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Digital Image Basics for non-Photographers • Blaise Tobia
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Image size is in practice a very imprecise term that is used to mean several distinct quantities
each of which may be referred to by a variety of interchangeable terms.
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document size: the dimensions at which the image will print
(or be displayed in virtual printed document such as a PDF).
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pixel dimensions: most basically, the horizontal # of pixels by the vertical # of pixels.
But, pixel dimensions can also be used to mean the size of the displayed image in MB.
In that case pixel dimensions and resolution can be interchangeable.
Note that, while Photoshop correctly uses pixel dimensions in the image-size dialog (above),
it incorrectly uses document size in its small display below the image!
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resolution: synonymous with pixel dimensions in the sense of the size of the image in MB.
Note that this is the amount of RAM
(temporary memory used by the C.P.U.) taken up when the image is being viewed
exclusive of any overhead using up additional RAM
(such as masks, layers, paths, etc.).
This use of resolution is different than its use in the first dialog box above, which is more correctly print resolution.
Image mode has an effect on the resolution of an image. If the size as a grayscale (single channel) image is 1 Mb,
then the size of an indexed color version would also be 1 Mb. The size of an RGB version would be 3 Mb.
The size of a CMYK version would be 4 Mb.
Color depth also has an effect on the resolution.
A 16-bit/channel image will be twice the size of the equivalent 8-bit/channel image.
open file size: not a widely used term, but it can be useful. It is basically an image's resolution (or pixel dimensions)
plus any image overhead such as layers, masks, paths, etc.
It is the total amount of RAM that will be occupied while an image is being displayed in a program such as Photoshop.
Chart of File Types and Displayed Image File Sizes
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stored (closed) file size: the amount of storage memory occupied by an image file (on a hard drive, CD/DVD or SSD).
For the uncompressed TIFF format, the stored file size will be very similar to the open file size.
For PSD format, with automatic lossless compression, it will be somwhat smaller.
For JPEG, it will be considerably smaller (dependent on quality level used).
Stored file size is also the key parameter in terms of transfer time across a network.
Highly compressed file formats such as JPEG and more compact file formats such as GIF
have been favored for the Web, email, etc.
JPEG format has been favored for uploading purposes.
Note that, when a maximum file size is specified, there may be some confusion.
Most likely the stated maximum is for the open file size (or pixel dimensions, or resolution).
But sometimes it is for the stored file size - which relates to both uploading time
and to storage space requirements on the recipient's drives.
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