From: sog@craycos.com (Steve Gombosi)
Newsgroups: rec.photo
Subject: Re: Novice questions: Reciprocity, EV
Date: 21 Jul 92 15:30:33 GMT
Organization: Cray Computer Corporation

In article <19389@optima.cs.arizona.edu> sham@cs.arizona.edu (Shamim Zvonko Mohamed) writes:
>Could someone please answer a couple of questions for a novice?
>
>Why does reciprocity failure occur? That is, for a grain of emulsion, why does
>the linear relationship fail if the photons arrive spread out over a longer
>interval? And is my thinking correct that for colour film, the different layers
>have different reciprocity curves, which is why there is a colour shift?

The exact mechanism of exposure is, surprisingly enough, imperfectly understood.
The last time I researched this topic, the theory (the "Gurney-Mott theory")
ran thusly:

Silver halide crystals consist of positivly chraged silver ions and 
negatively charged halide (Cl, Br, etc.) ions. When a photon strikes
the crystal, it liberates the "extra" electron from the halide ion.
These free electrons can migrate pretty freely within the crystal, but
they tend to accumulate at defects in the crystal lattice, causing an
accumulation of negative charge there. Alternatively, they may recombine
with the halogen atom to regenerate the halide ion. If they accumulate
in sufficient quantities at one point, the charge will build up to the
point where it will attract the positively charged silver ions which will
in turn combine with the electron to produce a small quantity of metallic
silver at the point. The more silver that accumulates at this point, the
bigger the defect, and the more pronounced the effect as additional light
strikes the crystal. When the emulsion is developed, this small speck
of metallic silver catalyzes the production of metallic silver throughout
the crystal and forms a nucleus around which the silver accumulates,
eventually causing the entire crystal to be converted to metallic silver.
The excess halogen atoms diffuse into the surrounding gelatin of the 
emulsion.

The reciprocity law (the "Bunsen Roscoe Law of Photochemical Equivalence",
to use its formal name) is:

	Exposure = Intensity x Time

There are two types of reciprocity failure - high intensity and low intensity.
The type most photographers are familiar with is the low-intensity variety,
although with the advent of 1/8000 sec (and faster) shutters and automatic
flashes, a lot of us may experience the other variety from time to time.
Why does this happen? Well, if the intensity of light is too high, large
quantities of electrons are released *very* quickly. The silver ions don't
have time to migrate to the charge concentrations. The halogen atoms don't 
have time to migrate out of the crystal. Some recombining with the
halogen atoms occurs, and the silver atoms that are formed are diffused
throughout the crystal. They don't form a large enough concentration of
silver to efficiently catalyze the exposure or development process. The result
is a loss of effective emulsion speed.

In low speed reciprocity failure, electrons are released very slowly into
the crystal. There's only a slight charge accumulation, so the silver ions
don't congregate very rapidly at the crystal defects. Very small quantities 
of silver are formed in the crystal
defects and these may donate electrons to the free halogen atoms, regenerating
silver halides.

Since exposure depends on this electron and ion migration, anything that
affects the charge distribution within the emulsion or impedes the migration
of halogen atoms out of the crystal will affect its reciprocity 
characteristics, including the presence of large, ugly, organic dye coupler
molecules in an emulsion layer ;-). Since the dye couplers are different
in each layer (one hopes!), each layer has different reciprocity character-
istics. It's amazing they can balance the emulsions as well as they do.



>How are EVs defined? I guess EV n+1 is twice the amount of light as EV n. So
>what is bright sunlight (f16, 1/asa)?

EV is an *exposure* value, not an *illumination* value. This is a common
misunderstanding, exacerbated by the tendency of manufacturers to use the
term EV when they actually mean "EV for ISO 100 speed film".

The definition of EV is from the ANSI standard for calibration of exposure
meters (ANSI PH 3.49-1971) [with a little help from donl mathis ;-)]:
	 
	EV = log2(BS/K) = log2(A^2)/T
where
	EV = exposure value.
	B = Field luminence.
	S = ISO/ANSI film speed (e.g. 100)
	K = exposure constant (reflected light); the luminance units
	    which B must be expressed in must be identified for any
	    given K value to be meaningful since B and K change
	    proportionally.
	A = Aperture (f-number)
	T = exposure time (seconds)

So, for the example you cite ("sunny f/16" rule):

	EV = log2(16^2/(1/ASA)) = log2(256*ASA).

Basically, EV is a shorthand way of referring to all combinations of shutter
speed and aperture that produce a given amount of exposure. A change of
1 in EV correspond to a change of one "stop" in exposure.

Whew!

Perhaps the keeper of the "FAQ-to-be" should include this...

>Also, does the EOS Rebel have any undocumented features? Or is there a more
>extensive user manual than came with the camera?

Of course is has undocumented features...there's a COMPUTER in it, right? ;-)
Sorry, couldn't resist. I don't know anything about these newfangled gizmos -
I'm an unreformed photographic troglodyte, myself.

Steve
Newsgroups: rec.photo