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Image Size and resolution



We continue with this short presentation where I will try to explain the relations between file size, resolution and print size.


Image Resolution

All of these images have the same resolution. Their print quality will be the same, however, each image has a different print and file size.
( 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. Resample Image: Refers to changing the pixel dimension ( and therefore display size) of an image. When off, allows you to change any of the dimensions or resolution options without altering the quality of the image. When on, will recalculate the pixel information when you adjust the settings, altering image quality and file size. When you down sample ( or decrease the number of pixels) information is deleted from the image and when you resample up (or increase the number of pixels) new information is added based on color values of existing pixels. You specify also an interpolation method to determine how pixels are added or deleted.

The most important thing to understand about resolution is the relationship between an image's resolution (ppi) and an image's print size (actual width and height). Image Resolution is represented by dpi. Dpi stands for dots per inch. It means the number of dots of information or pixels that are stored in a given file. A pixel is a dot on the screen the displays one color and is an abbreviation for picture element. A picture is made up of many pixels. For example a 6x4 image at 100 dpi has 600x400 pixels, or 240,000 in that image. A computer screen does not represent images in inches or other units, rather it displays in pixels. For example my monitor is set to display 1024x768 pixels. This means it displays 1024 pixels in width, and 768 pixels in height no mater what the size of my monitor or the print size of a picture on it.

Images are made of pixels, which are little squares of color. All factors being equal, the larger the number of pixels in an image, the higher the quality and the more defined that image is. Realistically, pictures will be different sizes; a small image may have few pixels and a large image many pixels. This does not mean that the larger image has higher quality. It is the density of pixels that impacts quality and file size. This is most often referred to as pixels per inch (ppi) and is the resolution value.
( So once again! When the Resample Image box is checked, any changes you make to an image's width or height will not change the image's resolution, and as such, any changes you make to an image's resolution will not affect the image's width and height. Keep in mind, however, when you increase width and height, or resolution, you are adding pixels to your image. These pixels don't actually exist so Photoshop must create them. As such, you will succeed only in degrading the quality of your image.)





print quality

Image information's probably speaks for it selves even here?


print size


image quality

We have changed image size from 720x576 pixels to 360x288 pixels and as we also now have marked "Resample Image", the prints size is also only half while keeping the resolution constant. So how getting the size you want without ruining the native resolution? By adjusting your print size in editor software and use PRINT size dialogs not IMAGE size dialogs




image resolution

(You should however be aware of 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. ("Dokumentstorlek" in the Swedish edition of Photoshop as on above screen shot) )

Additional information can be found at Understanding Resolution a site of Luminous Landscape


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