Image resolution

Content for printing requires a minimum level of detail, otherwise the human eye will notice the pixels within the image and see a clear lack of definition. The golden rule for pixel (aka bitmap or raster) based files is: 300ppi @ actual size

The physical image size (dimensions) is required to be set at 100% of the intended size for final use. An image at 300ppi (pixels per inch: within software this is commonly referred to as the same definition as the printing terminology dots per inch) will contain enough pixel information for high quality reproduction. Any image with a resolution lower than this value, that is desired to be printed on an offset or digital press, will suffer from minor to very obvious pixellation, depending on the actual file resolution and quality.

When scanning images, although most desktop scanners can produce files at 2400dpi, only 300dpi is required. Images above 300dpi will contain more information, but the constraints of standard printing will not allow this extra detail to be seen and the file size can also become prohibitively large. It is generally wise to retain an original scan at the largest physical size possibly required (or achievable) and then save further copies at the actual size to be used within a document – always keeping the resolution at 300ppi.

Any image content described as ‘line art’ (ie, charts, maps, type) will require a higher resolution of 1200dpi to avoid jagged edges often present at 300dpi for this detailed content. Black & white bitmap images will especially require this higher resolution, otherwise straight lines will feature very noticeable ‘stepping’ as the lower resolution attempts to create a hard edge with a limited amount of pixels.

Digital cameras:
Consult your user manual and set the camera to ‘high quality’ or a comparative high setting and also ensure the image size is set for the largest output possible. Also note to turn off date stamping if set as a default function. If purchasing a new camera consult with a specialist photographic or camera store for advice on which camera to buy, informing them about your intended use for print purposes.

Web images:
The resolution of a web image is generally not suitable for print output, unless the file is dimensionally very large. The computer monitor displays images at a much lower resolution – 72ppi – compared to the requirements for print, so most web sourced content will not contain the needed pixel information. Even though an image may look perfectly suitable on screen, the printable size will usually be very small and the quality of the file unsuitably low.

Resampling or resizing images:
Adobe Photoshop (or other similar image editing software) offers the ability to alter both the dimensions and resolution of an image. Resampling by increasing the amount of pixels within an image, is done by a process called interpolation, in which the software creates new pixels based on the colour information of surrounding pixels. This process is usually not advisable for images containing important sharp detail (faces, contrasting shapes, clean lines) as it will produce noticeable pixel edges and fuzzy tones in certain areas. If required, only use this method of increasing dimension or resolution by a small amount, which may hide any possible interpolation problems.

The following is a step-by-step guide for resampling in Adobe Photoshop:

  1. Select ‘Image’ from the menu, then ‘Image Size’.
  2. Make sure that ‘Resample Image’ is checked (or on) and select the appropriate method. The default method of ‘Bicubic’ is suitable for most images, however experimenting with ‘Bicubic Smoother’ when upsampling (creating a larger image) and ‘Bicubic Sharper’ when downsampling may produce better results.
  3. Make sure that the resolution setting is for pixels/inch.
  4. Within ‘Document Size’ the image width and height will now be unlinked to the resolution (as indicated by the links icon). Enter a new value for either width/height or resolution, based on your requirement to alter either the dimensions at the set resolution, or the resolution at the existing dimensions.
  5. Select ‘OK’.

If you are simply resizing an image saved at a resolution other than 300ppi, the following steps will change the dimensions and resolution in combination, without using interpolation:

  1. Select ‘Image’ from the menu, then ‘Image Size’.
  2. Make sure that ‘Resample Image’ is unchecked (or off).
  3. Make sure that the resolution setting is for pixels/inch.
  4. Within ‘Document Size’ the image width, height and resolution will now be linked (as indicated by the links icon). When you enter a new value for resolution, the dimensions will scale accordingly.
  5. Select ‘OK’.

Photoshop is now simply scaling the existing image size to produce a file matching your new resolution value. If your new file dimensions are very small, based on your requirement for a higher resolution, then only resampling will allow you to achieve an increased file size – with possible quality problems as previously described. Using this method to correct and save all your pixel files to the required print resolution of 300dpi, will then allow you to place images within page layout software at exactly 100%. When creating a design, significantly scaling any image beyond the actual file dimensions, will cause loss of detail as you enlarge beyond the available pixel data and must be avoided to retain image integrity.

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