Has anyone had any experience with raising the film speed through latensification?

[Doremus Scudder]

Pre-flashing (or post-flashing; same effect) is often touted as increasing film speed, but it really doesn't

What you are doing by flashing film (or paper) is to give it a very small, overall fog exposure. This has the effect of overcoming the film's "inertia," so to speak and getting it to the point where the very next photon or so will actually make a usable exposure. Film has a threshold exposure, below which nothing happens. It's not till enough exposure has accumulated that the next light hitting the emulsion will make the change needed for something to be developed.

So, by fogging the film a bit, you're ensuring that some more shadow detail will be recorded on the film, but because of the fog, you are decreasing the separation between shadow densities a bit.

Think of it in terms of AA's "exposure units." One exposure unit is what it takes to get a Zone I density on the film. So:

Exposure Units
1 - 2 -- 4 - 8 - 16 - 32 - 64 -128 - 256 - 516
Zones
I - II - III - IV - V - VI - VII - VIII - IX -- X

Now let's pre-flash our film by giving it a Zone I exposure overall, i.e., one exposure unit's worth:

Exposure Units
2 - 3 - 5 -- 9 - 17 - 33 - 65 - 129 - 257 - 517
Zones
I - II - III - IV - V - VI - VII - VIII - IX --- X

You can see that in the case with pre-flashing, the lower values get boosted. Zone I actually gets a Zone II exposure. Zone II moves to halfway between Zones II and III, Zone III moves up about a third of a stop. After that, the pre-exposure has a decreasing effect and makes no significant difference above a certain point.

So yes, you can get more shadow detail by pre-flashing but notice now that instead of doubling in exposure (and the corresponding density) the lower Zones now have less separation. Zone I to Zone II is now only 1.5x density change; Zone II-III less than 1.7x and Zone III-IV 1.9x.

Helpful if you need it and a really good tool (especially for paper in certain cases), but not a real speed increase for film, since whatever fog is introduced needs to be printed through to get a real black. The effect is boosted shadow values with less separation.

Best,

Doremus

[Greg]

In the mid 1970s latensification was one assignment in my Materials & Process class with Todd and Hill at R.I.T. Final determination was that it did work but very limited in increasing the film's ASA. In practice it wasn't worth the effort. Detailed article on Double Latensification in PHOTOGRAPHIC SCIENCE AND ENGINEERING Volume 8, Number 2, March-April 1964.

[Mark Sampson]

Just read the "Hoya Starscape filter" thread. There, forum member konakoa mentions "hypering" Tech Pan. So you might ask them about it.

[Andy F]

Thanks everyone. This was really helpful.