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Image Size and Resolution

Image size is the physical dimension of a digital image. The number of pixels in an image is fixed. The resolution and the image size are inversely proportional


In this short presentation I will try to explain the relations between file size, resolution and print size. In connection with that, I will first remind you what is also valid, when you scan your images


Resolution - How much to scan?

It will save you endless confusion to understand that scans don't really have any dimension apart from pixels. According to Wayne Fulton at www.scantips.com , I quote


We choose a scan resolution based strictly on the needs of the output device that will display that image. At home, this output device is normally a printer or a video monitor.

Video monitors and printers work very differently from each , and must be discussed one at a time. All of the rules are different for images intended for these two devices.

Properties of printed images Properties of video images
Image size is measured in inches Image size is measured in pixels
Image size does NOT vary with scanned resolution Image size vary with scanned resolution
Image size is modified on paper by scaling Image size is modified on screen by resampling
Image pixels are spaced on paper using the specified scaled resolution Image pixels are located at each screen pixel location, one by one
Several adjacent printer ink dots of three* colors are used to represent the color of one pixel One screen pixel location contains one image pixel, and can be any RGB value
B&W printers use only black ink or toner. Inkjet printers typically use three or four colors of Cyan, Magenta, Yellow or Black, Six color inkjets add a light Magenta and light Cyan"

End of quote

image size and resolution

File Size

All of the 3 following images have the same file size. However, the pixels are distributed (resolution) differently in each image thereby changing their resolution, pintsize and quality
( 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. Make sure that constrain proportions is checked. Constrain Proportions (Bibehåll proportioner in Swedish): When active, keeps to current proportions when resizing. When off, distorts from original proportions (i.e. stretch or squash) and resample is unchecked. So now you are scaling instead! See the difference? )


file size


file size

If you want to increase an image's width and height, or resolution, then uncheck the Resample Image box. Now any changes you make to the image's width and height will change the image's resolution, and vice versa as what now have happened with the image below


image size


image size

The resolution has now doubled while the print size now is only half compared with before


print size


image resolution

Again resolution has dabbled while the print size again and is only half compared with before. By changing the resolution of the image we have not changed the pixel data at all. You've only given Photoshop information about how you want the image to be printed. You can go to Print Preview again to verify this. In order for a print to reach the limit of what the human eye can discern you would need at MINIMUM 300 pixels/inch no matter what distance you were viewing it. To make an 8x10 inch image area at 300 ppi scan or digital image quality, you'll need 2409x3012 pixels, which is 6.9 Mpixels.)

Note: Above images have a pixel dimension (file size) of 1.19MB. How do one calculate this? (5x144) + (4x144) x3 = 1,244160 million of bytes , i.e. image size in inch x resolution in pixel/inch. As it is a color image (RGB) with 3 channels it is multiplied with 3 However now it is so that "megabytes" and "millions of bytes" are not quite the same units. (A million bytes is 1000x1000 = 1,000,000 bytes, powers of 10, or 106. But binary units normally are used for memory sizes, powers of 2, where 1K is 1024 bytes, and a megabyte is 1024x1024 = 1,048,576 bytes, or 220. So, a number like 10 million bytes is (10,000,000 / 1.048) = 9.54 megabytes. One megabyte holds nearly 5% more bytes than one million, so there are about 5% fewer megabytes.)

So therefore 1,244160 miljoner bytes/1,048576= 1,186523 MB. Rounded =1,19MB


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