Adobe Photoshop CS5 for Photographers - Martin Evening [136]
Resolution and viewing distance
In theory the larger a picture is printed, the further away it is meant to be viewed. Because of this you can easily get away with a lower pixel resolution such as 180 or 200 pixels per inch when making a poster print output. There are limits though, below which the quality will never be sharp enough at normal viewing distance (except at the smallest of print sizes). As my late colleague Bruce Fraser used to say, ‘in the case of photographers, the ideal viewing distance is limited only by the length of the photographer's nose’.
Image interpolation
Image resampling is also referred to as ‘interpolation’ and Photoshop can use one of five methods when calculating how to resize an image. These interpolation options are all located in the menu just below the Resample Image checkbox (see Figure 5.6).
Figure 5.6 This is a close-up view of the Image Resize dialog, showing the five interpolation options.
I generally consider it better to ‘interpolate up’ an image in Photoshop rather than rely on the interpolation methods offered by some scanner software programs. Digital files captured from a scanning back or multi-shot digital camera are extremely clean and because there is no grain present, it is usually possible to magnify a digitally captured image more successfully than you can a scanned image of equivalent size. There are other third-party programs that claim to offer improved interpolation, but there appears to be little evidence that you will actually gain any major improvements in image quality over and above what can be achieved in Photoshop. Here is a guide to how each of the interpolation methods works and which are the best ones to use and when.
Nearest Neighbor
This is the simplest interpolation method of all, in which the pixels are interpolated exactly using the nearest neighbor information. I actually use this interpolation method a lot whenever I need to enlarge dialog box screen grabs by 200% for use in the book. I choose this method because I don't want the interpolation to make the sharp edges of the dialog boxes go fuzzy.
Bilinear interpolation
This calculates new pixels by reading the horizontal and vertical neighboring pixels. It is a fast method of interpolation, and this was perhaps an important consideration in the early days of Photoshop, but I don't see much reason to use it now for anything.
Bicubic interpolation
This provides better image quality when resampling continuous tone images. Photoshop reads the values of neighboring pixels vertically, horizontally and diagonally, to calculate a weighted approximation of each new pixel value. Photoshop intelligently guesses new pixel values, by referencing the surrounding pixels.
The advanced bicubic interpolation methods
The Photoshop bicubic interpolation method has been sufficiently improved to provide the optimum resampling, especially with regard to downsampling image data. However, if you need to apply an extreme image resampling (either up or down in size), I suggest you use either the Bicubic Sharper or Bicubic Smoother interpolation methods described below.
Planning ahead
Once an image has been scanned at a particular resolution and manipulated, there is no going back. A digital file prepared for advertising usage may never be used to produce anything bigger than a 35 MB CMYK separation, but you never know. It is therefore safer to err on the side of caution and better to sample down than have to interpolate up. It also depends on how much manipulation you intend doing. Some styles of retouching work are best done at a magnified size and then reduced in size afterwards. Suppose you wanted to blend a small element into a detailed scene. To do such work convincingly, you need to have enough pixels to work with to be able to see what you are doing. For this reason some professional retouchers will edit a master file that is around 100 MB RGB or bigger even. Another advantage of working with large file sizes is