Carlpedia

Digital Image Size Info

Purpose/Overview: A brief explanation of sizing digital images, correctly for the four basic uses of digital imagery.

Platform: Mac or Windows
Where can I get help?: PEPS, A.T.'s

Image Size

Digital image sizing relies upon three different measurements.  All three are mutually dependent on each other.

    1.    Pixel Dimensions. - Width and Height in pixels.  
    2.    Inches (or Centimeters)
    3.    DPI Dots per inch

Pixel dimension is very important whether you’re working on the web, in presentation software, such as Powerpoint, or when one is printing.  But we’ll get to them in a minute.

DPI is VERY important for all reasons.  Here is a chart that shows some ‘basic rules’ to follow when dealing with DPI for various projects.

Let’s talk ‘scale’ now in pixels when dealing with an image that’s 9 inches by 6 inches, when using the chart above as a guide: Notice how the size (in megabytes) changes depending on the DPI.  This is the ‘pixel dimension’.

The colors in the above table reflect what is MOST important for each of the four uses of imagery.

For Printing you need to have the size in inches correct for your print, and also have the DPI at at LEAST 300 dpi. Here's a different handy table to give you an idea of how well images from a digital camera are likely to print.  

Camera
Capacity

Approximate
Resolution

File Size

Max Print
Size

2 Mega Pixel

1200 x 1600

1MB

16 x 20

3 Mega Pixel

1500 x 2000

1.6 MB

22 x 28

4 Mega Pixel

1800 x 2400

2 MB

24 x 36

5 Mega Pixel

1950 x 2600

2.5 MB

30 x 40

6 Mega Pixel

2000 x 3000

2.8 MB

36 x 48
(from MetalPix )

When using images on a website, you need to keep the size of the file ‘small’ so it doesn’t take a long time to appear for people with slow internet connections.  That’s why most ‘default’ settings for web imagery is 72 dpi.  In addition to the dpi, one must size the image according to pixels (not inches), as with HTML and most web-creation software (Dreamweaver or iWeb or Wordpress) sizes things that way.

Presentation size is dependent only upon what you’re using to show your presentation.  At Carleton, most of our projectors are 1024x768, so sizing your image to ‘fill up’ the screen is great.  Any bigger, and your Powerpoint file can become gigantic and cause slow issues with the computer.

For Archival copies of your work... You want the ‘biggest’ Resolution possible.  Scanning at the digital slide collection at Carleton is done at 1200 dpi.  Most cameras don’t make pictures with this resolution, but the bigger the better.

For information on changing or manipulating image size, please see these pages:

Resizing images with Photoshop
Resizing images with Preview (on a Mac)


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