DPI:er och andra konstiga djur !!
Det här med DPI upplösnig bildstorlek är inte självklart, fast allt hänger ihop. Jag hittade en ganska bra förklaring i en newsgrupp för ett tag sedan. På engelska visserligen, men jag skickar med den i alla fall.
Annars finns bra förlklaringar på
http://www.scantips.com/ där bla utskrift av scannade bilder mm tas upp.
/Lars-Olof
Här är den engelska texten:
One image, taked in a digital camera, whit 3.2 mega pixels, when opened
in PS5.5 will have how many resolution? (300x300 dpi, more?)
It will open at 72 pixels per inch.
That is because a digital camera captures only a certain amount of pixels. It
does not matter what the resolution of the image is, because resolution is
relevant only when the image is printed.
If the camera captures, say, 1600 by 1200 pixels, then it makes no difference
if it is 1600 by 1200 pixels at 72 pixels per inch or 1600 by 1200 pixels at
300 pixels per inch or 1600 by 1200 pixels at 3,000,000 pixels per inch. 1600
by 1200 pixels is 1600 by 1200 pixels.
Resolution, image size, and file size are all related.
Think of your image like a tile mosaic. Each tile is a single pixel.
Resolution determines how big those tiles are--that is, how big each pixel is.
If an image is 72 pixels per inch, then each "tile" is 1/72 of an inch across.
Likewise, if the image is 300 pixels per inch, the tiles are 1/300 of an inch
across.
Resolution and print size are related in a very simple way: The number of
tiles, multiplied by the size of each tile, gives you the total size of the
mosaic. If a tile mosaic is 100 tiles wide, and each tile is half an inch
across, then the mosaic is 50 inches wide.
Likewise, if you have an image which is 300 pixels per inch, and it's 600
pixels wide, it is 2 inches wide.
Similarly, a picture which is ten inches wide and is at 300 pixels per inch
must be 3,000 pixels wide.
When yu create an image for printed output, it is not always true that the
higher the resolutionis, the better. An ink-jet printer does not print a better
image from a 600 pixel per inch image than it does from a 300 pixel per inch
image, for a variety of reasons. Similarly, an image printed on a printing
press (say, in a magazine) does not benefit from a very high resolution; all
images printed on a press are broken up into halftone dots, and the ideal
resolution depends on how fine those halftone dots are.
When you look at an image on your screen, the image will seem to be very large,
because what Photoshop says is "100%" magnification is not the size the image
will print! When Photoshop shows you something at 100% magnification, what it
means is that one pixel on your screen equals one pixel in your image; you are
seeing "100%" of the number of pixels that will fit on your screen. An image
that is two inches wide at 600 pixels per inch is 1,200 pixels wide--it will
more than fill your 1,024x768-pixel monitor, even though when it prints out it
will only be matchbook sized.
You can change the image size of an image in two ways: by using interpolation,
which changes the number of pixels, or by not interpolating, which leaves the
number of pixels the same but changes the size of each pixel.
For example, let us suppose that you have an image that is 10 inches wide and
300 pixels per inch. It is 3,000 pixels wide.
If you shring it to 5 inches wide, but you have interpolation turned off, the
number of pixels will not change. The image is still 3,000 pixels wide, but it
is only 5 inches wide; therefore, it is now 600 pixels per inch.
Now, let's suppose you shrink the same image to 5 inches wide using
interpolation. Photoshop will change the number of pixels. The image will now
be 5 inches wide, and it will still be 300 pixels per inch, so it will now be
1,500 pixels across, instead of 3,000 pixels across.
Hope that helps...
