Every raster image has these physical attributes resolution, dimension, and size. Size takes on two personalities. You have the file size, which is a result of the pixel dimensions, and you have print size, which is the width and height of an image.
These attributes are interrelated. Change any one and at least one of the other attributes is affected also. The file size of a raster image is determined by the pixel dimensions — the number of pixels along the height and width of the image. Photoshop dialog box lists all the vital statistics of an image that takes up 176K of space and is 200 pixels wide and 300 pixels high. Just multiply these values:
200 x 300 = 60,000 pixels contained in the image.
How do 60,000 pixels lend a file size of 176K?
60,000 × 3 = 180,000
180,000 ÷ 1024 = 176
There are 3 bytes in a 24-bit color image. Multiply 3 bytes by 60,000 pixels, and the result is 180,000 pixels. Divide 180,000 pixels by 1024 bytes per kilobyte, and you get the file size value of 176K.
Remember: These are the basics that affect file size, but other factors contribute to size as well, such as layers and channels.
Resolution is usually measured in pixels per linear inch (ppi), although it can also be measured in pixels per millimeter. The image in my example has a resolution of 120 pixels per inch (ppi), or 14,400 pixels, which just happens to be 120 x 120, or 1202.
The dimensions of an image are its width and height when it is printed. To calculate these dimensions, all you have to do is divide the number of pixels by the image’s resolution:
200 ÷ 120 ppi = 1.667 inches (image width in print)
300 ÷ 120 ppi = 2.5 inches (image height in print)
Another way of calculating how much resolution an image requires is by multiplying the desired print size by the printer resolution. In other words, to print a 5-x-7-inch image at 300 dpi requires a resolution of 1500 x 2100 pixels per inch.
You’ll see recommended resolution settings somewhere between 72 ppi and 96 ppi even though resolution isn’t really a factor in preparing screen images. That’s just because monitors display somewhere in the 72 to 96 ppi range. So if you change the physical dimensions of an image, then it is always at a one to one ratio with the monitor. If you view an image whose resolution is higher than that of the monitor, the image appears larger on-screen than in its printed state. For example, try opening (or dragging and dropping) a 300 pixel-per-inch (ppi) JPEG file into a browser window. It explodes on your screen. Because the monitor can display only 72 to 96 ppi, it needs a ton of space to show all the pixels in the image.
These attributes are interrelated. Change any one and at least one of the other attributes is affected also. The file size of a raster image is determined by the pixel dimensions — the number of pixels along the height and width of the image. Photoshop dialog box lists all the vital statistics of an image that takes up 176K of space and is 200 pixels wide and 300 pixels high. Just multiply these values:
200 x 300 = 60,000 pixels contained in the image.
How do 60,000 pixels lend a file size of 176K?
60,000 × 3 = 180,000
180,000 ÷ 1024 = 176
There are 3 bytes in a 24-bit color image. Multiply 3 bytes by 60,000 pixels, and the result is 180,000 pixels. Divide 180,000 pixels by 1024 bytes per kilobyte, and you get the file size value of 176K.
Remember: These are the basics that affect file size, but other factors contribute to size as well, such as layers and channels.
Resolution is usually measured in pixels per linear inch (ppi), although it can also be measured in pixels per millimeter. The image in my example has a resolution of 120 pixels per inch (ppi), or 14,400 pixels, which just happens to be 120 x 120, or 1202.
The dimensions of an image are its width and height when it is printed. To calculate these dimensions, all you have to do is divide the number of pixels by the image’s resolution:
200 ÷ 120 ppi = 1.667 inches (image width in print)
300 ÷ 120 ppi = 2.5 inches (image height in print)
Another way of calculating how much resolution an image requires is by multiplying the desired print size by the printer resolution. In other words, to print a 5-x-7-inch image at 300 dpi requires a resolution of 1500 x 2100 pixels per inch.
You’ll see recommended resolution settings somewhere between 72 ppi and 96 ppi even though resolution isn’t really a factor in preparing screen images. That’s just because monitors display somewhere in the 72 to 96 ppi range. So if you change the physical dimensions of an image, then it is always at a one to one ratio with the monitor. If you view an image whose resolution is higher than that of the monitor, the image appears larger on-screen than in its printed state. For example, try opening (or dragging and dropping) a 300 pixel-per-inch (ppi) JPEG file into a browser window. It explodes on your screen. Because the monitor can display only 72 to 96 ppi, it needs a ton of space to show all the pixels in the image.
Photoshop: Resolution, dimension, and size in raster images
![Photoshop: Resolution, dimension, and size in raster images]()
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