How large an image do I need to print a 32″ by 18″ poster at 300 DPI?

Question

I need to have an image printed as a poster at 300 DPI, but am unsure of how big the file must be. Also, what file format is best?

Asked by Jake223

Answer

The straightforward answer to your question is very simple arithmetic: 32×300 = 9600 and 18×300 = 5400, so 32 inches by 18 inches at 300 dots per inch is 9600 by 5400.

However, it gets a little more complicated when you consider a more complicated relationship between pixels and colored dots in your output medium. For details on this, take a look at jrista's answer to a similar question: What DPI should I resize my image to for best printing quality?, at What does DPI mean?, and at Is there a general formula for image size vs. print size?

It sounds like you're doing something specific, though. In that case, the best answer is to ask your print service what they recommend — and the same thing goes for file format (and color space, a whole 'nuther can of worms).

Answered by mattdm

Check more discussion of this question.

What DPI should I resize my image to for best printing quality?

Question

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?

Answer

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:

• How do I generate high quality prints with a ink-jet printer

Check more discussion of this question.

Does the dpi number reported by camera in JPG have any meaning?

Question

I have two cameras, a 6mpix Panasonic FZ8, and 14mpix Canon A2200.

When I compare shots from two cameras, one of differences I notice is
dpi number that camera reports in JPG info:

• FZ8 reports dpi=72
• A2200 reports dpi=180

What physical meaning can these numbers have? I am at loss for guesses.

I am fairly familiar with notion of dpi in scanning and printing. I can calculate density of pixels on the sensor of the camera. But then, the linear density of pixels on the sensor will be hundreds times larger than number above. So what, if anything, does it mean?

Answer

The values are arbitrary and meaningless, and only serve to confuse people. The EXIF standard seems to imply that if the tag is missing, 72 is the (still-meaningless) default. However, it is apparently mandatory for the TIFF standard, from which the JPEG/EXIF format basically inherits everything. So maybe it has to have some value to properly comply with the standard.

You can still ignore it, though.

Check more discussion of this question.