Objectives of this Tutorial

Generally: To change the size and resolution of an image.

Specifically: On completion of this lesson, you will be able to:

• View and change image size and resolution
• Change the canvas size
• Crop an image
• Undo and redo commands
• Use the History palette
• Create a snapshot
• Use the History Brush

In this lesson you will learn how to control the resolution and the size of an image. The finer the resolution of an image the sharper the image will appear both on the screen and when it is printed. This lesson looks at how to optimise an image in order to keep its quality, but keep the storage space that the image requires to a minimum.

When you view an image on the screen each image is made up of square dots, called pixels. You can see each pixel in an image when you zoom in at a high magnification. Each pixel can be a different colour.

Change the Image Size

When you print an image, each inch is made up of a number of dots of colour. This measurement of dots per inch (dpi) is the image's print resolution. To obtain a good quality image, the resolution needs to be from 150 to 300 dpi, depending on the type of output (from desktop printer to professional print service).

When you view an image on-screen, the dimensions of the image are irrelevant, because the screen is also made up of dots (or pixels). Each pixel in the image takes up one pixel on the screen. The same image may appear larger on a small screen and smaller on a large screen, because small screens tend to use fewer, larger pixels. When preparing images for viewing on-screen (or on the web), set the resolution to 72 dpi.

Resampling

Images can be resized with or without resampling the resolution. If you do not resample the image, the resolution will change in opposite proportions to the size of the image; that is, if you make the image larger, the resolution will decrease and vice versa.

When an image is resampled pixels are either added (if the image is made larger) or subtracted (if it is made smaller). Photoshop can do its best to preserve the quality of an image when it is resampled, but making a picture much larger than its original size will cause it to blur.

To change the image size and resolution

• From the Image menu, select Image Size...

OR

• Right-click on the title bar of the image window and select Image Size...

The Image Size dialogue box is displayed.

• To change the size, but not the resolution, select the Resample Image: check box and make sure the list box is set to Bicubic

OR

• De-select the Resample Image check box to fix the resolution to the image size
• Optionally, select the Constrain Proportions check box to ensure that the image's proportions remain the same

Changing the proportions of an image will cause it to distort. From the Document Size drop-down boxes to the right of the Width: and Height:, select the measurement units Enter new measurements in the Width: and Height: boxes Optionally, set a new Resolution

For example, you may want to reduce the resolution of a scanned picture that is going to be used on the web. Note that increasing the resolution will not usually improve the quality of an image. If an image looks blurred or "pixellated" when printed, re-scan it at a higher resolution (150 - 300 dpi, depending on the type of printer).

• Click OK

Change the Canvas Size

Often an image is the correct size, but you have little space surrounding the image to work with. In order to overcome this problem you can increase the canvas size of the image. When the canvas size is increased the file size increases without enlarging the elements that make up the image. The new space that is added to the image is filled with the background colour of the image.

To change the canvas size

• From the Image menu, select Canvas Size...

OR

• Right-click the image window title bar and select Canvas Size...

The Canvas Size dialogue box is displayed.

• Select one of the boxes in the Anchor: area to indicate where you want to add the additional space to the image
• Enter the new width and height for the canvas in the Width: and Height: boxes
• Click OK

The canvas size is increased.

Continued...