For those who wrongly believe that "dpi" is the resolution of a digital image


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mpenza

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#1
check out the faq here:
http://www.cs.duke.edu/~parr/photography/faq.html#dpi

Think it explains pretty well. Here's an extract:

The first thing that you need to understand about digital pictures is that they don't have an inherent DPI rating. That's right, the resolution of digital images is not measured in DPI.. Digital images are collections of pixels. The DPI of an image depends entirely on how you decide to display it or print it.
 

denizenx

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#4
Originally posted by ckiang
About time man. This DPI thing is the single most confusing concept among the starters. It took me a long time to grasp it myself.

Regards
CK
after that is CoF hor? I realised CoF is something like dpi... but let's not rush in where angels fear to tread... LOL
 

denizenx

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#6
Originally posted by ckiang
CoF? You talking about DoF or CoC? :bsmilie:

Regards
CK
CoC lor
but Ruminous NandScape calls it CoF... which is the sound u hear when u think too much CoC... :D
 

jasonpgc

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#8
Originally posted by mpenza
check out the faq here:
http://www.cs.duke.edu/~parr/photography/faq.html#dpi

Think it explains pretty well. Here's an extract:

The first thing that you need to understand about digital pictures is that they don't have an inherent DPI rating. That's right, the resolution of digital images is not measured in DPI.. Digital images are collections of pixels. The DPI of an image depends entirely on how you decide to display it or print it.
Thought we've sorted that out long ago :bsmilie:
 

willyfoo

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#12
DPI stands for Dots (pixels) Per Inch

dpi * size (in inches) = pixels
Conversely,
size = pixels / dpi
dpi = pixels / size

Therefore a 2 Megapixel Camera which produces an image of 1600x1200 pixels can print an image of

8 inch wide at 200 dpi (Magazine Quality) 8 * 200 = 1600
5.3 inch wide at 300 dpi (High Quality Print) 5.333 * 300 = 1600

You can use Image.Size in photoshop and uncheck 'Resample Image' and see how the inch/dpi changes in relation to each other
 

willyfoo

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#13
Additional Info

Screen resolution is typically 72 dpi
Printing from IE is 96 dpi
Newspaper is about 100-150 dpi
Magazine is about 200-300dpi
Quality Deskjet Print starts from 300dpi
LaserJet prints usually is at 300/600 dpi

However printing photos beyond 300dpi does not show noticeable difference. (Disclaimer: your mileage may vary)
 

blurblock

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#14
Originally posted by willyfoo
Additional Info

Screen resolution is typically 72 dpi
Printing from IE is 96 dpi
Newspaper is about 100-150 dpi
Magazine is about 200-300dpi
Quality Deskjet Print starts from 300dpi
LaserJet prints usually is at 300/600 dpi

However printing photos beyond 300dpi does not show noticeable difference. (Disclaimer: your mileage may vary)
Actually, a magazine's photograph is nearly 600 - 3000 dpi ;) depending on the quality of the magazine. Take a magnifying glass and look at the pictures in it and you will not even see a pixel ;).

Current Laserjet printout is ranging from 300 - 1200dpi, there are even 2400 dpi printers (Mid Magazine Quality) but on the higher end.

Deskjet has the problem of smearing (Even though they claim a quality of 2400dpi (espon)), although the technology now make it almost impossible to see with naked eyes.

I don't know about newspaper print, never read about that before.
 

#16
Do not, I repeat, do not, confuse image resolution (expressed as PPI / PIXELS per inch) with the final printer dpi (DOTS per inch).

The latest Epson printers run at 2880 and 57600 Dpi, but doesn't mean your image has to be at that kind of size. Going beyond 300Ppi gives no noticeable improvements. Going beyond 300Dpi on the inkjet printer DOES give noticeable improvements. But above 1440dpi on the printer gives no improvements other than to waste ink and time.*

Regards
CK

* Unless you are printing B&W prints using ONLY the black ink
 

akdwivedi

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#17
How does the DPI concept work while scanning.. If I remember most of the scanners specs say 1200x2400, 2400x2400, 4000x4000.... Doesn't it translate to pixels as well...

Can you pros, explain that to enlighten me a bit as well...

thanks in advance :dunno:
 

Hobbes234

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#18
hi guys,

i am using a DOF calculator program for my ppc, and there's this option which stated resolution as lines/mm.
litttle confused...
is resolution commonly measured by lines/mm? or is that for film only?

regards
 

mpenza

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#20
akdwivedi said:
How does the DPI concept work while scanning.. If I remember most of the scanners specs say 1200x2400, 2400x2400, 4000x4000.... Doesn't it translate to pixels as well...

Can you pros, explain that to enlighten me a bit as well...

thanks in advance :dunno:
you have an input size. if you scan a 4 inch by 6 inch at 1200dpi, you get an image which is a collection of 4800x6000 pixels with no inherent dpi rating.
 

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