Document Actions
Image Size
Explanation of terms and values of measurement involved in an image's size.
Measurements of Image Size
The "size" of a digital image has four components which measure its different, possible incarnations as digital data, as an image on a monitor, and as a printed image. It can be somewhat confusing to deal with at first because the measurements are relative to each other and to the medium displaying the image. For example an image which appears large on a computer monitor (pixel dimensions) might print out the size of a postage stamp (document size).
File Size
The file size refers to the file’s size in terms of data. For example: 5.19 MB (megabytes). This will vary depending on the amount of data (pixel dimensions) and type of compression applied (J-PEG is one type of compression).
Document Size
The document size refers to its “physical” size. For example, a typical photo slide is 1.4” x .9.”
Because they are so small, at the Teaching Enhancement Center, we scan the slides at 200% their original size. Therefore your TIF files will have a document size of approximately 2.8” x 1.8.”
Resolution
When they are scanned or created, digital images are assigned a pixels-per-inch / dots-per-inch value. (ppi refers to pixels on a monitor; dpi refers to dots of ink on a printed page – however “dpi” is often used to refer to both).
Very high resolution, professional printing is typically between 1200 and 6000 dpi.
For most commercial applications 600 – 1200 dpi is sufficient, particularly for inkjet printing.
For consumer purposes, photo files are typically created at 300 dpi, which is ample for desktop inkjet printing.
Screen resolution is 72 ppi/dpi. Images intended for website, PowerPoint, or other computer display applications should have a resolution of 72 dpi.
There is no reason to have a resolution lower than 72 dpi. This would result in very poor quality in display and in print.
[NOTE: Some computer applications display all images at 72 dpi regardless of their document size. In Photoshop, an image displayed at 100% size is displayed at 72 ppi (pixel dimensions – see below) regardless of its resolution. Applications designed to create printed documents, such as Microsoft Word, tend to display images according to their document size. Thus an image with a 600 dpi resolution will appear much larger in Photoshop (at 100%) than in Word.]
Pixel Dimensions
Pixel Dimensions refers to the size of the image in screen pixels (document size x resolution). Unlike Document Size and Resolution which are relative, this is an absolute measurement.
An image with a document size of 2.8” x 1.8” at a resolution of 600 ppi has Pixel Dimensions of 1680 x 1080 (as above). This is the approximate size of your scanned slides in their TIF file versions.
Changing Resolution
To get an idea of how this works in Photoshop, we'll start by changing the resolution of an image without changing its pixel dimensions. In other words, it will retain the same number of pixels and thus the same file size, but we will change how that information will be displayed or printed.
Choose Image > Image Size
First we must uncheck the Resample Image box in order to preserve our Pixel Dimensions.
Now if we change the resolution to 72 ppi, the document size changes in relation to 23.33” x 15.”
This is the approximate size of your scanned slides in their JPG file versions.
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