How to Resize Images

It sometimes is surprising how something seemingly simple can end up becoming immensely complicated. And in the world of digital photography, resizing photos is exactly one such thing. A first glance, one does not realize the potential pitfalls of this step if done incorrectly. Let me guide you all through the process of resizing images, so that you always get the best images for posting on the Web, as well as for printing.

Let me first introduce you to a couple of terms which will help you understand digital images better.

Pixels: Images are displayed in tiny blocks of color that together form an entire image. Such a ‘picture element’ is called a ‘pixel’. So, how we measure an image size in inches, say 5” x 4”, the computer measures it in pixels - 1024x738 pixels. To learn more about pixels, please click here.

Resolution: A measure of how tightly the pixels are packed together in an image, and is measured in dots (pixels) per inch or dpi. So a 1024x738 image can be of resolution 30 dpi, 300 dpi or even 3000 dpi. A 300 dpi image will have more pixels packed tightly together in the same inch of space than a 30 dpi image. So if we print the image at 30 dpi as well as 300 dpi, the lower resolution 30 dpi print having loosely packed pixels appears ‘boxy’ or ‘chunky’, whereas the higher resolution 300 dpi print having tightly packed pixels appears sharp and smooth. And no points for guessing that the size of the 300 dpi image, in megabytes (MB), is going to be far higher than the 30 dpi one. Click here for more on resolution.

The rule goes that for printing good quality images, the minimum resolution required is 300 dpi. Computer monitors are less fussy, and manage quite well with 72 dpi. You really can't make out the difference on-screen between a 300 dpi image and a 72 dpi one!

So, the take-home message is:

1. If you want to print the image, resize to a minimum of 300 dpi.

2. If you intend to share the image over the web, around 72 dpi should do.

The below steps are for Photoshop, but no matter which photo editing program you use, the steps should be very similar.

1. Open the image in Adobe Photoshop. Click on Image>Image Size.

2. The dialog box which opens shows the following information:

• Image Size in MB

• Pixel Dimension (Size of image in pixels)

• Document Size (Size of image in inches/cm)

• Three checkboxes for Scale Styles, Constrain Proportions and Resample Images

3. Un-check ‘Resample Images’. This keeps the size of image in pixels constant and prevents the software from changing the number of pixels. Hence, the image quality is preserved.

4. Enter ‘300’ in the ‘Resolution’ field making sure the units is ‘pixels/inch’. This would change the Document Size, but keep the Pixel Dimension constant. In other words, we’ve changed the resolution to 300 dpi, keeping image quality intact. But the image size (in inches/cm) got altered, as now the pixels are more tightly packed together.

5. Save the image under a different name, so as to preserve the original.

The point to remember here is to keep file size in MB low to ensure faster loading. As digital cameras usually output files at 72 dpi, it's not advisable to further reduce resolution. Here, lowering Pixel Dimension would help. You can essentially use the same method described above while entering ‘72’ in the resolution field instead of ‘300’, and altering ‘Pixel Dimension’ to your needs. Photoshop also provides a quicker way to do it.

1. Open the image in Photoshop.

2. Click on File>Save for Web.

3. A dialog box opens. You can choose format (JPEG/GIF), quality using a slider (I usually keep it as high as possible) and an option to resize Pixel Dimensions. Here I feed in the required dimensions. For simply sharing family photos over Flickr or Orkut, I resize to 1024x764, a size which fits most 17” monitors. You may choose a different size according to your needs.

4. Save the image under a different name.

Photoshop provides a nifty little tool, by which you can increase Pixel Dimensions, while maintaining both Resolution as well as Image Size. It does this by a software algorithm that adds pixels of the correct colours throughout the image. Such an artificial addition of pixels is not the recommended way of ‘upsizing’ an image, but can be a life-saver in emergencies.

1. Open the image in Photoshop.

2. Click on Image>Image Size.

3. Leave ‘Resample Images’ checked. From the drop-down box, choose ‘Bicubic Smoother’.

4. Save the image under a different name.

Use it with utmost caution, as over-applying it would lead to a very unnatural looking image!