The very first thing you must understand about photography is that it is totally based on illusion; you choose to believe what you perceive. This concept didn’t originate with photography’s pixels and dots; it is the very basis for human sight. Your brain chooses to believe something to be true well beyond what your eyes can verify or recognize to be true. The very word “resolution” gives light to this concept. The resolving power of a lens is its ability to distinguish small elements of detail. This same issue is true concerning the human eye and its perception of images on a computer screen and the printed page. Each of these “interpretations” relies on a mechanism to carry out an illusion. The eye’s mechanism is rods and cones, cameras use photo receptors, computer screens use pixels, and printing machines use spots and halftone dots. The degree to which each device succeeds in their illusionary quest is dependent upon the resolution of the mechanism and the resolving power of the device.
Each system requires two elements – a transmitter and a receiver. Just as a magic trick requires both a salesman (the magician) and a customer (the viewer), each “visual” process requires a good presenter and a willing observer. The common phrases, “seeing is believing,” and “perception is reality,” pretty much define the benchmark of success. Now let’s get image resolution explained and show you where it’s is most effectively used.
There comes a finite distance when viewing any image where your eye can no longer distinguish individual colors. Beyond that point, your brain must sell the idea that detail indeed exists beyond that point of distinction. The detail you see when viewing an object at close range continues to be perceived long after that object is too far away to verify that detail. There are limitations to the normal resolving power of the human eye with “normal” defined as 20-20 vision.
In the image reproduction process, delivering an image with excess resolution becomes useless when the result of that extra resolution has no purpose. Thus, the gauge of all visual resolution must ultimately be framed by resolving capabilities of the human eye. Producing more image resolution than the eye can perceive doesn’t increase the detail or improve the definition, it just creates bigger files.
While you feel more confident when you pass massive amounts of pixels on to your printer, your printer doesn’t appreciate the excess. It throws all those extra pixels away. More ain’t better; it’s just more.
Dots, Pixels, Lines, and Spots
Beware of the numbers game that is played by manufacturers in the imaging industry. There is ample misinformation and misused terminology floating around that causes significant confusion about imaging resolution. Allow me to clarify some very foggy air beginning with terminology.
DPI (Dots per inch)
The term DPI is probably the most misconstrued acronym in the digital imaging world as it is loosely cast about in digital imaging and applied to just about every device. DPI, or dots per inch, is a reference to printing device’s resolution and describes the dots and spots that each technology uses in various combinations to simulate “tones.” Dots are neither pixels nor halftone dots. We’d all be a bit better off not using this term as it has little practical application.
PPI (pixels per inch)
The basic structure of every digital image is the pixel. Pixels are the square blocks of tones and colors that you see when images are enlarged on computer screens (see the Eye illustration below). The measure of those pixels (typically in a linear inch) determines an image’s resolution and should always be addressed as PPI, or pixels per inch. This setting is affected by the Image Size dialog box in editing software. The higher the number of pixels in an inch, the higher the image resolution. Scanners, digital cameras, and paint programs all use the PPI terminology.
Of all the resolution terms in the industry, this is one that deserves top billing. While the rest of the terms need to be recognized, rarely will they have to enter the conversation.
LPI (lines per inch)
LPI refers to the halftone dot structure used by laser printers and the offset printing process to simulate the continuous tones of photographic images. LPI refers to the number of “lines” of halftone dots used by various printing processes. “Lines” is a throwback reference to the days when actual lines were etched in glass plates to interpret photographic tones in early printing processes.
This LPI number is specific to the printing industry. Lower numbers refer to larger, more visible halftone dots (newspapers) while higher numbers refer to much smaller and less visible dots (magazines and artwork). I’ll get into the numbers later.
Spots and SPI (spots per inch)
A spot is a rarely used term that refers to both inkjet and imagesetter processes. With inkjet, it is the measure of micro-droplets of ink sprayed during the inkjet printing process. SPI, or spots per inch is a User-Selectable issue concerning the resolution choices when using some inkjet printers. Higher SPI also affects the quality of the printing process by slowing the speed at which the paper is fed through the printer. The spot “marking” size of both plate and imagesetters determines the quality of the shape of halftone dots produced and only applies to high-end lithographers and service bureaus.
Device real-world requirements for optimal resolution
Now we’ll look at each device’s real-world requirements for optimal resolution. How much is too little and how much is too much? The answers require a bit of explanation because there are some variables involved in the projects and the printing devices. First I’ll clarify some misconceptions about digital camera files, then I’ll address three specific printing technologies and give you some concrete examples.
The most common reference to camera resolution relates to the camera’s image sensor. These sensors contain a grid of cells called photosites, each cell measuring the light value (in lumens) striking it during an exposure. The actual number of cells contained in an image sensor varies depending on the camera model. When the number of horizontal cells gets multiplied by the number of vertical cells on the sensor, the “size” of the sensor is defined. The Nikon D5000 sensor measures 4,288 x 2,848, or 12,212,224 pixels, making it a 12.3 mega (million) pixel camera.
These pixels can produce any number of different size pictures for various purposes. Each printing process requires a different number of pixels per inch (PPI) to deliver optimal quality prints at a given size. This is because the technology used for each type of printing is different. For example, high-quality inkjet printers spray liquid inks onto paper using very small nozzles (usually 1440 spots per inch).
Most laser printers are either 600 or 1200 dpi devices meaning that a solid line printed horizontally will be composed of either 600 or 1200 dots. Type is printed using all these dots while halftone images can be effectively reproduced from 220-300 pixel-per-inch (PPI) images.
Inside these laser printers is a raster image processor (RIP) that generates halftone dots from square pixels. The value of each image pixel gets transposed into a halftone cell. The formula for exchanging this grid of square pixels into a diagonal pattern of variable-size dots goes way beyond explanation in this article, but it’s kind of like magic.
Laser printers simulate gray tones using the halftoning process provided by the printer’s RIP.
Inkjet printers use totally different technology to translate color pixels into printed images. Tiny spray nozzles distribute ink to specific parts of the image to deliver their version of the imaging illusion. The resolution (PPI) required to deliver accurate inkjet images differs from laser printers. This is because they do not use the geometric mechanism of halftone cells but instead, spray microscopic amounts of each ink to precise locations as determined by the pixel values.
Inkjet printers require significantly fewer pixels per inch (PPI) than laser printers to carry the illusion. Typically 150-200 PPI is quite sufficient.
Offset printing includes newspapers, magazines, and brochures. Each requires a slightly different lines-per-inch (LPI) pattern of dots. Newspapers are typically 85 LPI, magazines are 150 LPI, and high-end brochures and other collateral material require up to 200 LPI resolution.
Each line screen value is produced by a different PPI formula. While all these types of printing can be produced from 300 PPI files, all that resolution is certainly not required and is technically overkill. Even those high-end brochures technically don’t require this much resolution, but the early-adopted myth of 2xLPI persists yet today. The actual requirement for all high-end printing is only 1.4xLPI. Any more resolution simply gets discarded by the platesetter’s RIP.
In this calculation, newspapers (85LPI) need only 120 PPI, magazines require only 212 PPI, and even the best quality print is ideally produced with just 283 PPI.
In case you’re thinking that this is splitting hairs and irrelevant, consider this… using the 1.4 rule totally meets the mathematical requirement and saves a whopping 50% of the file size in storage real estate and transfer time.
I fully expect to hear some pushback about these numbers, but science and math don’t lie. Phobias about resolution are long entrenched, respected, and expected. However, in the end, it really doesn’t matter that much.
There are two unforgivable sins in preparing your images for proper resolution. Low-res and up-res.
The biggest sin of all is sending files to the printer/publisher with too little resolution.
That is a certain formula for poor results and shows up in the form of soft detail and bitmapped edges caused by normal sharpening.
Every form of print technology requires a minimum of pixels to produce fully-detailed and sharp images. So do not shortchange your project in this respect.
Remember, size your images for the final appearance and assign the PPI at that final size. If you want to see an 8”x10” image appear in print, make sure you address the issue of PPI in the Image Size dialog and before you save the file.
Make it a rule never to increase your image size as it is a sure-fire recipe for disaster. You can’t create detail; you can only destroy it. Whatever size file (pixel count) you begin with is the largest pixel count you should print unless you’re okay with soft images.
Pixels are not rubber, and you cannot stretch them to a larger size without sacrificing the sharpness of the image. Your digital camera most likely provides you with ample original pixels to print most projects, try to stay within that original ratio.
You can increase the image size, but you can’t increase its detail. Every time you enlarge an image, you distort the pixels. So if you want to print sharp images, don’t enlarge them!
The major advantage to maintaining higher resolution files for an archive is that if an image ever needs to be cropped or enlarged, that extra resolution will undoubtedly come in handy.
It remains standard operating procedure in the printing industry to send all files to the printer with 300 PPI resolution. Cloud services, backup systems, and storage media sales folks certainly want you to continue the 300 PPI trend and rent more parking space on their sites.
Make it your goal to make the best of this visual illusion called photography. Your camera, your computer, and your printer provide all the tools you need to perform your magic with great success. Enjoy.