Image Resolution - Achieve the Best Quality
Proper resolution is central for high quality printing. The following tips will help you extract the best quality of your images when printing them through optimalprint.com.
Resolution is a measurement of the quality of an image, usually in terms of samples, pixels, or dots per inch. The terminology varies according to the output device: SPI (samples-per-inch) refers to scanning resolution; PPI (pixels-per-inch) refers to screen resolution; and DPI (dots-per-inch) refers to print resolution.
DPI is the relevant measure for printing.
DPI, used worldwide, defines how many dots of ink are placed in a square inch when the image is printed. 1 inch equals 25.4 mm. Often images are referred to as high resolution or low resolution. High resolution would be an image intended for print, generally having 300 dots per inch or more.
For high quality image printing, you should always strive for at least 300 dpi resolution for your image!
If your image has embedded text in it, a 400 dpi resolution is strongly recommended!
Keep in mind that there is a tradeoff between resolution and size. If you enlarge an image, you lower its resolution and vice versa.
Please choose a topic from the list below:
- The Basics of Image Resolution
- Using Images from the Internet for your Printing Material
- Using Images from a Scanner for your Printing Material
- Using Images from a Digital Camera for your Printing Material
- Purchasing Images from Stock Photography for your Printing Material
Resolution refers to the degree of sharpness of your image
Whether printed on paper or displayed on your computer screen, an image is made up of tiny little dots called pixels. Each pixel represents the color of a small part of an image. Pixels can be pulled from a palette of anywhere from two colors to 16 million colors. "Text only images", can be two colors (background and foreground). Web images are usually 256 colors and photos are usually 16 million colors. The more colors of pixels you have, the more your image looks like a photo.
Resolution refers to the dots of ink or electronic pixels that make up a picture whether it is printed on paper or displayed on-screen. Low resolution images have lines that look jagged and you can actually see the individual pixels that make up the image. High resolution images have many more pixels. That means that each pixel makes up a much smaller part of the image - making them so small that they become invisible. But there is a point where the pixels become so small that adding more (increasing resolution) won't get you any better image quality.
The human eye cannot detect the squares of color if there are 300 or more in an inch. For clear and crisp printing, images in their final output layout should be at 300 dpi - or at 400 dpi if the images include text.
Every scanner, digital camera, printer, computer monitor has a maximum number of dots it can process and display no matter how many dots are in the picture. For example, a 300 DPI laser printer can print up to 300 dots of picture information in an inch. A computer monitor can typically display only 96 (Windows) or 72 (Mac) dots of picture information in an inch.
|High Resolution||Low Resolution|
When a picture has more dots than the display device can support, those dots are wasted. They increase the file size but don't improve the printing or display of the picture. The resolution is too high for that device. A photograph scanned at both 300 DPI and at 600 DPI will look the same printed on a 300 DPI laser printer. The extra dots of information are not used by the printer; the result is only a larger file size.
When a picture has fewer dots than the display device can support, the picture will not look as clear or sharp. For example, pictures on the Web are usually 96 or 72 DPI because that is the resolution of most computer monitors. If you print a 72 DPI picture to a 300 DPI printer, it won't usually look as good as it does on the computer monitor. The printer doesn't have enough dots of information to create a clear, sharp image.
Interpolated resolution is a way to enhance resolution of a scanning device that is computed using a software algorithm. Also called the "digital resolution", it makes an image appear as if it were scanned at a higher resolution. So, if your scanner is limited to a lower resolution than desired, it is possible to circumvent that issue by using the interpolated resolution. For example, if your scanner has an optical resolution of 800 dpi and an interpolated resolution of 1600 dpi, then the scanner actually scans at 800 dpi. The scanner's software can then be used to interpolate additional pixels between two adjacent pixels; therefore artificially increasing the scanning resolution. If the first pixel is light blue and the second is medium blue, the software fills in a pixel that is halfway between the two. These guesses usually get you a pretty good image but can sometimes result in odd patterns showing up in continuous tone areas of the final image.
Size reduction can achieve good quality
A computer monitor can typically display only 96 dpi for Windows or 72 dpi for Mac. Therefore, images on the Internet are typically compressed to these resolutions in order to achieve smaller file sizes for a quick download of a webpage. If images are saved at a resolution of 72 dpi, then such images will not print nicely on a 300 dpi printing press.
However if the size is adequately adjusted, files with a 72 dpi resolution can still be altered for a quality printing. Image size (dimension) is inversely proportional to resolution. If the size of an Internet image is reduced without re-sampling, then the resolution of the image increases by the same ratio. For example:
|Original image||72 dpi||5 x 7 inches|
|Desired output||300 dpi|
|Calculated multiplier||300 / 72 = 4.17|
|Steps required||Multiply original by 4.17||Divide original by 4.17|
|Final image||300 dpi||1.20 x 1.68 inches|
Determine the image size that you want to print before you scan it; if unsure, guess a larger size
To reach your final image resolution, you need to:
- Figure out the size of the original image
- Determine the size of the printed image (output)
Original image is larger than the output size
A 1200 dpi scanner is capable of slicing up one inch of paper into 1200 separate pixels to represent it digitally. If you set your flatbed or film scanner to 300 dpi and upload your scanned image to your account on optimalprint.com for printing without increasing its size, the image quality will look the same as the original one. The human eye cannot detect printed images with resolutions above 300 dpi.
Original image is smaller than the output size
An original image can print well if sampled at the right resolution. The scanner should be set at a higher resolution so when the scanned image is increased to fit the desired dimensions, the final output resolution is at least 300 dpi for your image or 400 dpi if your image has embedded text.
To find the resolution setting for your scanner, simply increase the resolution above 300dpi by the same percentage you will be enlarging the original photo. For example:
|Original image||4 x 6 inches|
|Desired output||5 x 7 inches|
|Calculated multiplier||Largest of (5 / 4 = 1.25) and (7 / 6 = 1.17) = 1.25|
|Steps required||Multiply 300 dpi (minimum resolution for non-text images) by 1.25|
|Scanner resolution setting||375 dpi|
If you need to enlarge by 200% or more, it is recommended to contact a supplier for a professional scan.
Because of pixel loss during image editing, it is often better to scan at higher resolution than you'll eventually need. Just remember, the higher the resolution, the larger the file size. Don't go overboard!
Always use the highest quality setting available on the camera
Camera resolution is not represented in dpi or ppi - just in pixels. A camera with one megapixel resolution will create an image with one million pixels. To find out how many megapixels are required by your camera to produce a specific image, multiple the length x width of that image (in inches) x 300 dpi x 300 dpi. Virtually all digital images can be scaled down and compressed for print purposes.
A 2 megapixels camera can produce images that are 1200 x 1725 pixels, measured in size that equals 4 inches (1200/300) x 5.75 inches (1725/300). This size measured in centimeter is 10 cm (4 x 2.5 cm) x 14 cm (5.75 x 2.5cm).
The dimensions of the output image depend on the camera resolution. For images with no text, you should divide the pixel width and height by 300 to determine the dpi. Otherwise, divide by 400. For example:
|Digital camera original image (with no text)||1200 x 1725 pixels|
|Desired output||300 dpi|
|Steps required||Divide original by 300|
|Final image||300 dpi||4 x 5.75 inches|
Note that images taken with a digital camera use the RGB color space. When RGB is converted to CMYK, images tend to darken. It is recommended that you brighten and sharpen your image using an image editor software for clearer printing.
Know the image characteristics before you purchase online
Prior to purchasing an image, read the specifications of the image carefully:
- Color (CMYK if possible!)
- File size
- Resolution and quality
- Other factors depending on your need.
and make sure it coincides with the desired output specifications. Don’t forget that you should aim for at least 300 dpi resolution for your final output! If your image has embedded text in it, a 400 dpi resolution is strongly recommended.
Beware of the file size as well:
a 1 x 1 inches image at 300 dpi will result in an 0.3 MB file.
a 3 x 3 inches image at 300 dpi will result in an 3.1 MB file.
a 4 x 6 inches image at 300 dpi will result in an 8.3 MB file.
a 5 x 7 inches image at 300 dpi will result in an 12.0 MB file.