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Understanding resolution

Keep in mind, they're only pixels

The Question:
Help me change resolution
My digital camera only produces 72 resolution on its highest quality but the pictures are huge. I was wondering if I change the resolution at its largest size is it really making a difference?
Our reply to a question sent in by: Jenn
Judging from the number of letters we get with similar questions, 'resolution' is a phenomena not well defined or understood in the computer graphics field.
    Resolution is the number of units that occupy a linear inch in an image. It's measured in terms of ppi, or 'pixels per inch' when viewed in an image or on the monitor. It's referred to as dpi or 'dots per inch' in terms of printing. Any way you call it, resolution determines how the image will appear on the screen and how it will print -- more importantly, how the pixels are distributed in the document.
    The resolution 'scale' is not as important to us as the number of pixels the document actually contains. This is the crucial information.
pixels is pixels
The fact that your digital camera produces only 72 ppi resolution is irrelevant to your needs because it refers only to the pixel count of the ccr in the camera. The file size in terms of pixels by pixels is what you're most interested in.
    Imagine your camera has captured an image and saved it to a file which is 1,440 pixels wide and 1,152 pixels tall. (The literal size of the file is approximately 20 inches by 16 inches.) There are 72 pixels in each linear inch.
    Now, if you were to tell your printer that you want to print twice as many pixels per inch, say, 144, and the printer is capable of printing pixels that small, then the finished image when printed would probably be very close to 10 inches by 8 inches.
    Even though there are still 72 pixels per inch in the actual file, you've 'increased' or doubled the resolution of the printed image by using shooting the pixels on the paper half as large (as 72) but twice as many. This is basically what Photoshop does when you increase the resolution.
    Let's take this lesson one step further. This time, let's say that your printer is capable of printing from 300 to an unlimited number of pixels per inch. Now you could tell Photoshop to make the image 300 pixels per inch. Try that and see what happens. The printer now jams 300 pixels into each linear inch and and prints a stunningly detailed image where you can no longer 'see' individual pixels. The trade off is the resulting image is now 5 inches by 4 inches.
    You've discovered how output devices change your "huge" image at 72 ppi into a high resolution image.
If you change resolution...
Getting back to Jenn's question, "... if I change the resolution at its largest size is it really making a difference?" let's instruct Photoshop to make this a 288 pixel-per-inch image, but retain the 1,440 pixels wide by 1,152 pixels tall aspect ratio and pixel count. We're asking Photoshop to do what's referred to as "up-sampling." Suddenly the resulting file will balloon to a whopping 5,760 pixels by 4,608 pixels -- 80 by 64 literal inches and roughly more than 27 megabytes. If printed to this size, it would appear unacceptably fuzzy and pixelated. Why? Because Photoshop had to invent the pixels that were needed to accomplish the size you asked for. Most interesting however is that if you then printed this image back to the 1,440 by 1,152 pixels resolution which the camera originally produced, you would see a nicely printed 72 ppi image.
    This is a basic, and mechanical lesson in the reasons why we never like to 'sample' up. It's also a convincing lesson in why you should set your digital camera to produce the largest possible picture (most number of pixels, width by height) so that it can be comfortably reduced and printed at a visually pleasing resolution.
    This little exercise also reinforces the rules of rastarizing images for high quality printing.
* The image needs to be at least 2.5 times larger than the desired final print.
    If your brochure calls for an image to bleed the entire front cover (4 x 9) then you'll need an image that is at least 720 pixels wide by 1,620 pixels tall.
    Once you understand the relationships between the number of pixels printed per inch, and the 'threshold' where the human eye can no longer see pixels, everything else becomes simple no matter how many pixels per inch are produced by the camera.
    I hope this helped. If readers hit me with a flood of letters, I'll have to revisit this and attempt a more clear explanation.
Retrieved from Photoshop 911: 10/01/2002
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