Alley in Pisa, Italy

by Lars Kotthoff

submit your photo


Hall of Fame
View past winners from this year

Please participate in Meta
and help us grow.

Take the 2-minute tour ×
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It's 100% free, no registration required.

And my apologies if this sounds a bit too basic but I can't get my head around this.

I have a digital image I took with my camera. 4000x3000 pixels, and GIMP claims that its resolution is 72x72 DPI.

I would like to print a thumbnail of this picture in the highest quality possible. What I was told is that the printer which is going to be used works optimally with images set for 300 DPI.

In the printed document, I would like my image to be exactly 166 pixels wide, or 3.32cm (as Microsoft Word sets it).

And now to the question: how do I calculate how to resize my 12MP image, so when I take that image and import it into my word processor, it will end up printing the best?

My initial thought was this: Since the image on paper is going to end up being 3.32cm wide (1.31"), I should resize my image to be 300x1.31 = 393 pixels wide and set its DPI headers to note "300x300".

However, I'm pretty clueless about photography in general so I'm afraid this sounds as if I'm smoking something cheap.

Am I missing anything?

share|improve this question
add comment

2 Answers 2

up vote 13 down vote accepted

When it comes to print, terms like DPI, resolution, PPI, etc. get thrown around without much care or concern as to what they truly mean. So, before I send you off to a more in-depth answer about DPI, PPI, resolution, and print, a quick summary:

  • DPI: Dots Per Inch
    • A 'dot' is a single element of a pixel
    • On a computer screen, a dot is a single 'sub-pixel' element, and may be red, green, or blue
    • On a print, a dot is a single droplet of ink expelled by the print head
  • PPI: Pixels Per Inch
    • A 'pixel' is the smallest element of an image, "PIcture ELement"
    • On a computer screen, every pixel is composed of three 'dots' or sub-pixels, one red, one green, one blue
    • On an ink jet print, every pixel is composed of numerous dots of varying ink colors, usually a mix of cyan, magenta, yellow, and black, although modern printers often have several other inks as well
    • On a dye sublimation print, every pixel is a single dot from a blend of varying in colors, such as cyan, magenta, yellow, and black.
  • Resolution: Variable meaning
    • Regarding an image, the resolution usually means the width and height of an image in pixels
    • Regarding a print, the resolution usually means the number of pixels in an inch (or cm, if you are from a country that uses metric.)
    • Regarding a computer screen, resolution usually means the width and height of the screen in pixels, but can also mean pixels per inch (i.e. 72ppi is the common "resolution" of the average LCD screen, while higher end screens often have a resolution of 100ppi.)

To answer the rest of your question, I've written up an extensive article here on Photo-SE that answers that question in great detail:

share|improve this answer
1  
+1. I had forgotten that monstrously detailed article! –  John Cavan Nov 9 '10 at 20:10
    
I keep adding to it. ;) I have another answer coming up soon that covers whether you should resize to the printer's native resolution (or multiple thereof) or let the software/printer decide for you. –  jrista Nov 9 '10 at 20:13
1  
Unfortunately, there's a very long tradition of referring to whole monitor pixels (all three sub-pixels) using DPI, both in applications and in operating system monitor profiles. The 72ppi you refer to is very, very commonly referred to as 72dpi. If only everyone would get together and agree to make sense, this would all be less confusing! –  mattdm Nov 10 '10 at 0:33
2  
@mattdm: Indeed. There is also a TERRIBLE tradition of referring to printed pixels as "DPI", which is an incredibly misleading figure. I can't begin to count how many times I've seen numbers like 360dpi and 720dpi thrown around supposedly "professional" printing forums, when in reality DPI for common photographic works is usually in the range of 4800-9600dpi, but resolution is only 600ppi. Its created a huge mess regarding printing, and the reason why I decided to write my own articles to correct the issue. –  jrista Nov 10 '10 at 7:17
    
at some point you're going to need to update those bits about displays and the "three" subpixels to deal with displays that add a forth (yellow) subpixel into a square arrangement. –  cabbey Feb 22 '11 at 21:01
show 1 more comment

What you're suggesting in terms of resizing sounds correct, and you can use any number of tools to do it. If you're on Windows you can use IrfanView, and naturally almost any of the pro line of tools will do it (such as Photoshop).

It's also worth noting that most of these tools will also let you set the width as a function of their physical size (cm, inches), and will calculate the dimensions in pixels using the DPI that is set in the image.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.