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Resolution for Dyslexics by Mike McNamee - part 3 of 1 2 3 4 5

by Mike McNamee Published 01/12/2013

Scaling Images - why do it?

This is a valid question. Scaling an image is almost invariably called for by the eventual use or intended output of the image. Output can be many things including:

for scale down:

• for use on the web
• to discourage illegal copying
• for projected image competitions
• to ease transmission over the internet for scale up:

• enlarging for inkjet output
• enlarging for murals, posters, or wallpaper
• enlarging to meet the requirements of a picture library
• enlarging to span a double-page spread in a magazine layout Within the above there are combinations of mix and match, for example you might need a set of small images to email to a client for approval or selection followed by delivery of full-resolution images. Traditionally this could include placing low-resolution files in a layout then substituting full-resolution ones before going to press (the availability of high-power computers seems to have killed this tactic off though!).

As well as scaling, the photographer might wish to perform other tasks on an image or set of images. Converting to sRGB for web use while retaining high-resolution files for print use in ARGB is a typical example.

Watermarking and appropriate sharpening are other common tasks.


Why is it confusing? Many people struggle to grasp the difference between image resolution and size on the one hand and print resolution and size on the other especially when printer resolutions such as '1,440 dots per inch' pop up.

The best was around any ambiguity is to work in pixels.

Images as files contain a defined number of pixels, eg a Nikon D700 is 4,256 x 2,832 pixels. If you crop an image you lose pixels, they are discarded, so a 50% crop on a D700 image would reduce its size to 2,128 x 1,416 pixels and the total number of pixels would be quartered.

It is only when resolution is applied to this collection of pixels that the physical size of an image becomes a reality. This addition of a resolution value has to be carried out before an image may be printed on paper or shown on a screen. The setting of resolution may be carried out by the user in a conscious way or, for example, the printer driver may set the value according to the scaling of the print that is being asked for.

In a printer driver scaling the output to a smaller size will increase the resolution, scaling to a larger size will decrease the resolution. Either way though, the printer driver has to convert the pixels into ink dots and these are usually resolved at 1,440, 2,880 or (today) 5,660 dots per inch. These values are larger than pixel resolution because as many as 12 dots may be employed to make a single pixel (and they are different sizes as well).


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1st Published 01/12/2013
last update 21/07/2022 08:46:24

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