From: cjk4800@ritvax.isc.rit.edu (Ludwig van Halen Rocks RIoT)
Subject: Re: Novice questions: Reciprocity, EV
Reply-To: cjk4800@ritvax.isc.rit.edu
Organization: Rochester Institute of Technology
Date: Wed, 22 Jul 1992 03:31:32 GMT



>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?

You can think of grain catching light like water in a bucket.  If you pour a
normal amount of water into the bucket at a normal rate, you will find
that amount of water in the bucket when you are done.  Makes perfect sense,
right?  Now for reciprocity...

If you pour water into the bucket at 2 Liters/minute for one minute, you expect
there to be two liters in the bucket.  No problem.  If you poured in 1 L/m for
two minutes, you would still have two liters in the bucket.  This is
reciprocity.

Now, let's look at the extremes... if you poured in the water at 1200 L/m for
.1 seconds, you should find two liters in there, but what will really happen? 
You should have some pretty substantial splash-over.  Take a look at the other
extreme...  If you dripped in the water all day, you would have some of the
water evaporate on you and you will not find as much water in the bucket as you
dropped in.  

In these cases, our calculations may be correct, but in practice, physical
limitations make the water collected less than what we expected.  This is what
is known as reciprocity failure.

Without getting into the very intricate aspects of silver halide technology,
this is kinda how it applies to the AgX Crystal.

The Silver Halide Crystal (AgX) must receive a quantum number (between 3-6,
depending on who's theory you beleive) of photons in a certain time window for
the crystal to be sensitized.  When the crystal receives a photon, it creates a
partial charge to which a free electron is attracted to.  Since that too
creates a partial charge (now negative), the free silver is then attracted to
it and creates a sensitivity speck.  After a number of these, the crystal
becomes stable (see note on image stability).

Since the partial positive charge created by the photon is transient, there is
a limited amount of time in which the crystal can receive enough of these
photons to become stable (the partial positives are "evaporating" like the
water in the bucket).  Thus, when you are exposing with low light for long
periods of time, you will lose some of the effience of the system.  The same
goes with the other extreme... if you have a very short burst of intense light,
the partial charges don't have a chance to attract free electrons and Ag+ ions
in the crystal matrix, so you have a loos there too.

I hope that I answered (and not over-answered) your question. :-)

>
>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)?
>

Ugggh.  I need to be at work in seven hours and I need to squeeze some sleep
and drive time in there somewhere so I will leave it for another willing netter
to answer.  :-)  Besides, it is too late for me to contine making sense.

>Also, does the EOS Rebel have any undocumented features? Or is there a more
>extensive user manual than came with the camera?
>
>Thanks for any responses!
>
>-s
>--
>  Shamim Mohamed / {uunet,noao,cmcl2..}!arizona!shamim / shamim@cs.arizona.edu
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+Christopher J Kohanek          \\         CJK4800@RITVAX.isc.rit.edu 	      +
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