Our world is visual. From the images you see on screens to the things you read every day, photographs play an important role in conveying emotion, illustrating a point or explaining a concept. Photographs improve our comprehension and add to our enjoyment. Because of this, they are an integral part of printed material and web sites.
As camera technology has advanced, the quality of the photography we see every day has reached new levels. Couple that with the ease of using a digital camera, and we now see businesses and organizations becoming comfortable with handling their photographic image needs without the services of a professional photographer.
Our job is to reproduce those photographs in printed brochures, sell sheets, marketing material, newsletters and direct mail pieces; on signs and banners; and even business cards. Since we want the photographs to reproduce in the best possible quality, we’re offering some technical tips on the characteristics of digital photo files to use for printing and web applications. These aren’t tips on how to take photographs, but rather what to do once the image is captured and you’re ready to submit the photo file to us.
What is a pixel?
A pixel is a contraction of the words picture element. A pixel is a square that contains a series of numbers that describe its color (for color photographs) or intensity (for black and white photographs).
The greater the number of pixels in an area, the smaller each one is, the more tightly they are packed, and the better our eyes can blend the edges together to create a complete image. As the number of pixels decreases, the larger each one must be to fill the area, the more space there is between them, and the more evident the square shape becomes. If there are too few pixels in too large an area, we will see the individual squares just as we see the individual tiles in a mosaic. One telltale sign of a low resolution photo print is pixelation, known informally as the jaggies – a curved or diagonal line made up of stair-stepped pixels.
Pixels and resolution
Resolution is the term for the amount of detail in an image file. You can think of resolution as pixel density. One specification of a digital camera is the resolution it is capable of, expressed in megapixels – one million pixels. A smart phone’s 1.5 megapixel camera can capture roughly one-and-a-half million pixels of information per inch, while an 8 megapixel digital camera can capture eight million pixels per inch. The amount of detail is directly related to photo quality.
Likewise, the resolution of a photo file or print, whether offset, digital or inkjet, is the amount of detail it contains. Photo file resolution is expressed as pixels per inch (ppi), while print resolution is expressed as dots per inch (dpi).
In general, the higher the ppi value, the more detail in the file and the higher the image quality. But when the number of pixels in the image equals the number of pixels that can be used by the output device – photo printer, offset printing press, digital printer or inkjet printer – then adding more pixels does not improve image quality and may make file handling and storage cumbersome.
For offset and digital printing, the recommended resolution is 300 dpi when the photograph is in the size desired on the printed page. By multiplying the size in inches that you want the printed photograph to measure by 300, you’ll be able to determine the resolution in pixels you’ll need for the photo file. For example, if you want to print a 8x10 photograph, multiply 8 x 300 and 10 x 300, yielding 2400 x 3000 pixels in the photo file.
To determine whether your camera has sufficient resolution for this size of file, multiply the two pixel dimensions (2400 x 3000 = 7,200,000). To translate into megapixels, divide by 1 million, which in this case yields 7.2 megapixels.
Resolution for web-based photographs
Like digital cameras, image files and photo prints, computer screens have a resolution, expressed as the number of pixels wide and high in the viewing area. As a standard, most computer screens are set to display at between 72 and 96 ppi. This means that image files displayed on a computer screen do not need as high a resolution as files for print. The resolution standard for photographs on the web is 72 ppi since any additional information contained in extra pixels is ignored. One benefit of the lower resolution requirement is that photo files are smaller, meaning they load faster on the web site.
Using web-based photos for print
If your business uses product shots in its advertising and marketing materials, you may be tempted to download product photos from the manufacturer’s or distributor’s web site. Considering the resolution requirements for web (72 ppi) and print (300 ppi), it becomes clear that using a web-based photo will not result in a high quality photo in the printed piece when the two photos are of identical size. In fact, you’re likely to see the individual pixels and the jaggies on curves and diagonal lines.
What may be possible, however, is to reduce the size of the web-based photo until the resolution nears 300 ppi. A rule of thumb is that web-based images print with acceptable quality at about half the size they appear on screen. So a photograph that measures 2x2 inches on screen can be printed as 1x1 inches.
File formats for photographs
File formats for photographs are a means of organizing and storing an image. The data may be stored in compressed or uncompressed format, then rasterized into a grid of pixels whose bits define color and color depth. The grid is sometimes called a bitmap. Raster image processing (or RIPping) is a requirement for offset and digital printing. Because raster images are hard to resize and manipulate, they are best used at the size and orientation of the original. Raster images can be cropped, colorized, converted to grayscale or monochrome by using an image editing program such as Adobe Photoshop.
File compression is a way to reduce file size without compromising image quality. There are two types of image file compression. One, called lossless, keeps all the pixels of the original image but finds more efficient ways to represent recurring patterns of pixels in the file. Another, called lossy, eliminates pixels that aren’t needed to maintain quality. Lossy compression results in smaller file sizes than lossless but may trade that for lower image quality.
For photographs, the preferred file format is Tagged Image File Format (TIFF). TIFFs can be either lossy or lossless, and some digital cameras can save in TIFF format using Lempel-Ziv-Welch (LZW) compression.
Another format for photographs is Joint Photographic Experts Group (JPEG). JPEG compression is usually lossy and produces a significant reduction in file size. Repeated opening and saving of JPEG files results in continual loss of pixels which degrades file resolution over time.
Using photographs in print
Our aim is to give you the highest quality possible photographs in all your printed materials. If you are uncertain as to what resolution or file format to use when taking photographs for us to print, give us a call. We’ll give you advice based on what you are trying to accomplish, balancing photo quality and file size. For more information, call Millennium Printing Corporation at your convenience.