Resizing (Resampling) Images

Resized Image

Image Credit: Flickr Creative Commons. By lirontocker

Resizing vs Resampling

Usually, when people use the word resizing, they actually mean resampling. Technically, resizing means changing the ppi (pixels per inch), or the resolution, of your images. Resolution and ppi become significant when you wish to print photos. For example, let’s say you have a 12 megapixel camera. This means the photos you take will have 12 million pixels (4,000 pixels by 3,000 pixels). If you want high quality prints, your photos should have a resolution of 240-300 ppi. At a resolution of 240 ppi, your photos will print at a size of 16.6 inches (4,000/240) by 12.5 inches (3,000/240). At a resolution of 300 ppi, your photos will print at a size of 13.3 inches (4,000/300) by 10 inches (3,000/300). Notice how the print size of photos decreases as the ppi increases. Changing the ppi of images will change the sizes the images will print, which is why it’s referred to as “resizing.”

Most people are interested in “resizing” images for the web, not for printing, in which case is actually resampling. When reducing image dimensions by resampling, your image program is intelligently throwing away pixels, hence the smaller file size. When enlarging images by resampling, your image program is adding pixels to fill in the gaps by guesswork, or algorithms. So resampling changes the size of images by changing their number of pixels.

For images on the web, a resolution of 72, 96 or 100 ppi will suffice. Most laptops have a display resolution of 96-100 ppi.

Here are some more display resolutions:

  • Iphone 4, 5: 326 ppi
  • iPad Original, 2: 132 ppi
  • iPad with Retina Display: 264 ppi
  • MacBook Pro 15-inch: 110 ppi
  • MacBook Pro 15-inch Retina display: 220 ppi
  • iMac 27-inch: 109 ppi

How to “Resize” Images in Photoshop

To “resize” images in photoshop, all you have to do is change the images’ pixel dimensions (width and height in terms of pixels). Go to the Image menu in the Menu Bar, and select Image Size from the Image menu options. Once the Image Size dialog box pops up, make sure the Scale Styles, Constrain Proportions and Resample Image boxes are checked. Having the Constrain Proportions option checked ensures that changing one of your images’ dimensions will result in the automatic adjustment of the other dimension. You can also “resize” images by percent. You should get the best results for 50%, 25% and 12.5%. Read a more detailed tutorial here:

Resampling Options

  • Bicubic (default) = Best for smooth gradients
  • Bicubic Sharpener = Best for reduction
  • Bicubic Smoother = Best for enlargement

Tip: You can also batch resize images with Photoshop:

If you don’t have Photoshop, you can use IrfanView, FastStone Image Viewer (or FastStone Photo Resizer) or PhotoScape, the three most popular free photo editors on Cnet.

Here’s a tutorial on how to resize images with IrfanView:

And here’s a tutorial on how to resize images with FastStone Photo Resizer:

A quick and dirty way you can make your images smaller is to zoom out and take a screen capture.

Zoom Out For Sharper Web Images
March 28, 2007 by Corey Barker

“This is a tip we use almost daily when we have to greatly reduce the size and/or resolution of an image. Sometimes when you make a drastic size/resolution change, it can really make the resulting image blurry, so what we do is simply zoom out on the image so that the window and image are at either 50% or 25% view. Then, we take a screen capture of our image window at the new smaller size. That way, the image still looks sharp, but it’s much smaller when we open the screen capture in Photoshop. The trick to making this work is using either a 50%, 25%, or 12.5% view size for making the capture. If you view the image at 66.7%, 33.3%, or 16.7%, the image won’t be as crisp (because of the way Photoshop draws the image at those views)” (Planetphotoshop). 


Difference Between Image Resizing and Resampling
Wiki: List of Displays by Pixel Density
Zoom Out For Sharper Web Images

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