photoshop3

Image Basics - resolution

 
Digital Workflow Issues

Two of the first questions everyone wants answered -
1. How do you define resolution? 2. How big a file do you need?
Digital images are made up of picture elements called ‘pixels’ ( this comes from pix for picture and el for element). Every digital image contains a finite number of these blocks of tonal information. In the digital world resolution relates to the number of pixels, the more pixels in an image the finer its resolving capacity. The digital resolution therefore refers to the fineness of the output - the number of pixels per inch or centimeter used to construct the final print. So if we have a digital image with a dimension of 3000 pixels, the same image can be output either 10 inches at a resolution of 300 pixels per inch or 12 inches at 250 pixels per inch.
This can be expressed in the following formula: pixel size = physical dimension x (ppi) resolution. In other words, there is a reciprocal relationship between pixel size, the physical dimensions and resolution.
   
pixels
Pixels versus vectors
Digital photographs are constructed with pixels and as such are resolution-dependent. You can scale up a pixel image, but as you do so the finite information can only be stretched so far before the underlying pixel structure becomes apparent (it bitmaps). Adobe Illustrator is a vector based program. Objects drawn in Illustrator are defined mathematically. If you draw a rectangle, the proportions of the rectangle edges, the relative placement on the page and the colour it is filled with can all be expressed using mathematical expressions. The rectangle can be output at any resolution, whether it is at 72ppi on the screen or at 300ppi on an A1 blowup print. It will always be rendered as a crisply defined shape.

TIP

Object orientated applications such as In-Design & QuarkXpress observe the resolution setting even though you can resize the image in the application, therefore, the image needs preparing in Photoshop as completely as possible before importing into In-Design & QuarkXpress. Ideally, you should never have to resize, rotate, or crop an image in any other program.

   
pixel
Repro Considerations
The structure of the final print output appearance bears no relationship to the pixel structure of a digital image. A pixel in a digital image does not equal a cell of halftone dots on the page. To explain this, the black cell in a CMYK set is normally at 45 degress. If the Black Screen is at a 45 degree angle (which it normally is) the narrowest horizontal width of the black dot is 1.41.

If we extend the width of the data creating the halftone cell, then multiplying the the pixel sample by factor of 1.41 would mean that there would be at least a pixel width of information with which to generate the black plate. For this reason, you will find that the image output resolution asked for by printers is usually at least 1.41 times the halftone screen frequency used, ie multiples of x1.41, x1.5 or x2. This multiplication is also known as ‘the halftone factor’, but which is best? Some will say the 1.41 or 1.5 multipliction produces crisper detail than the higher ratio of x2. At No Duff Stuff we use the x2 factor (133 line screen x 2= 266 pixels per inch). The reason for this is there is enough data in the supplied file to survive any extra unforeseen adjustments (ie enlargement) without compromising the print quality. At the same time the file will not be unnecessarily large.
Conclusion:
Minimum resolution for Grayscale High Res would be using a 133 line srceen,133 x 1.41 = 188ppi. At No Duff Stuff our standard is to use a factor of 2 (not 1.41)ie 133 x 2 = 266ppi. Minimum resolution for Color High Res would be using a 150 line srceen,1150 x 1.41 = 212ppi. At No Duff Stuff our standard is to use a factor of 2 ie 150 x 2 = 300ppi. or in the case of 175 line screen 350ppi.
Note:
You can get away with a minimum ppi ppi but this allows no room for any adjustments that might need to be made especially enlargements.

   
  What is low resolution?
What happens here is there are not enough pixels (digital information) to fill the area so Photoshop has to guess what the missing information is so detail is lost and the image becomes blocky.

low res

Low resolution (bitmapped) image

high res

High resolution

 

Anything below 1.41 x the output devices Lpi can be considered low resolution

Laser Printer 60 x 1.41 = 85 ppi
imagesetter (grayscale)100 x 1.41 = 141 ppi
imagesetter (grayscale)133 x 1.41 = 188 ppi
imagesetter (grayscale and color)150 x 1.41 = 212 ppi
imagesetter (grayscale and color)175 x 1.41 = 247 ppi

   
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