How Does Direct Positive Film Work?

[Donald Qualls]

Wow, looks like either (or a combination of) insufficient first dev, or bad bleach. I'd vote for the latter. What should come out, if everything works right, is a reasonably clear slide with the orange mask.

 

[drmoss_ca]

It was all complete guesswork - strength of Rodinal, time in it, how long to re-expose the film etc. I was happy anything happened at all!

 

[Donald Qualls]

It was all complete guesswork - strength of Rodinal, time in it, how long to re-expose the film etc. I was happy anything happened at all!

No doubt. If you cared about results, it shouldn't take more than a half dozen rolls to get to something at least reasonably viewable.

 

[flavio81]

Just once, out of curiosity (or perhaps stupidity), I took a colour negative film and did a homemade basic reversal processing. I think I used Rodinal as the B&W developer, flashed the film on the reel for a guessed 20 seconds in front of a bright light, then ran it through a C-41 presskit. The film came out almost black, but with a very bright light you can just see there are positive colour images. The Flextight managed to extract something more than my eyes could:

View attachment 292168

I can't say I recommend doing it! But it was a fun experiment.

Interesting!!

Perhaps you already know this, but you can do actual reversal development using a high contrast b/w developer as first developer, light re-exposure,and C41 kit chemistry. We have a thread on that.

 

[NedL]

Yes, for 2 hours in a warm water bath, flash the film and then process in a diluted developer.

Thanks!

Dr_moss and Donald:

Fascinating stuff! I'm slowly working on something right now and realized that I'm going to need an interpositive on film. I'm going to use regular film ( I will contact print a paper negative onto 5x7 FP4 ) but it got me wondering if there is any direct positive film available any more. The stuff at Ultrafine is out of stock...

Edit: photrio editor weirdly chopped off the end of what I wrote.. anyway I just was noting that it might be fun to put some direct positive film into a pinhole camera and it might get to what I'm trying to do in one step...

 

[Bronson Dugnutt]

The FPP sells their 'Super Positive' (a Svema Micrat-orto relabel) stock in 35mm & bulk. At 0.8 ISO it's an extremely finicky film to shoot and process, but is the only direct positive option of which I'm aware.

POTA, 11'

 

[NedL]

Thanks! 0.8 ISO would be perfect for me, but I need large sheets.... 5x7 minimum, preferably larger. I'll have a look at FPP anyhow... haven't been to that site in a few years. Regular 5x7 film is too expensive to go through a long process of it dialing in reversal processing ( and probably fussy enough that I should stick with contact printing normal film )... cool photo!

 

[Donald Qualls]

I suppose you could work out your reversal process with short clips of available slow film (like Pan F?) in 35 mm, then you'd only have minor fine tuning when you start on 5x7 -- and could also verify with the (significantly cheaper per sheet) 4x5 of the same stock first.

 

[Bronson Dugnutt]

but I need large sheets.... 5x7 minimum, preferably larger.

There's a more comprehensive list of stocks on Astrum's site (http://www.astrum-ltd.com/en/kino-foto-materialy.html) that includes sheet sizes for some films. Specifically the "Positive b/w MZ-3L «Svema» 13х18х200" seems suitable for your use case but would appear to be the ISO 3 stock which isn't a direct positive. FPP is the only retailer I'm aware of that offers any of these stocks to state-side consumers, albeit with obfuscated origins.

Echoing Donald's comment, I think every photographer with a sufficient level of sophistication should have at least one specific B&W reversal process in their repertoire. The advice we give folks just beginning to develop their own film (pick a film, pick a developer, and stick with them) is doubly true for reversal with it's higher degree of difficulty.

 

[pbromaghin]

The core emulsion, of whatever type is fogged during sensitization and then a second normal emulsion is precipitated on top. The developer or the coating usually contains a 'nucleating' agent that prevents development on the surface where there is exposure, but reveals the fogged silver where there is not any exposure.
PE

Is PE saying that Ilford DPP operates essentially on the same principle as a tintype?

 

[NedL]

Thanks guys... You're right of course, with some time and effort I could dial in reversal processing, and it could be done without wasting too much film. If I pick up some inexpensive litho film I'll probably give film reversal a try. Normal contact printing will work nicely for the time being for my current project, but the idea of one-step enlarged internegatives is very appealing.

 

[AgX]

Is PE saying that Ilford DPP operates essentially on the same principle as a tintype?

No, not at all.

The direct positve effect of a tintype is caused by reversed reflectivity by coating a standard emulsion on a black base.
What PE described is a direct positive effect by reversed transmission density due to a special emulsion technology.

 

[pbromaghin]

No, not at all.

The direct positve effect of a tintype is caused by reversed reflectivity by coating a standard emulsion on a black base.
What PE described is a direct positive effect by reversed transmission density due to a special emulsion technology.

Oh, shoot. And I had it all squared up in my mind, life was so simple, and the planet was revolving on it's axis with a tilt of 23.5 degrees. Now I'm not so sure.
Can you please explain "reversed transmission density" a bit further? I don't think I dare even attempt to understand the "special emulsion technology" part.

 

[AgX]

Well, in this case PE already said it better than I could do.
In other words: By that special-emulsion technology max. exposure does not lead to max. density but to max. transparency, a effect that I called reversed transmission density.

(I had to include the term transmission as this technique is basically different from the tintype technique.)