Tuesday, July 19, 2011

Image Size/Resolution - Resize Images for Power Point

I will attempt to explain image size and resolution using screenshots from the Image Size window (Image Menu/Image Size) in Adobe Photoshop to illustrate. There is a handy summary at the bottom of this post with output resolutions and a couple of easy equations.

Image size is the spatial resolution of an image, or simply, the x/y pixel dimensions. The image shown here is 3044x4357 pixels. But how big is that? The answer is, “That depends”.

Size = Pixel Dimensions (X/Y)

How big a picture appears to you is determined by what you are using to look at the picture. Are you looking at it on a screen at full size, in a PowerPoint document or in print? In the Image Size window in Photoshop, this is called “Document Size”.

Document Size

The size of the “Document” is how big the picture is on the output medium (screen, print, etc.). The size of the document is determined by the “Resolution” of the output.

Resolution of output/display

Here are the common output resolutions you will want to know:
  • Monitor resolution=72pixels/inch
  • Microsoft Word and PowerPoint=150pixels/inch
  • High Quality Photo Print=300pixels/inch
(For the following demonstration purposes it is important to make sure that “Resample Image” is unchecked. “Resample” means resize, or change the x/y pixel dimensions and we don’t want to do that right now.)

Resample Image unchecked

With “Resample Image” off, change the Resolution to 72pixels/inch. You can see that the 3044x4357 pixel image will be 42.2x60.5” on screen. Notice that the Pixel Dimensions don't change. You haven't changed the size of the image, you have selected the display resolution!

Height and width at 72pixels/inch

Of course, most monitors are not 60 inches high, so you won’t see the entire image on screen when viewed at full magnification. Here’s how big that picture looks on screen at full magnification. Notice the ruler on the side of the image reflects the "Document Size".

Change the resolution to that of a high quality photo output, 300pixels/inch. You can see that the 3044x4357 pixel image can be printed at extremely high quality well up to 10x14.5”.

Height and width at 300pixels/inch

You may notice the pattern here. Here’s the handy equation:

  • Document size = Pixel Dimension/Resolution

Using this equation, and knowing that PowerPoint resolution is 150pixels/inch, we can tell that a 3044x4357 pixel image will be about 20x29” (3044/150 x 4357/150).

PowerPoint resolution=150pixels/inch

But that’s too big of an image for PowerPoint, considering that the typical PowerPoint page is 8.5x11”. Turn on “Resample Image” to re-size the image for PowerPoint. (Make sure that “Constrain Proportions” is on, too or your picture will get all stretched out of shape.)

Constrain Proportions and Resample Image are checked

With “Resample Image” on, you can now resize your image for PowerPoint. Make the image 7 inches high at 150pixels/inch.

Notice that this makes the pixel dimensions smaller. You can figure that out by reversing our equation from above. Resolution * Inches = Pixel Dimensions (150*~5 x 150*7 = 750x1050 pixels). Now you really are resizing the image.

Since you have made the image smaller, make sure to “Save As” a different document and not over-write your original.

To summarize:
  • Size = Pixel Dimension/Resolution
  • Monitor resolution=72pixels/inch
  • Microsoft Word and PowerPoint=150pixels/inch
  • High Quality Photo Print=300pixels/inch
I hope this is helpful. Size and resolution are very poorly understood even by seasoned professionals. Unfortunately, it is often easier to just "get it" then to understand or explain it.

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