"Michael Jakobsen" <admin@goose.dk> wrote in message
news:h05oku8s3fuib049sl4epkjig2ba71klmc@4ax.com...
> Hvordan opnår man den slørede baggrund på billeder taget med et
> digital kamera, der er mange af dem på
www.fotokritik.dk ?
Løsningen:
Brug en maske i dit digitale fotolab .. Fritlæg forgrunden i et lag for sig
(se fx "Extracting objects from their background" i photoshop
online-hjælpen) - og "blur" (fx bare 1.5 pixel gaussian blur) så ellers løs
på baggrunden til du får det resultat du ønsker dig. Det er supernemt når
først det ligger i hånden - og resultatet bliver overraskende godt. (Har du
mange penge, så er der mange værktøjer der også speeder den slags op, fx
http://www.extensis.com/maskpro/). Men er der mange "dybder" i dit billede,
der alle skal blurres i forskellig grad, kan det dog blive en noget træg
proces!
Chit-chat:
Digitalkameraer er anderledes bygget op end alm. kameraer og den effekt du
ser er sværere at opnå med et digital-kamera (det handler om at få så meget
depth-of-field som muligt, dvs linsen lukket godt op og f-stop ned). Men
hvis du zoomer så langt som muligt ind på dit motiv (det hjælper at have et
7x eller 10x digital zoomkamera), og sørger for at der er lidt afstand til
baggrunden, så skulle du automatisk have en bedre chance selv med fuld
automatik på (det med zoomen er desværre selvsagt ikke praktisk muligt altid
. - Men se fx et billede som
http://twalker.d2g.com/forweb/crw_4197.jpg - der er det lykkedes flot,
simpelthen ved bare at zoome (Canon Pro90 kamera, zoom'et ind på 17mm).
// Steffen
P.S.: Kan du nøjes med en lidt hurtigere, men knap så stilren tilgang til
tingene (simpel lasso tool fritlægning) er en turorial som fx
http://www.eastofthesun.com/pi7/blur.htm udemærket.
--
Lidt engelsk cut & paste for den ihærdige læsere:
There is a relationship between lens aperture, exposure time, and depth of
field.
In general, a large aperture (which means a small F-stop), and the amount of
the subject which is in focus (depth of field). If, for example, you use a
very
wide aperture, and you focus on the tip of the model's nose, her nose may be
in
focus, her eyes may be very slightly out of focus, and the background will
be a
featureless blur.
On the other end of the scale, a tiny aperture means that the model's nose,
her
face, the trees behind her, and the mountain range on the horizon will all
be
in razor sharp focus.
The aperture also determines exposure time. A large aperture lets more light
into the lens, which means you need to use a faster shutter speed; a small
aperture allows only a small amount of light to pass, and the shutter must
therefore stay open for a correspondingly longer time.
So depth of field, aperture,and shutter speed are all interrelated. Wide
aperture=shallow depth of field and fast shutter. Narrow aperture=good depth
of
field and slow shutter.
The effects of a shallow depth of field are not the same as the effects of
selecting and blurring the background. For one thing, blurring the
background
will drag parts of the unselected area into the selected area, creating a
fringe around the shbject; and fo another, light soources int he background,
when blurred in the camera, tend to assume the shape of the aperture, and
tend
to make the background lighter, not darker. (Think of it as a combination of
blurring in Screen mode and dialation of light areas of the background;
that's
a crude analogy, but the point is that the blur is not a simple Gaussian
blur.)
So the effect can only be approximated with Photoshop's tools; selecting and
blurring the background does not give the same results you would get if you
shot the image with a wide aperture.
Matematik:
http://hobbymaker.narod.ru/English/Articles/blur_eng.htm