Using a DSLR for digitizing older 35 mm film slides ?

[DaveW]

As it was explained to me years ago, for a silver grain to start exposing it required a certain number of photons to strike it simultaneously. One photon hitting it would produce no reaction. Once this critical threshold point has been triggered, then every additional photon adds to the exposure.

What pre flashing film, or printing paper, does is activate all the silver grains so even the weak light coming through the densest parts of the film will add to the exposure, whereas before the light was too weak to initially trigger exposure in the silver grains it fell on.

That is one reason why you should not leave exposed printing paper near to the enlarger as it has in effect been pre-flashed so will be more liable to fog than unexposed printing paper at the same distance from any light source. I presume the effects will be different on negatives to positives since their densities are reversed?

I may have got it wrong but that was my limited understanding of the process involved?

http://www.ilfordphoto.com/aboutus/page.asp?n=125

http://www.rhdesigns.co.uk/darkroom/htm ... gging.html

However Betty, as you are using a digital camera for copying this will not apply. It is only for copying film onto film, or printing using silver based printing paper. You can do all your contrast manipulation or bring up the shadows in post processing on the computer if you shoot RAW, even reclaim some highlight detail.

Found a good multi-part article here for digital shadows and highlights see:-

http://www.ronbigelow.com/articles/shad ... light1.htm

DaveW

[AndrewC]

Well you wouldn't do it all in digital post processing. There are a couple of approaches commonly used to make sure you get good data to work with:

1) Basically follow an HDR approach and if necessary expose for shadows, midtones and highlights

2) Use a multipass scan to reduce noise in the shadows and then run a curve correction / shadow recovery

I happen to use Vuescan for driving my scanner as it can automatically take care of a lot of scanning issues.

http://www.hamrick.com/

Andrew

[Harold Gough]

I have some older film slides, that I would like to digitize.

Is the word "older" significant?

Harold

[Harold Gough]

According to DaveW's account this seems to be a nucleation process.

It is one thing to give paper a pre-flash but film in the camera would need a carefully calibrated, manual, pre-exposure, depending on film speed and aperture. At least the former would have been less of a concern for Ansell Adams. And the camera would have to allow double exposure on each frame.

Turning to HDR, this is very successful in capturing a wide range of tones but the result can be rather surreal, if sometimes pleasingly so.

Harold

[DaveW]

If I have understood this correctly it requires 10 photons simultaneously striking the silver crystal to activate it?

http://discovermagazine.com/2000/aug/featchemistry

"Most photons that strike film are squandered: The electrons they liberate, one electron per photon, fall right back into the "holes" in the halide ions. Film today thus requires 10 or more photons to make its quota of four silver atoms per crystal— that is, it wastes 60 percent of the light."

The above article is a little outdated, being written in 2000. It is stated that film would equal a 20 Megapixel camera and so digital cameras of its time could not compare. However we now have 24 Megapixel cameras coming out rather than the 3-4 Megapixel cameras of that time, whilst film has not really moved on.

I wonder what happened to the extra sensitivity the link talks about? Was the idea not practical, or did the manufacturers of film consider digital had taken over the market so the expense of developing it was no longer worthwhile?

DaveW

[g4lab]

My understanding of what I was doing which was explained to me by the guy that taught me how to copy slides was that it was what you are calling fogging. Trying to move the exposure curve back into a linear range.

I never thought about quantum efficiency in those days.

What I did not like about the copies was that although flashing did seem on comparison to improve the luminance channel (so to speak) of the copies,
the color saturation was less in the copies. I suppose I could have flashed the film with a color enlarger but I did not have one available nor was I good enough to do that.

[Charles Krebs]

Wow... this takes me back...

I always knew this process as "latensification". It does indeed work!

If you google "latensification" you'll get plenty of hits.

Here's one...
http://www.holography.ru/les19eng.htm

[Harold Gough]

Here is some discussion from another forum:

http://forum.pbase.com/viewtopic.php?f= ... 1610a18ac1

Harold