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**Harold Gough** · #18 March 10th 09, 04:29 PM

In the pre-digital days the rule of thumb was to slightly under-expose reversal film and to slightly over-expose print film. To me, that would mean 1/3 to 1/2 stop and no more.

Quote:

Originally Posted by Peter Irwin

Michael Benveniste wrote:
On Thu, 5 Mar 2009 01:51:17 +0000 (UTC), Peter Irwin
wrote:

It is going to take more than the listed results from filmscan.ru
to convince me that Kodak is rating their films inaccurately.

Then pick up some old film shootouts from the various photo magazines,
or go shoot some and ask your lab to measure the densities for you, or
talk to people who rate their own film, and see if they use the same
EI for all films with the same ISO rating.

That's another question entirely. What is optimum to use is a different
question from whether or not films are rated accurately. Film
manufacturers always tell users to find their own optimum EI by testing,
and say the ISO rating is only a starting point.

Kodak isn't rating films inaccurately -- the spec is simply loose
enough to permit them to choose the nominal rating. Don't believe me?
Compare the characteristic curve of Kodak 400NC to that for Kodak
Vision2 250D.

Cine negative film speeds are covered by a different ISO standard than
still negative films. Both will be rated to the nearest standard value
to the measured speed.

My own experience with 400UC and 400NC show them to be at 2/3rds
of a stop apart. Similarly, I rate Fuji 160S at either 125 or 160,
but I typically rated Portra 160 at 100.

I bet that difference is to gain more contrast on the Portra by putting
more of the exposure off the toe.

Within 1/6th of a stop for _all_ existing films? Not very likely.

From _Photographic_Sensitometry_ by Todd and Zakia (1974 ed) p.162
"Manufacturers' published speed values necessarily include some
tolerance. Thus an ASA Speed value of 64 represents in fact a range
of speed values. The present standard permits a total range, at the
time of testing, of 1/3 of a stop, so that a film rated at 64 could
have a tested speed between 57 and 71."
"Beyond this tolerance, any given sample of film will, by reason
of its age and usually unknown storage conditions, have an effective
speed perhaps considerably different from the value obtained when
it was tested."

You're right on Kodak's spec sheets, although they've been less
precise in consumer ads.

In their specifications tab B&H lists both TMAX-3200 and Delta 3200 as
ISO 3200 films. Adorama does the same.

Neither B&H nor Adorama make film. I haven't seen anything actually
from Kodak which claims 3200 as the ISO speed.

Specifically with Ektar 100, I found an overexposure of about 2 stops
was sufficient to blow out the blues, resulting in the dreaded C-41
cyan sky. And you?

I haven't used Ektar 100 yet. I don't doubt that you saw what you
describe. I do doubt that it had anything to do with overexposure.
I think it highly probable that this is a side effect of moving
the shadow values up off the toe thus increasing the overall
contrast with the result that the same printing contrast didn't
give enough room on the paper for the highlights.

Then please feel free to provide your own data or test shots rather
than stating generalities.

My three generalities were useful, and were things I did not
know when staring out.

With colour negative film
- a small increase in exposure gives better detail in the shadows
- a small increase in exposure results in a decrease in apparent grain
- a small increase in exposure reduces the risk of underexposure
and that the risk of underexposing negative films is greater than
the risk of overexposure thus giving more useful latitude.

I should also have noted that increasing exposure increases the
contrast in the shadows which can sometimes have undesirable
consequences and that increased exposure also makes film very
slightly less sharp, although this is scarcely noticeable unless
you go seriously overboard.

Peter.
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