Resizing Images

 5-15 min

  

There are times when you may want to use an image but it is either too big, or too small, to use. The way around the problem is to resize (or resample) the image and we'll show you how in this tutorial.

By the end of this tutorial you will be able to:

Resizing vs Resampling

A typical monitor resolution is either 72 ppi or 96 ppi (pixels per inch) and this is what controls the image size on screen. When you print an image, the pixels are printed as small dots. The number of dots per inch gives an image its dpi setting. A high quality photo print will be printed at 300 dpi (dots per inch).  

There are really two ways to change the size of an image—resizing and resampling. What's the difference? Although often used interchangeably, there's actually an important difference between the two.

Whichever method you need to use, you're going to need the Image Size dialog (and of course an image!).

Let’s begin...

Resizing images for print

The only time that you'll really resize (scale) an image is when you want to print it. Remember, resizing does not modify the image in any way. Obviously, to get a good print you'll need to maintain a certain dpi—200dpi will produce and acceptable print, aim for 300dpi for a professional print.

To resize (scale) an image:

  1. On the Image menu, click Image Size... The Image Size dialog opens.

  2. Clear the Resize layers option. All resampling options are greyed out.

  3. In the Print Size section, the current Width, Height and Resolution dimensions are displayed. The native resolution of this image is 96 dpi. At this resolution, the dialog tells us that the image will print at approximately 28.2 x 18.8 inches.

  4. To specify that the image must print at 300dpi, simply type 300 in the Resolution box and press the Tab key. The Height and Width update automatically. You'll see that the image will now print at approximately 9 x 6 inches at 300 dpi.

  5. To specify exact print dimensions, type the Width (or Height) you want to print at and press the Tab key. The corresponding values update to match.

  6. When you are happy with the print size, click OK.

The following illustrations show how the image will print with the above settings.

2710 x 1806 pixel image at 300 dpi on A3 paper

2710 x 1806 pixel image at 200 dpi on A3 paper

The images dimensions have been ‘squashed’  or 'stretched' to change the size the image is printed, but the image itself has not been resampled (the pixel dimensions are the same). Try printing a copy to see the effects.

Resampling images for print, screen, email and the Web

There are many times when you want to create a larger or smaller version of your image—to create a large, high quality print, to create a small file to share via email, or to create lower resolution image for your website. In each case, you'll need to change the number of pixels in the actual image, or to use the correct term, resample the image.

In the next part of the tutorial, we'll show you how to do this while preventing the distortion that resampling can sometimes introduce. We'll start by showing you how we could create a 300dpi A3 poster print of our image. To ensure that the print goes right to the edges, we'll increase our image print size to approximately 12 inches x 18 inches.

To enlarge an image to a 300 dpi print by resampling:

  1. On the Image menu, click Image Size...

  2. In the Image Size dialog:

  1.  PhotoPlus resamples the image to the new size ready for printing.

  2. After resampling an image, you may need to sharpen the details. Try using the Unsharp Mask on the Effects > Sharpen menu to sharpen your image before printing. For more on using the Unsharp Mask, see Help and the Key Adjustments tutorial.

To change image dimensions for screen:

  1. On the Image menu, click Image Size...

  2. In the Image Size dialog:

The Export Optimizer

Most images taken straight from the camera have dimensions much bigger than the average monitor display. This means that we can reduce the file size by resizing the image to fit the screen. Also, as the resolution doesn’t need to be as high for a screen image, it means that we can further reduce file size by exporting the image as a lower quality JPEG, without affecting the appearance too much. We can do all of this in one step with the Export Optimizer. Let’s do that now.

We’ll demonstrate this with one of our 12 megapixel images taken from the 2010 Isle of Man TT. The image measures 4288 x 2848 pix and has a file size of 7.4MB. This image is great for printing, but too big to share or display on a website.

To resize an image with the Export Optimizer:

  1. On the File menu, click Export Optimizer...

    The dialog displays your image along with the estimated exported file size at the selected quality settings.



    We’ll set the size of the image first.

  2. In the Size section:

  1. Click  Zoom In to zoom into your image.

    At the lower quality settings, the file size is tiny but the image becomes blocky as can be seen in these examples even at 100% zoom.

  2. Increase the quality slider until the image no longer appears distorted. As a rule of thumb, a setting of between 90% and 95% normally produces a small file but with an acceptable quality image for the Web.

  3. When you are happy with the image preview, click Export.

  4. In the Export dialog, browse to the folder that you want to save your file to. In File name, type a new name for your file. Click Save.

    Your image is saved to the chosen folder and is ready to be uploaded to your website or sent via email. (The original remains unchanged.)

  5. Thumbnails

    Thumbnails are smaller versions of the same picture. They are very small, in both file size and viewing size, and are often used on websites to link to the larger, high quality version of the image. The small file size means that even a page containing many thumbnails will load very quickly in a browser. The example below is a 150 x 99 pixel thumbnail created from our original image with the Export Optimizer. The file size is only 12.8K!