Adobe Photoshop CS5 for Photographers - Martin Evening [133]
The optimum number of pixels
The only thing to concern yourself with is that you have a sufficient number of pixels to make high quality prints. What then are the minimum number of pixels required to print at a particular size and intended viewing distance? Plus, what is the relationship between the pixel dimensions and image resolution? These questions crop up time and time again. Digital cameras are usually classified according to the number of pixels they can capture. If a sensor contains 3000 × 4500 pixel elements, it can be said to capture a total of 13.5 million pixels, and therefore be described as a 13.5 megapixel camera. When we talk about the resolution of an image we are principally referring to the number of pixels that are contained in the photo. Basically, every digital image contains a finite number of pixels and the more pixels you have, the greater potential there is to capture more detail.
The pixel dimensions of an image are an absolute value. Therefore, a 2400 × 1800 pixel image contains 4.32 megapixels and this is an absolute measurement of how many pixels there are in the image. A digital image with this number of pixels could be printed at 12″ × 9″ at 200 pixels per inch, or it could be printed at 8″ × 6″ at 300 pixels per inch (see Figure 5.7). So if you want to know how big an image can be printed, you simply divide the number of pixels along either dimension of the picture by the pixel resolution you wish to print at (see Figure 5.3). This can be expressed clearly in the following formula: the number of pixels = physical dimensions × (ppi) resolution. In other words, there is a reciprocal relationship between the pixel size, the physical dimensions and the resolution. If you quote the resolution of an image as being so many pixels by so many pixels, there can be no ambiguity about what you mean.
Figure 5.7 The above table shows a comparison of pixel resolution, megapixels, megabyte file size (for flattened 8-bit TIFFs) and output dimensions at different resolutions, both in inches and in centimeters.
Figure 5.3 In this diagram you can see how a digital image comprised of a fixed number of pixels can have its output resolution interpreted in different ways. For illustration purposes let's assume that the image is 40 pixels wide. The file can be printed small at a resolution of 40 pixels per cm, or printed big (and more pixelated) at a resolution of 10 pixels per cm.
Repro considerations
The structure of the final CMYK print output bears no relationship to the pixel structure of a digital image, since a pixel in a digital image does not equal a cell of halftone dots on the page. To explain this, if we analyze a CMYK cell or rosette, each color plate prints the screen of dots at a slightly different angle, typically: Yellow at 0 or 90 degrees, Black: 45 degrees, Cyan: 105 degrees and Magenta: 75 degrees. If the Black screen is at a 45 degree angle (which is normally the case), the (narrowest) horizontal width of the black dot is 1.41 (the square root of 2) times shorter than the width of the Yellow screen (widest). If we want the frequency of the number of pixels to match the frequency of the halftone cells, then we can work out the ideal pixel resolution to use by multiplying the line screen frequency by a factor of 1.41. This is because using a multiplication factor of 1.41 for the pixel resolution will match the spacing of the 45 degree rotated black plate (see the example on the page opposite).
The relationship between ppi and lpi
1.
The halftone screen shown here is angled at zero degrees. If the pixel resolution were calculated