absorbed during the first bath) the developer is exhausted before all the exposed silver in those areas is developed so the highlights maintain detail and don't block up.

The shadow areas have less exposed silver so there is sufficient developer in the emulsion to develop those areas completely.  This results in excellent shadow detail.

Combining those two effects means that contrast is controlled and the resulting negatives will have "normal" contrast even if the original scene had extremely high contrast.  Normal contrast scenes are also rendered beautifully.  In my testing I found that low contrast scenes don't lend themselves to development with Diafine.  The resulting negative can be quite flat.  (Update:  If low contrast scenes are to be developed using Diafine I have found that shooting the film at the manufacturer's recommended ISO setting will allow the resulting negatives to achieve more normal contrast.)

So when should you use Diafine?  The short answer is "it depends".  If you are using roll film where you can't develop each negative independently and the contrast range of the original scene was high or normal then Diafine will give very good results for each negative on the roll.

If you are using sheet film then Diafine can also be used for those negatives of normal to high contrast scenes.  For negatives of low contrast scenes another developer will probably yield a better result.

Now I would like to make a comment about film speed.  The instructions with Diafine state that most films will receive a speed increase when used with Diafine.  For instance TMax 100 (and now 100 TMax) can be used at ISO 160 rather than 100.  In my testing I generally found this to be true, but I also found that because of the compensating effect of Diafine if film is exposed at the manufacturer's ISO rating shadow detail is improved and highlights still don't block up.  (Using the manufacturer's ISO rating also allowed me to use Diafine with negatives of low contrast scenes and get acceptable negatives.)

Because of this I generally shoot at the manufacturer's stated ISO (or at my personal EI determined by testing with my "regular" developer).  This gives me more flexibility than following the Diafine directions would allow.  I don't have to decide ahead of time that I want to used Diafine.  I can shoot as I would for my normal developer and if I later decide to use Diafine I can do so without worry.

One last note- Diafine makes developing film almost fun.  I have the two solutions mixed and ready to go in my tempering tray.  I just load the film into the developing tank and pour in solution A.  I don't have to worry about time, temperature or agitation (other than a couple of knocks to dislodge air bubbles after pouring in the solutions).  After three to ten minutes (yes I've tested times up to ten minutes) in solution A I pour it back into the bottle and then pour in solution B.  After more than three minutes it is time for the fixer.  Simple, fast and actually fun.

The batch I'm currently using has been stored in brown plastic partially full bottles for over seven months.  The remaining solution is poured back into the appropriate bottle after use.  The solutions definitely get discolored from the anti-halation dye in the film but it doesn't seem to affect them at all.  There are stories of news photographers keeping Diafine solutions in bottles in their desks for years.  I don't doubt it!  (Update:  I used my original batch of Diafine for over a year.  It was working just as well at the end of that time as it was at the beginning.  The solutions were stored in regular plastic bottles at darkroom temperature, typically about 70 degrees F in my darkroom.  I made no effort to keep the bottles full or to expel air after use.)