17 aug 2007 kl. 17.35 skrev ADavidhazy:
...In any case, I was pondering whether one can get a fair idea of
whether an image file has
sufficient digital "resolution" so that when printed it will look
"good" by looking at the
image at a larger size than what it will be reproduced at. So if I
have a 5x5 cm image
file at 300 dpi but I look at it on my CRT or LCD screen at 200% or
300% or 600% or more
magnification and at 300% the image on my screen looks OK ... but
at 600% it starts to
fall apart ... is that an indication of anything?
I think you´re using several different ways to measure resolution
here. First, dpi and ppi are two different things: ppi is the
resolution in pixels, and dpi is the number of primary colour ink
spots that a printer needs to lay down in order to give the
impression of a continuous, smooth tone scale in the print. Thus,
the dpi number is normally much greater than the ppi number (but only
the latter has any real relation to the detail resolution in the image).
Supposing then that you really mean "a 5x5 cm image at 300 ppi", this
image will consist of 600x600 pixels (if we conveniently set 5 cm
equals 2 inches), and the same image file could be described as e. g.
"a 10x10 cm image at 150 dpi". Of course, if you want to print with
300 dpi true resolution, the info in the image file is only
sufficient for a 5x5 cm print, and I suppose that´s your premise.
Now, if you display this image on a monitor, the percentage setting
in software like Photoshop relates to the pixel density of the
monitor, which is far lower than 300 ppi, typically 70 - 100 ppi (I´m
sitting in front of my spankin´new 30" Apple Cinema Display right
now, and it has almost exactly 100 ppi resolution).
So, a 600x600 pixel image at 100% would be 6x6", or 15x15 cm on the
monitor. At 300%, it would be 45x45 cm (and slightly clipped
vertically, even on this screen). But it would still contain the
same info as when it was a 5x5 cm 300 ppi print (assuming a very good
printer and using good paper). Now, if I tried to make the image on
the screen 5x5 cm, it could only contain 200x200 pixels, because that
´s what there is within 5x5 cm of screen surface. So it would be
downsampled somewhere on its way from Photoshop to the screen (and
that´s why it is not a good idea to judge noise and sharpening at
less than 100% screen resolution).
Personally, I don´t find that displaying at higher screen resolutions
than 100% really contributes anything; I just start to see the
pixelation as tiny squares. But I have good eysight (glasses..) and
sit close enough to the screen. 100% seems to be the optimum for me:
if it looks good at 100%, it IS good (technically).
So, for your final question, if the screen image still looks good to
you at 300%, but falls apart at 600%, it just means that your
eyesight/working distance is such that you can´t resolve 3x3 pixel
squares on your screen, but 6x6 pixel squares are visible to you...
I don´t really see what this fact could tell you about a 5x5 cm paper
image that you probably look at from a closer distance than the screen.
Well, I suppose I´ve just contributed to the confusion, as usual...;-)
Per Öfverbeck
http://ofverbeck.se
"In a world without walls or fences, who needs Windows or Gates"
