Re: EI question

[Ben Kreunen <ben@xxxxxxxxxxxxxxxxxxxxxxx>]

>Technically speaking, a film's speed is measured through the specific
>density change in the negative over the base of the film.  HOwever, what
>type of light should be used in exposing the film to measure the density
>change?  What percentage of infrared radiation?  As soon as you answer
>that question, you make inappropriate all other mixes.

Providing different speeds for different lighting mixes...  Colour film 
data sheets usually provide different film speeds for different mixes of 
light (daylight/tungsten with various filters)... Panchromatic BW films 
also come with film speeds for different developers/concentrations... and 
the IR data sheets provide speeds for different IR filters.

Film speed... I had used "shadow detail" because I didn't want to confuse 
the issue by mentioning 0.1OD above film base + fog... which is nice if 
you're using a sensitometer and densitometer but since most of us don't 
have either we rely on more agricultural tests. In a 'normal' scene this 
comes from the shadow detail....  areas where there is little light (either 
visible or IR) reflected from things that don't reflect much light (either 
visible or IR) so in this respect the 'mix' should be the same as the 
illuminating source (daylight).

So it is possible to produce a reasonably accurate EI for infrared film (in 
daylight). What they say in their product info sheets is of course up to 
them, I just feel that it is a bit of a cop out to suggest that the numbers 
they provide may vary greatly. I've worked out a speed for my film and I 
haven't found any conditions yet where it changes.

>No big deal, however.  Just do some film
>testing, something too few people bother performing.

True for any film (and many people)... I'm in the middle of working on a 
page going through some theory and ending up with some agricultural tests 
that will provide not only a more accurate film speed but a better 
understanding of the film and it's processing at the same time... The hard 
part is to describe the test assuming the least knowledge of photography 
(and I have to go and shoot some samples, since i threw out my earlier tests)

Cheers

Ben

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Gallery and Photography Tips (including IR)
http://www.bigbenpublishing.com.au/gallery/photography/
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