How exactly does direct positive film work?

[frobozz]

Kodak has made some film they call "Direct positive" (eg Direct Positive Panchromatic Film 5246) but they aren't anything more than films intended for reversal processing. If you toss them in D-76 they produce a negative. But they have some films that really are direct positive - toss them in developer, stop, and fix, and you get a positive image. For instance, LPD-4 and LPD-7 (the number is the base thickness) which are extremely high contrast; 2360 Direct MP and SO-291 (which seems to be more or less the same thing in its earlier prototype form) which are closer to continuous-tone; maybe some others.

My question is, how exactly do those latter films work? The developer turns the emulsion black anywhere light has not hit (eg the rebate is all black, just like with E-6 film) and turns it completely clear anywhere lots of light has hit (eg the leader) and of course a range of tones in between. It behaves like you would expect film to behave in developer - the more time in the soup, the denser the black areas become. But of course that means you can't really "push" the film by extending the development time, you have to cut the developing time...which means the blacks just get grey... while I suppose "pulling" it by developing more would work nicely. How did they make an emulsion which turns blacker in developer the *fewer* photons have hit it??

I conceptually understand normal reversal processing. This stuff however leaves me scratching my head. I'm still playing around with it to nail an EI and a developer/time combo that gives me the happiest results, so I think understanding how it works would help me on that path. So far I can develop the LPD-4 to very nice looking negatives in terms of density, but I'm way underexposing it. And on the 2360/SO-291 it looks like I've got the EI right, but the negatives are overall much thinner than I'd like, so the developer choice or time need some work.

Duncan

 

[Nicholas Lindan]

The HD curve - when the entire thing is plotted - looks like a haystack. Normally the 'front' of the haystack is used. Direct positive film is pre-exposed so that any subsequent exposure is on the backside of the HD curve. Solarization does the same reversal trick.

For more information see Mees or Haist.

 

[nworth]

Kodak Direct Positive Film (5246) was a reversal film. The processing was similar in most ways to E-6, but a different kind of bleach (dichromate) was used along with a high contrast black and white second developer. TMX has replaced this film. It can be used as either a negative or a reversal film. There are also at least two other kinds of "direct positive" film which yield a positive image with ordinary processing. I don't know the details of these films, except that they are used for special industrial processes. Most of the "direct positive" films that work with ordinary processing have fairly high fog levels and are not suitable for slides.

 

[Steve Smith]

I recall seeing an advertisement for a Rollei positive film which would give a positive image when processed as an ordinary black and white film. Did this ever come to fruition?


Steve.

 

[frobozz]

The HD curve - when the entire thing is plotted - looks like a haystack. Normally the 'front' of the haystack is used. Direct positive film is pre-exposed so that any subsequent exposure is on the backside of the HD curve. Solarization does the same reversal trick.

For more information see Mees or Haist.

Ah, OK. I don't have any photo theory books - would you indulge a few more clueless newbie questions?

I've only ever seen solarization as a printing technique. Obviously these films don't give that very recognizable "solarization look" - is that the difference between doing the flashing post-exposure or pre? And does this effect require two different exposures to light, or could you actually overexpose the living heck out of a normal negative and get it over to the other side of the curve? Interesting stuff.

Duncan

 

[frobozz]

Kodak Direct Positive Film (5246) was a reversal film. The processing was similar in most ways to E-6, but a different kind of bleach (dichromate) was used along with a high contrast black and white second developer. TMX has replaced this film. It can be used as either a negative or a reversal film. There are also at least two other kinds of "direct positive" film which yield a positive image with ordinary processing. I don't know the details of these films, except that they are used for special industrial processes. Most of the "direct positive" films that work with ordinary processing have fairly high fog levels and are not suitable for slides.

Right, 5246 was supposed to be processed in that reversal kit Kodak used to sell. It's got frame number markings and still camera perfs. Are you saying it's T-grain, or just that once they killed it as a separate product they recommended TMX as the substitute?

Here is what Kodak says about 2360 Direct MP film (2360 is the Estar 35mm stuff; 5360 is the normal acetate 35mm version, and 7360 is the acetate 16mm version):

EASTMAN Direct MP Film 5360 (35 mm) and 7360 (16
mm) is intended for making black-and-white direct
reversal duplicates of black-and-white negative or positive
film, or of color print films. Its primary application is in
producing workprints and musical scoring prints for
editing purposes. The duplicates, however, will not
necessarily be of release print quality. This low-speed,
orthochromatic reversal film is characterized by ultrahigh
resolving power and adjustable contrast. Although it is
reversal film, it should be processed in a conventional
black-and-white process rather than a reversal process.
The image produced has the same tonal-scale orientation
as the film from which it is printed, i.e. a negative from a
negative, or a positive from a positive.

Duncan

 

[nworth]

Right, 5246 was supposed to be processed in that reversal kit Kodak used to sell. It's got frame number markings and still camera perfs. Are you saying it's T-grain, or just that once they killed it as a separate product they recommended TMX as the substitute?
...
Duncan

TMX is a very different film, and probably a better one, than Kodak DP. It just happens to work as a reversal film as well as a regular negative film.

 

[Mike Wilde]

The direct reversals work on the 'back slope' of the H-D film response curve. I will leave it to you to research on the web what an HD curve is. You will come away a wiser photogrpaher.

Typically we work between the 'toe' and 'shoulder' of the postive slope of a film density 'D' in response to light exposure 'H' graphed plot.
There are logarithms involved. Don't fret them at the moment.

The goal of most photogrpahy is to use the relatively linear response of the curve between the toe and the shoulder.

Past the shoulder, as you add more light, the negative actually begins to become (relatively) less dense.
This is what direct reversal films exploit.

In manufacturing the films are exposed to enough light to take the density, once developed, up to the shoulder of the HD curve.
Your exposures push the light that has struck the film over the shoulder.

 

[frobozz]

OK, I get the concept now, I just wasn't aware there was a back side to the curve. So it seems like if I leave a chunk of film out in the sunlight for a few hours (or whatever the amount of light is, that gets to the other side of the curve) then when I develop it and fix it, it should be totally clear. Unless it's not just a humped curve, but sinusoidal, in that too much light gets us back on the upslope again?

I'm kind of not surprised by the behavior of the LPD-4, where the slope of the curve is quite steep, and therefore the images are insanely contrasty (as was its intended use.) But this 2360 stuff is much more interesting, where it appears to have a reasonably normal (but inverted) slope, albeit with an orthochromatic frequency response. More testing ahead...

Duncan

 

[holmburgers]

There is a kind of meandering discussion on this topic from a while back ((there was a url link here which no longer exists)), but with a few nuggets of goodness.

Here's what Ron had to say...

There are at least two types of direct reversal emulsions. The process is Develop, Stop or Rinse, and Fix using normal chemistry.

The two types in common use are Reversal F and Reversal P which differ considerably. These emulsions are VERY complex and beyond the scope of any discussion so far on APUG, but to make it very simple, A reversal F emulsion is usually say a pure chloride cube which is then treated with bromide and made into an octahedron kind of shape. The cur of this, the chloride, may be fogged, sensitized some way or just left as is before shelling. This process is called conversion and does not make a true core shell emulsion.

That final emulsion then is sensitized as normal but uses a special chemical called a nucleating agent added to the mix.

At exposure, exposed areas will not develop, but unexposed areas will develop thus giving us a positive image directly with a normal developer.

There are two disadvantages. One is that the nucleating agent can build up in the developer changing both direct reversal results with time, and also it can affect negative emulsions so the developer is best used one-shot or only for direct reversal emulsions with lower capacity. The second disadvantage is that these materials have a shorter tone scale and higher fog than normal materials. This results in re-reversal or combined negative and positive images in the same frame. This is rare, but very ungood when it takes place.

Reversal F was used in Kodak PR-10 instant films and in Ektaflex R material.

PE

 

[frobozz]

There is a kind of meandering discussion on this topic from a while back ((there was a url link here which no longer exists)), but with a few nuggets of goodness.

Interesting, I missed that one! (I guess because it was about paper.) Here is another quote from Ron in that thread:

Remember though that all normal emulsions can be Solarized by the method of overexposure and as described by Spence. We normally repress that possibility by chemical means so that it is not normally seen.

I wonder if he means "repressed by chemical means" in the makeup of the emulsion itself? If so, that would explain why you can't take normal negative film and just expose it to death until you get to the other side of the curve. There must not really be another side of the curve for normal B&W negative emulsions, by design, and for obvious reasons.

At least it's now confirmed in my head that the developing process is working as I imagined it: more development equals blacker grains of emulsion, it's just that which grains get blacker is reversed.

I can also see where "fog" of a sort would be a problem, thinking back to the recent discussions of latent image-keeping. If the manufacturing pre-flash inexorably starts to fade as this stuff ages, then the image is going to start to head back to the wrong (front) side of the curve... Even in the LPD4 that had enough development, there was just the eensiest bit of mottling in the deep blacks of the unexposed portions of the film. I guess that's what fog looks like on this stuff, eh? But that was some LDP4 that was found in a used bulk loader I bought. pretty much guaranteed to have suffered many years of awful storage conditions. I have some still-sealed LPD4 I'll break out and see if it performs better.


Duncan

 

[holmburgers]

It'll be interesting to see what you get out of it fer sher.

 

[Photo Engineer]

The "haystack" method that Nicholas mentions is brought up in old texts, but AFAIK it has never been commercialized by Kodak. The Reversal F and Reversal P methods have been commercialized by Kodak and a few other companies though. Basically though, the actual curves of direct reversal emulsions are the reverse of a haystack being more like a valley between two hills.

The reversal curve is on the left, the dmin is in the center, and the negative is on the right. The problem is to get the dip low enough to give good dmin and to obtain a wide enough valley to separate the two hills. Otherwise you get high dmin, and a phenomenon called re-reversal. In this phenomenon, you get a ghost negative image in the center of light spots, so for example if you have a bright reflection off something, you get a dark center.

We called this the fried egg effect.

Chemical reversal by design is more efficient than physical reversal, and thus specially designed emulsions were always the best and had the higher speed. All are known for being rather coarse grained for their speeds though.

PE

 

[frobozz]

Ah, OK, the valley concept makes sense too.

I would say there is an extremely wide valley on these films. I am not only not seeing the dark spots in highlights (slight overexposure) I'm also not seeing the leader turn dark again (massive overexposure.) Though I suppose I could even more massively overexpose it to see if I can reach the other side...

You call them the Reversal F and Reversal P "Methods" rather than emulsions. I assume that means that pre-flashing the film is part of the recipe? And does that mean that it can "fog" with age from latent image-keeping issues?


Duncan

 

[Photo Engineer]

Pre-flashing has never been used this way commercially. The method is in the precipitation techniques when the emulsions are made. As for fog with age, well AFAIK they age like any other emulsion. Kodak PR-10 instant film used a Br/I Reversal F emulsion with an incorporated nucleating agent. It seemed to keep well.

Kodak Ektaflex R print material used a Cl/Br Reversal F emulsion, and the European Kodak product Directachrome used a similar reversal F emulsion. Both used incorporated (ballasted? I've forgotten) nucleating agents. Directachrome was originally designed in Rochester and Chalon together (at that time Pathe). Directachrome was cancelled in Rochester and some of the people went on to work on Kodachrome film and the new (last) process.

Just a bit of history thrown in.

PE

 

[frobozz]

OK, so this emulsion is designed so that from the moment it's coated (with no pre-flashing), its grains will turn dark when exposed to developer...an effect that diminishes the more those grains have been hit by photons? Interesting. LPD4 has been around quite a while, did you have any experience with it? I gather the 2360 stuff is a little newer product.

The unexposed LPD4 looks like more or less normal film. Well, it looks a bit more brown than grey compared to most films. The 2360 however is a bizarre translucent red. The red must be an antihalation dye or something because it washes right out. In fact I've taken to prewashing it until the water runs clear, just so I don't get it in all my other chemicals like I did the first time.

Duncan

 

[Photo Engineer]

I have very little experience with the direct reversal emulsions themselves. I have seen the formula for the Directachrome emulsions, but that was in the 60s. I have worked extensively with the coated materials though, but that is all I can say. I have not worked with the product you mention.

PE

 

[ath]

I recall seeing an advertisement for a Rollei positive film which would give a positive image when processed as an ordinary black and white film. Did this ever come to fruition?


Steve.

It's called Rollei RSD and you can buy it. Here's the MACO page for the film.

It seems to be intended for copy work (i.e. making a direct copy of a negative or positive).

 

[frobozz]

That Rollei RSD sounds exactly like 2360 - slow speed, polyester base, antistatic coating, intended for direct copying, etc. Pretty pricey though, at about $6.50 a roll.

Duncan

 

[Nicholas Lindan]

I'm glad PE stepped in here ... always good to have someone in the thread who really knows what he is talking about.

For an example of haystack reversal with regular film there is Ansel Adam's famous 'Black Sun' photograph:
http://worshipthelight.blogspot.com/2008/06/ansel-adams-black-sun.html

Leaving the film out in the sun for a few hours and then developing it will indeed result in black, not clear, film. Sunlight is capable of turning silver salt to metallic silver all on it's own.

 

[frobozz]

Leaving the film out in the sun for a few hours and then developing it will indeed result in black, not clear, film. Sunlight is capable of turning silver salt to metallic silver all on it's own.

OK, sunlight has some special properties, bad choice by me for an example. Then what about cranking up my big-momma Canon 577G flash to full manual power and dumping it onto some HP5 repeatedly? Will that put it over to the other side of the curve? Or are modern emulsions made to not have the other side of the curve? Just trying to figure out how all this works? Though this particular line of thinking turns out to not have much bearing on the films in question, since that's apparently not how they work after all.

Duncan

 

[Photo Engineer]

Modern films have addenda in them to prevent solarization!

Come to one of my GEH workshops and I'll explain it to you!

PE

 

[richard ide]

A little off side but I used very low speed as well as camera speed high contrast direct positive engineering films and papers made by Kodak, Dupont, Fuji and Agfa in the past. Some were exposed through a yellow filter to give auto reversal (5000w light source). I remember screwing up a couple of exposures (contacts) and re-exposing to white light which made the film usable again. The overall white light exposure reversed and neutralized the yellow light exposure. Do you have any idea how these materials worked?

 

[Sparks n Fire]

I may have some insights on this. I recently found a patent that describes the mechanism of direct positive paper. Silver chloride will fog completely to black in a developer given there is no free bromide present. If you use a really slow chloride emulsion with just a little bit of a silver bromide emulsion of much higher sensitivity, then the emulsion is exposed and developed, that small amount of silver bromide develops into silver and releases bromine which can inhibit the fogging of the silver chloride, effectively rendering that area white. You create a direct positive this way.