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Image Size and Resolution

 
image print size
 

We now continue with the last three images of this presentation. They should illustrate the relations between file size, resolution and print size.

 

Image Print size

All of these images have the same print size. However, each image has a different resolution so each file size and print quality is different. (The image size in pixels divided by the print size in inches tells you how many pixels per inch (PPI) you are printing, i.e. the quality
( Note 1. To further reduce file size for the web presentation I have reduced the images with 50% compared with what is indicated on the screen grab above. You can download a more printer friendly version of this presentation as a "zipped" PDF-document hear.

Note 2. Print size is the size of the representation of an image when it is printed. The size that it is represented on the screen is found by multiplying the size in inches by the dpi (size * dpi = pixel size). An image of 720x576 pixels that is set to 288dpi would print at 2.5x2 inches.

Note 4. At scantips.com you will find a handy "Printing Resolution Calculator - Scanning Calculator"
)

 

 

image print size

 

image resolution

Image information's probably speaks for it selves again?

 

image quality

 

 print size

While the image size and resolution have been halved the print size is still the same

 

 

image print size

Again the image size and resolution have been halved still the print size is the same.
(Note that the width and the height of the image as you view it on your monitor is not necessarily representative of the image's actual width and height — the size it would print out at (Print Size). Average monitor resolution is 72 dpi. If you view a 72 ppi image at 100% in Photoshop, chances are that it will appear on your screen in its actual print size. However, this is not true when viewing a 300 ppi image. A 300 ppi image viewed on-screen at 100% will be enormous. Don't get tricked into believing that what you see on your monitor is what you'll get when you print or place the image into another application. The only way to determine what your image's actual print size will be is through the Image Size dialog box or by calculating it manually from the formula (size x dpi = pixel size) i.e. print size= Pixel size divided by the resolution in dpi)

This has covered the typical uses of the Image Size dialog box, but I am going to encourage you to go back and spend some time experimenting with all the options in Image Size, just to get a feel for what they do. For instance, try all three of the interpolation methods for resampling, change the pixel dimensions without the constrain proportions box checked, resample an image repeatedly and then compare it to an original to observe the effects of repeated resampling.

Note 1: Which of the methods in the "Resample Image" drop down list should I use?
Bicubic interpolation, the default method, anti-aliases edges to soften them, but retains contrast by darkening dark pixels and lightening light pixels edges, much like a sharpening filter. Although it is usually the best method, if you need to sharpen the image or enhance its contrast significantly afterwards, the edge contrast may become to pronounced. In those cases, you should use Bilinear interpolation, which also anti-aliases edges, but creates less edge contrast. Nearest neighbor interpolation does not anti-alias edges. Therefore, it is the fastest method of interpolation and creates the sharpest lines. It is most useful for retaining sharp edges on images with primarily vertical and horizontal lines, such as screen shots of a computer application. However, it creates obvious stair stepping on diagonal lines, so it should not be used for photographic images. You can set a global preference for resampling method in the General Preferences dialog box. The method you choose becomes the default in the Image Size dialoged box, and is used when you scale, rotate, or otherwise transform all part of an image>

 

resolution
image resolution

 

All Rights Reserved Lars Ekdahl© 2002-2003;

Updated last time May 12 Year 2003