MEGApixels, PPI, DPI: who knows?
a digital photography reader asks:
I am confused! I have an 8 megapixel Canon Powershot Pro 1 The photos are great. But when sending to a magazine, they said they needed a minimum of 300dpi, and mine came out at about half that! How can I send them by email to reach 300dpi minimum? And why is it failing to deliver this anyway? Will I need to buy another camera?
I too have the Canon camera... but an older model that captures only up to 7 megapixels. I love it -- it fills all my digital photography needs.
Yes, it is sometimes confusing when the digital practitioners began talking to the printing practitioners -- and I've quite frequently gotten the very same question as you've asked.
MEGAPIXELS
This term refers to the number of actual "pixels" your camera is capable of capturing, depending on the capture settings you use. (Pixels, as you know are tiny bits of digital information, interpreted by the device or monitor, laid out om a grid of squares in order to compose an entire image.)
This is measured in terms of number of pixels in width, times the number of pixels in depth. (Or height.) But it really doesn't affect the traditional printing equation.
DPI and PPI
These are the terms popularized to refer to the actual reproduction of an image, not its creation.
DPI = dots per inch in reproduction like magazines (Learn More)
PPI - pixels per inch or 'pixel' density. (Learn More)
For all practical purposes, DPI and PPI are the same -- the industry being based on the 72 points-per-inch standard established over a century ago. (Same as used in typography sizing.)
Pixels vs. Points/Dots
Here's where the rub comes in. A "Pixel" is a finite object, as produced by the specific capture device. As you probably already know, you can add or subtract pixels to your photographs.
In any software capable of showing pixel measurements, a 500 X 500 image is always a 500 X 500 pixel image unless you add or subtract pixels. Changing the pixel measurements forces the software to enlarge or reduce the image by taking away pixels or splitting existing pixels to achieve the new dimensions.
The best analogy is a tile floor. So many tiles by so many tiles. Make the floor bigger = add tiles. Make the floor smaller = remove tiles. (No matter how the software accomplishes it!)
How we perceive pixels
Resolution, on the other hand, refers to HOW those pixels will be generated in a final display or print.
When changing the resolution of the image, you don't add or subtract pixels, you display or print them at a larger or smaller size.
Using our tile floor analogy again, now the "viewer" is on a ladder. When standing on the floor, the tiles are large and distinct. Viewing the overall floor is difficult because the individual tiles distract from the overall image of the floor.
But if the "viewer" climbs up a tall ladder to view the floor, the tiles visually appear smaller and less distinct. They lose their individuality; allowing any patterns which run through the tiles to become more clear and more easily viewed as the "whole floor" -- and of course the "whole floor" appears smaller.
View any image on your computer ZOOM in 200% and then ZOOM out 200% and you'll see the transition from individual pixels to an overall image.
View your image at 72ppi, then ZOOM out 400% and you'll be seeing it as if it were 300ppi.
RESOLUTION
When your magazine editor asks for "300 dpi" this refers to the number of dots per inch the reproduction methods and equipment is needs for the very best quality of your image. (Learn More)
Usually there's a dimension associated with that measurement. For instance, they may want to reproduce your photo in the magazine at 5 inches by 4 inches. He knows that if your image has 300 pixels per inch, then the photo will look great. (The camera needed to capture at least 1500 X 1200 pixels.)
The equipment actually doesn't modify the pixels at all. It just shrinks the size of each pixel, while retaining all it's color information, thus getting more bits of information into each inch of human viewable surface. (The tile floor viewed from high up on the ladder!)
MAKING IT HAPPEN
All this information really isn't much good, and really has no impact until you see it in action. Open any Photo in a software program capable of displaying the actual pixel dimensions in relation to the "resolution" of the image.
Here's an 800 x 600 image, set at 250dpi resolution. Note the physical size the image will print is: 3.2 X 2.4 inches. Open: Dialog #1
Now we'll change that resolution to 300dpi -- but we'll turn OFF any resampling of the image. (Remember, resampling causes the software to remove or add pixels to accomplish the change, thereby degrading the original pixel map the device captured!) Open: Dialog #2
Now, you'll see we've retained every bit of digital information originally captured by the camera ... still 800 x 600 pixels, but the SIZE of the reproduction changes to: 2.6 X 2 inches.
This is what I call "True resolution change", because it does not disturb the quality of the original pixels captured. It is the preferred method, so long as the final reproduction size is equal to or greater than the final size desired by the printer.
If your image does NOT have the appropriate pixel dimensions to achieve the Editor's desired reproduction size, then you would turn ON resampling and hope the change does not degrade the image.
This is the WHOLE REASON the digital camera industry has been driven to increase "Megapixels" ... so that the actual captured image would have sufficient "pixels" to print good, crisp, data-laden images at high resolutions and large sizes.
THE ANSWER THAT SATISFIES
Your 8-megapixel camera is capble of very high pixel counts.
Here, your 8-megapixel at 72dpi/ppi is a whopping 33 inches wide. (we're standing very close to that tile floor) Open: Dialog #3
Now increase that resolution to 300 and watch what happens. (We now climb that ladder 4-times away from the tile floor) Open: Dialog #4
Now our printed image will be 8 inches X 6 inches, yet retain all 2400 X 1800 pixels, including the embedded color information for each of those pixels.
This photo will print sharp, clear and stunning. Guaranteed.
(If you were sharp, you noticed the "viewing" zoom of the two above images. If not, go back and see how resolution affects viewing on the monitor. Photoshop shows the viewing zoom in the title bar after the name of file.)
And, that's about all there is to the PPI, DPI, and megapixels question.
Thanks for reading...
- DT&G Magazine: Design, Typography & Graphics
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- Photoshop Tips & Tricks - First online Photoshop Zine (Since 1990)
- Photoshop 911: - Where to go for Photoshop Help
- Showker Graphic Arts & Design, Harrisonburg, VA (Since 1972)
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