Somebody else will give you a better explanation of true film
speed.  Its basically how much light you need for something to show up on
the negative.  

All I know is that with developers like Xtol, I can shoot ISO 400 film at
ISO 400.  

If I use Rodinal, the the negative is underexposed unless I shoot it at
something like ISO 200.  When I was a newbie, roll after roll of film at
shot (at the given ISO) came out dull and slightly underexposed.  I
developed with HC-110.  Then I started rating ISO 400 film at 250 and got
much better results.  Nowadays I use Xtol which for me, is the perfect
developer.

On Sat, 29 Jan 2005 06:38:44 -0800, blut4rsky wrote:

Short answer: It lowers the threshold of light, film requires to produce
detail in a given area. The amount of light needed to produce
a specific density value is thus lessened.

You have to think in terms of what it is that "speed" describes and why it's
useful with regards to photography, it's not just a number on the camera
dial (ok it is to some). Speed or the film's "rating" describes how the
manufacturer predicts the emulsion will react when struck by light and then
developed with a certain developer and the results then graphed
scientifically on an exposure scale (the H&D curve). All developers are not
created equal. You wouldn't except the same results processing two
identically exposed frames if one was developed in D-76 and the other in
coffee developer(Cafenol?) or horse piss for that matter. A developer can be
mixed at different strengths too, a strong developer is described as
"aggressive" a mild developer as "soft" or "gentle" think of this in
woodworking terms. If you used a coarse rasp to round off a corner you'd get
the job done quick but the wood would be rough, if you used 180 grit sand
paper to round off the corner it might take a while longer and the corner
would be smoother, which you choose depends on how much time you have and
what is acceptable in the finished product -- both are correct in their own
context. It's often advantageous to be able to shoot a subject with a faster
shutter speed, smaller aperture or longer lens. Carrying a tripod around
allows you to do quite a bit but isn't always practical. If you can bumped
up the shutter speed by declaring "this film is faster than that film" or
"this developer rates my film 1 or 2 stops faster" then you extend the range
of what you're able to do with the camera.
 For example, if I shoot the same scene, let's say a grey card, and develop
a frame of  TMAX 400 in developer "A" then measure (with a densitometer) how
dark (or dense) the image of card is on the negative and I get a value of
0.95 Then develop the same scene in developer "B" and the grey card measures
1.25 in density it means (or might mean) developer "B" is capable of
creating 1 stop more density on the negative than developer "A" because a
0.30 increment on the "photo density" scale (H&D curve) equals 1 stop. To
some degree "density" equates to data or usable "information" on the film so
a better developer means you have more negative to work with better detail
in the shadows might be one such benefit.

Its helpful to know what ISO film speed is. The ISO has a standard
for measuring film speed which specifies the conditions of exposure and
development of the film. There was once a standard developer (actually
two) but the standard no longer requires these to be used. Any
developer can be used provided the developer is stated with the speed.
The standard is supposed to give a number that when set into the
computer of an exposure meter will give an exposure resuting in good
tonal rendition. The problem comes when the conditions of actual use
are not similar to those assumed by the standard.
One of the variables is the developer.
The standard specifies a range of exposure for the test and a range
of densities to be generated on the film. In effect, it specifies a
contrast index but it is indirect. If the film is developed to a
different contrast the effective speed will be different than given by
the standard.
Developers vary in their ability to select low values of latent
image. The range is not very large. D-76 is considered a standard for
"full" film speed for comparison to other developers. D-76 is not quite
optimum for maximising speed although close. Some Phenidone developers
are closer to being optimum, they give an increase in speed, that is, a
lower exposure will result in a given density on the film at the same
degree of development which is another way of saying contrast. So, when
one of these developers is used for the ISO test it will result in a
higher speed than if D-76 was used. Developers like Microphen, Xtol,
T-Max RS, fall into this class. Some developers destroy some of the
latent image or are not able to differentiate it from unexposed silver
halide crystals. These developers give a lower density for the same
value of exposure and development. Extra fine grain developers, like
Microdol-X or Ilford Perceptol, when used full strength, fall into this
class. A few developers fall in between somewhere.
The overall range of speed in comparison to D-76 is about plus or
minus 3/4 stop, not a lot.
Since the ISO standard effectively specifies a contrast index
develolping to a different contrast will affect the speed. The contrast
index of the standard is fairly high, suitable for diffusion printing.
If film is developed to a lower contrast, say for use in a condenser
enlarger, the speed will become less. In this case by about 3/4 stop.
The ISO speed is based on the idea that one should give film the
minimum exposure possible and still get good tonal rendition. The
reason is that grain and loss of sharpness increase with density. So,
there is a benefit in making negatives on the thin side, provided the
tonal rendition is good. Increasing exosure will sometimes result in
better tonal rendition with little increase in grain or loss of
sharpness especially with modern films which have thin, fine grain,
emulsions.
Because the ISO speed is designed to give minimum exposure, and
because it has little safety factor, film does not have much
underexposure latitude (room for exposure error) but usually has
enormous overexposure latitude.
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com

"Greetings.  I have read many posts stating that a particular developer
could "give true film speed increase," and wonder what is meant by
this.  What particularly confuses me is that the same photographers
shoot the film at the factory recommended ISO, so where does the speed
gain come into play?

Thank you for your time."

Certain developers give more or less speed, almost always balancing
other desirable qualities. This has been known for decades. It depends
on the formulation. The ISO speed is measured using a specified
developer, which is similar to D-76.