"____" <internetphobic@[EMAIL PROTECTED]
> wrote in message
news:internetphobic-9BE424.14082724022008@[EMAIL PROTECTED]
> In article
> <55c237f9-dc18-4ff2-8f02-5a00f838164a@[EMAIL PROTECTED]
>,
> Monica Schulz <monica.schulz@[EMAIL PROTECTED]
> wrote:
>
>> On the risk of beeing pushing I must still insist on the
>> relationship
>> between neg contrast and paper contrast.
>
> Neg contrast= 1.45 max
> Paper contrast .85 <
>
> Gamma is not a good indicator of true paper capability try
> .85 or if you
> must use film gamma subtract .20 from the bottom and top
> of the
> Characteristic curve your using, use a shoulder around
> 1.45 or less more
> like 1.25
>
Gamma is a measure of the slope of exposure vs: density
of the straight line portion of a film or paper curve. Since
nearly all film or paper characteristics are curved and have
either small or no straight line portions gamma can be
misleading about overall contrast. More modern measures are
average contrast or G-bar and Contrast Index or CI. These
two are very similar but CI has a specified interval of log
exposure.
In any case, whether one is using gamma or G-bar or CI
the numbers combine by multiplying, not addition or
subtraction. That is, a gamma of 0.5 will combine with a
gamma of 2.0 to result in a gamma of 1.0. A gamma of 1.0
means that the contrast is the same as the original subject.
>> As we agree that both must balance to about 1 and color
>> negative film
>> is normally developd to about 0,6, the paper should show
>> gamma of 1,7.
>> But almost all color printing papers have gamma values
>> far in excess
>> of 2. So how does the steeply raised contrast of the
>> print fit into
>> your above statement? Does the contrast have to be raised
>> that much so
>> offset the unavoidable flare? If thatīs true paper
>> contrast cannot be
>> calculated by simply using the reciprocal of the neg.
>> Maybe thatīs all
>> what blocked me from recognizing the true connection?!
>>
>> Best regards!
>> Monica Schulz
>
> Flare should not be an issue when using a modern lens if
> using something
> older producing flare you should replace it with a modern
> Apo lens.
>
Flare comes from other sources than just lenses, for
instance light scattered in the optical path of a projector.
Also, ambient light in a room where slides are being shown
has an effect similar to flare. Projection transparencies
and motion picture prints are often slighly higher than a
gamma of 1.0 to compensate a bit.
Another note: The prefix APO on most modern lenses in
meaningless. It is permitted in German made lenses because
of an error in the German DIN standard which mis-represents
the term "apochromatic" and allows its use for lenses which
are not so corrected. I think this has been changed but the
terminology continues.
A true apochromat is corrected to bring three colors to
a common focus. The curve of longitudinal chrmomatic
aberration will be S shaped crossing the zero line three
times. Most lenses are achromats, that is they are corrected
for two colors. The curve for an achromat will cross the
zero line twice. More important than the number of zero
crossings is the deviation from focus between the corrected
points: the less the deviation the better the lens is
corrected. Its possible to have an achromat that has better
overall correction than an apochromat. In any case, nearly
all of the current crop of lenses with APO as a prefix are
NOT apochromats but are simply well corrected achromats. I
think the term APO used for such lenses is misleading and
should be eliminated.
In any case, an APO lens, regardless of its true color
correction, has no less flare than any other. Flare is
mainly a function of the number of glass-air surfaces but
other things can contribute such as reflections from the
inside of the lens mount. Surface reflections from the glass
can be controlled by anti-reflection coating. Such coatings
have been known for over a century but were made
economically practical only after WW-2. Modern coatings are
complex and are effective over the entire range of colors of
interest in photography. Modern complex lenses, especially
zoom lenses, would be impossible without advanced coating
techniques.
Even single coated lenses have substantial reduction in
flare and modern multiple coated lenses are nearly flare
free. Probably the use of a modern lens would pretty much
eliminate having to increase print gamma to compensate for
flare but one might still want to compensate for other
factors such as the viewing conditions.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
