Photoshop: Understanding image size & DPI

Why does image size matter?

Before starting any project within Photoshop, it is critical that your image is correct. Understanding image size and DPI (Dot per inch) will ensure that your image is ready for print or screen output. If you image is too small, the quality will fail. If it’s too high, you may find it eats into your computer’s resources.

In saying that, it’s often wise to work with an image slightly bigger than you require. Images for screen output tend to be small, but if you then wish to share such an image for print, you’ll not be able to effectively up size.

Image size dialogue boxThe Image Size dialogue box provides you with the opportunity to modify an image’s physical size. This is an important feature to gain knowledge of. Whether you use Adobe Photoshop for print, web or video, at some point the actual dimensions of your image will need to tally with functionality

To mobilise this dialogue box, visit (menu) ‘Image-> Image Size…’ or right-click on the blue info bar at the top of your image and select ‘Canvas Size…’ from the drop-down list.

Explanation of settings (with ‘Re-sample Image’ activated)

  1. Pixel Dimensions. This section is primarily used for altering pixel dimensions for web, email or video output. At the top of the field-set you will see the current file-size of your selected image without compression. If you were to modify any measurement within this dialogue box, the former file-size would become bracketed with an updated file size proceeding it (see image).
  2. Pixel Width and Height. You can select either ‘Pixels’ or ‘Percent’. Pixels enable you to recalculate the current size in relation to the amount of pixels across the width or height. This is useful for setting dimensions for internet or screen use. Percentage settings permits you to change dimensions starting from 100%.
  3. Document Size. This field set is used primarily for print setup. All standard units of measurements can be found within the drop-down menus.
  4. Resolution settings corresponds to the DPI (dots per inch or pixels/inch) resolution set for print. Although you can select pixels/cm, it is not a widely recognised industrial standard. The correct resolution for print will vary dependant on technology and output use.
  5. Scale Styles. Select if you have used layer or vector styles and wish for the styles to accurately reduce/increase during resizing.
  6. Constrain Proportions. In order to keep the proportional aspect of an image, you will select this feature. Without constraints your image will stretch or squash during resizing.
  7. Re-sample field-set Image. This allows you to physically change the size of your image. Without this setting, you will only be able to change the manner in which your image is outputted during print processing.
  8. Link. This symbol signifies that the ‘Constrain Proportions’ option is currently selected.

Click on the ‘OK‘ once you are happy with your settings to apply changes.

Understanding Resolution & DPI

The resolution of an image is particularly important when preparing an image for print output. It is imperative that it is understood that by up-sizing an image from a low to a higher resolution, there will definitely not be an increase in image quality. If an image is too small for a given purpose, then attempt a re-scan.

In order to ascertain the required resolution for a certain print process, you would be better off seeking further advise from the printer’s manufacturer. The average desktop printer will expect an optimal dpi of 288. The human eye is incapable of seeing beyond that setting. Larger format printers used in exhibition output may require a dpi of 72, a quarter the size of the desktop printer.

Resolution is not a requirement of images elected for screen output. Be concerned only with pixels. If designing for the web, take into consideration the average monitor resolution of your visitors. Currently, at the time of writing this lesson, the common screen resolution is 800 by 600 pixels (taking into account accessibility rulings).

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