Newsgroup: it.arti.fotografia
Titolo: Nitidezza

Leggendo diversi articoli sulla profondita' di campo ho notato come venga
considerato ragionevole un circolo di confusione di 0,0508 mm per una
pellicola 35mm.
Si tratta di mezzo decimo di millimetro , misurato pero' sulla pellicola.
In una stampa 20x30 questo circolo di confusione diventa dunque di quasi
mezzo millimetro , giusto?
Quindi vuol dire che se i bordi di una immagine 20x30 non sono nettissimi ma
leggermente sfumati per mezzo millimetro , questo e' da considerarsi normale
?
Io ho fatto una stampa da diapo 30x40 e diciamo che i contorni sono
leggermente sfumati per una zona di circa 1 mm , e' normale ?
Ho anche sentito dire che in generale gli obiettivi Canon sono "morbidi"
mentre i Nikon sono molto "contrastati".
E i Minolta ?
Purtroppo non ho ancora potuto visionare foto fatte da qualche esperto per
avere un' idea dei limiti di nitidezza che si possono ottenere col 35mm.
Ho visto foto 20x30 di una nitidezza eccezionale , erano foto di un
matrimonio : di solito chi fa matrimoni usa il 35mm o un medio formato ??
Grazie anticipati.
Gabriele.

About the Circle of Confusion
Imagine a perfect white point in an empty black room. The point has no
height, and no width. If you focus an optically perfect lens on that
point, it forms a perfect point on the film as well. If, however, you
focus slightly in front of or behind the point, the point will image on
the film as a small blurry circle. If that circle is small enough, it
will still look like a point when enlarged for printing. The "circle of
confusion" is typically calculated as the largest on-film circle that
you see as a point when you make an 8 × 12 print and view it from a
"normal" viewing distance, typically 2-3 feet. Anything larger is seen
as a small circle, and is therefore perceived as out of focus.
f/Calc calculates the CoC using the "Zeiss formula": d/1730, where d is
the diagonal measure of the film, in millimeters. This formula yields
acceptable values for most uses.
If the CoC is Such a Slippery Value, Why Is It Reasonable for f/Calc to
Calculate It?
Many lenses have depth of field markings engraved on the barrel: the CoC
affects DoF, so obviously these manufacturers are comfortable giving you
a very fixed definition for CoC. This is because the main factor
affecting CoC is the film size; lenses are designed for a particular
camera, and thus a particular film size.
The film size is important because you don't have to enlarge large
negatives as much to get a particular sized print. So, a 6 × 4.5 cm
frame, being roughly twice the size of a 35mm frame, will have a CoC
that's roughly twice the size of that for a 35mm frame. In other words,
if a fuzzy disc 0.025 mm wide looks like a point when printed from 35mm
film, you can have an 0.043 mm disc on 6 × 4.5 cm film and still have
the same apparent degree of sharpness if you enlarge it to the same size
print as you made from the 35mm frame.
What Are Some Good Reasons to Override f/Calc's Calculated CoC Value?
Technically, the print size and the viewing distance are factors in
choosing an appropriate CoC value. For most purposes, these factors are
not important, because larger prints are generally viewed from farther
away than smaller prints, and because people making large prints
generally use large film sizes. Nevertheless, there are exceptional
conditions, like prints hung in an art gallery: a gallery setting often
demands large photos which will be viewed relatively close-up.
The CoC value also takes into account imperfections in the camera and
enlarging lenses used, and the resolution of the film and paper used to
make the image. Since most people take pictures with typical cameras and
have the photos developed at generic photo labs which print their photos
on standard paper with typical photo printing machines operated by
technicians of average talent, it is usually not important to take these
various quality issues into account: they're effectively built into the
Zeiss formula. Your situation may be atypical, however. If you're using
a digital camera and an inkjet printer, for example, you might need a
smaller CoC to account for each device's relatively low resolution.
Cheap lenses will also require smaller CoC values than sharp ones to
achieve the same degree of apparent sharpness. (Smaller CoCs can only go
so far in improving sharpness, however.)
Dalle note di fCalc
Saluti
EF
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