Jeff Spirer wrote:
At 02:26 AM 2/13/2006, howard wrote:
I saved a complicated image from its original 6 mp size, setting it
to 800 by 532 at 72 dpi (CS2 lingo) a
One more time...there is no "dpi" for a screen image. There is no
"dpi" until an image is sent to the printer. Up until that time, dpi
is meaningless - it's a value in the header for the printer.
If you want to check this out, take the same original image and save
it at a fixed image size (800x532 is fine) at 1 dpi. Then go back to
the original image (do not use the 1 dpi image as it has been
compressed) and save it at 10000 dpi. The file size and the image
display on the screen are identical.
Jeff Spirer
Photos: http://www.spirer.com
One People: http://www.onepeople.com/
Jeff - thank you for the, uh, lesson. I was being careless with my
language. I can assure you I do know the difference and have for a long
time.
Apologies: I will write out 1000 times
"Remember to distinguish clearly between dpi (printers) and ppi (screen
resolution) when writing to Photoforum."
However, regardless of that distinction the point I was making remains
correct if you substitute ppi for dpi.
As for Emily's query, the file as posted is in fact 1080 by 720 pixels
and the file size if uncompressed is 2.2 M in RGB (8 bits) when opened
in Photoshop.
Correct me if I'm wrong but each pixel = 1 byte in greyscale and 3 bytes
in RGB
Thus 1080*720 = 777600 bytes.
and for RGB, this becomes
1080*720*3 = 2332800 bytes, which is roughly 2.2 MB.
This is 15" by 10" at 72 ppi.or 9" by 6" at 120 ppi or 1080" by 720" at
1 ppi
As you said quite correctly these are all the same size and will all
generate the same file size assuming the other parameters involved are
the same.
Obviously changing to CYMK increases the files size (by about 0.7 MB
because each pixel must record 1 of 4 colours)
And, not surprisingly, RGB in 16 bit mode doubles the size to around 4.45 MB
Changing to sRGB apparently makes no difference.
Indexed color however cuts files size considerably - to around 750 KB.
Not sure why...not sure if I'm that bothered either.
Howard
