Re: ATM film

[Brian Nestel <blast@aloha.net>]

Chuck Hards wrote:
> 
> Hi all:
> 
> While shooting Hale-Bopp, I ran into an old acquaintance on the mountain,
> who used to run a couple of big telescopes at Kitt Peak.  (Mr. David
> Chamberlain).  I was surprised to find him shooting the comet with ASA 100
> film!  He explained that in practice, the slower films eventually "catch up"
> to the faster films in terms of speed, and the finer grain allows a sharper
> print.  So unless you are planning exposures that are fairly short (one or
> two minutes), you will get better results with a slower film.   The fast
> films are "quicker out of the gate", but reciprocity failure hits them more
> severely as the exposure times lengthen.  The slow films don't start out as
> efficiently, but like the tortoise racing the hare, they hang in there
> longer, and eventually catch back up.  I was shooting Fuji 400 and 800, and,
> while my shots looked pretty good, I have since seen prints made from slower
> films, and I have to admit, they were sharper, and showed just as much, if
> not more detail.  I can't comment on color balance because I don't know what
> corrections the lab plugged in, but I figure he must know what he is talking
> about, if he was in charge of a 90-inch telescope!
> I haven't yet had the chance to try it out myself, just passing on some
> wisdom from the pros.
> 
> Chuck Hards
Hi:
To say that 100 ASA film 
has a better reciprocity failure index then faster film is very misleading,
T-Max 100 has a better exposure characteristics than Tri-X, 
PlusX 125 is close to the exposure rating of T-Max but it is doesn't have
near the same reciprocity rating due to chemical composition and manufacturing 
not speed. its basically what the film is designed for that determines what
you can use it for. Some films are designed for portraits,  some for low
light and action, some for industrial and scientific purposes, and a bunch
for general use by the public. The examples I used were B/W but the same
applies to color, New color film and reformulation have been popping
with monotonous regularity the past few years.
Another thing that makes a difference is what you are photographing,
point objects or extended deep space objects.
The effect of reciprocity failure is calculated by the following formular:
Actual Speed = Rated Speed X Tp-1 (p-1 is superscript)
where T= exposure in seconds and p is The Schwarzchild exponent which 
will vary with film types and manufacture. The following is a partial table
of some films. The exponent varies with each film due to external varibles
such as temperature ("cold camera" for instance)
B/W
Gas hypered film                .95 to 1.00
Tech Pan                        .80 to .90
T-Max                           .08 to .90
Tri-X                           .65 to .70
Plusx                           .65 to .70
Color Positive
Ektachrome200                   .75             bluish background 
Ektachrome100HC                 .75             better color
Fujichrome100                   .90             runs slightly green
Fujichrome400                   .75             runs slightly green
Color negative
Fujicolor NPH 400               .85             good color, fine grain
Kodak Pro PPF 400               .92             good color, fine grain
Kodak Pro PMZ 1000              .90             good color, medium grain
Kodacolor Gold 1000             .78             reasonable color

Most of the above are can be different now as manufacturers change formulars
and manufacturing technics, Faster films with finer grains
are coming out all the time. With a film like PPF 400 you can make
long exposures without hypering or prefogging. 
Data sheets are available for most films from their makers and you can
check the exposure ratings.

-- 
Aloha   159 30 W, 21 55 N
Brian   Middle of the Pacific,
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