NOTE: This post was prepared and edited from a book I designed, wrote and attempted to publish in 2017. -P
This is the one area of most confusion and misinformation that I come across when speaking with anyone wanting to understand or learn more about Photoshop. The issue of resolution still seems to confuse many digital artists and fine artists alike that use Adobe Camera Raw and Photoshop to process their digital images.
An “image resolution” query on Google will return a great many hits about this most basic and essential step in producing quality reference images, so I thought I would present my own take on the subject. This is a rather longish but important post, and some background information is in order beforehand. I’ll attempt to set the record straight on resolution but I warn you it might get a bit technical. Please bear with me, because its extremely important that you understand these concepts and explanations and use them properly. Although resolution issues matter in advance of image editing, what you do with your images AFTER you’ve completed your image editing – how you repurpose your images – is of equal if not greater importance.
To help you remember the acronyms: spi/ppi is for input resolution (scanning or other digital capture device; dpi is for output resolution (laser or inkjet printer); lpi has to do with the way images are reproduced in print media using the halftone process to simulate a continuous-tone image.
A glossary of important acronyms you need to know:
SPI (samples per inch). This is scanner and/or digital image resolution. The scanning or capture device takes a sampling of portions of the image. The more samples taken per inch, the closer the scan is to the original image. The higher the resolution (set by you or someone), the higher the SPI.
PPI (pixels per inch). The number of pixels (picture elements) displayed in an image. A digital image is composed of samples (SPI) that your screen displays in pixels. PPI is the display resolution and not the image resolution but the two are frequently interchanged.
DPI (dots per inch). The term DPI is probably the most misconstrued acronym in the digital imaging world, loosely cast about and applied to just about every device. DPI is a measure of the resolution of a PRINTER. It refers to the dots of ink or toner used by an imagesetter, laser printer, or other printing device to print text and graphics. In general, the more dots, the better and sharper the image. DPI is about PRINTER resolution ONLY. Please DO NOT refer to the resolution of your image file in DPI. It is wrong.
LPI (lines per inch). This has to do with the way printers reproduce images in print media, simulating continuous tone images by printing lines of halftone spots (another and different SPI). The number of lines per inch is the LPI, also called line frequency or halftone resolution. A typical line frequency (LPI) for a high-quality magazine printed on glossy paper would be 175 lpi; a newspaper would typically be 75 lpi. The higher the line frequency the more the printed image will look like a continuous tone (photographic) print.
So then, just what is resolution anyway? The American Heritage® Dictionary of the English Language:Third Edition, 1996, in its definition of resolution states:
…6. the fineness of detail that can be distinguished in an image, as on a video display terminal.
In other words, resolution – input or output – is a measure of the ability of a device to render fine detail. And as can clearly be seen from the previous descriptions, in all cases, the higher the number, the better the image will appear.
Choosing an input or scan resolution based on the ability and limitations of the output device that will process the image is VERY important. Understanding what happens between image input and final output is critical. For those of us who are creating digital images, the scanner or digital camera is most likely the first tool that you will use to get your ideas into Photoshop and onto the display monitor. The quality of a scanned or captured image is contingent upon the capability of the output device. For digital artists creating content for the world wide web, for example, the output device is a video monitor. For those artists working primarily in print, the output device would most likely be an inkjet printer, a laser printer or some other type of imagesetter. But monitors and printers work quite differently from each other, and the rules are different for images intended for both devices. And, as is often the case these days, the images we create will most likely need to be “repurposed” and often. What follows then, is an attempt to detail the significance of these differences and, in the process, demystify it. But first, there are a few things to consider about your image.
An informational record only
First, your image has two states, an informational record, and a physical size. Attributing physicality to your raw or virgin image is a mistake. Its best to just think of your image as an informational record of a grid of pixels (short for picture elements), repositing on a hard drive in your computer as a file. Nothing more. When these pixels are small enough so as to not be individually discernable, a digital image can achieve true photographic quality. Increase the magnification enough however, and the individual pixels will become readily apparent.
Secondly, forget for the moment, a number such as 72 dpi (dpi is wrong anyway – more about that below). This is a number without any meaning – an invented number, a relic from the days when computer screens often had a resolution of 72 ppi. A misunderstanding took hold that image resolution on a screen or on a website is always 72, but nobody has a monitor that really displays at 72 pixels per inch. For an onscreen presentation 72 is a good number yes, but so is 96 or 120 (what I use).
The number 72 is there at the bottom of Photoshop’s Image Size dialog because a box (field) has to be filled in, so the originator of the device that produced the image simply invents a number. It is usually 72, but is different from different originators. My camera, a Nikon D750, for example, defaults that number to 300. An image file stored on a Flash Card in a camera, on your computer, or wherever, has no size in inches or centimeters, hence no ppi. Such a file only has a resolution, in pixels, referred to as Pixel Dimensions. Only when you are going to publish your file, whether online, to an inkjet printer, or traditional print media (magazine, newspaper, direct mail, etc.) does the issue of physical size matter, and only then do you ascribe a dimension of physical measurement, normally given in inches or centimeters. And only then does the PPI – number of pixels per inch – (regrettably still called dpi), really matter.
This number, say 72 pixels per inch, is also known as Spatial Resolution or Spatial Sampling and represents a variable property of an image file. It means the angle of view, or light (photons), entering the camera is broken down into a rectangular grid of pixels. It only becomes a fixed property of an image once it is output in some permanent form, like published in print. Tonal Resolution or Tonal Sampling means the continuously varying tones of brightness in nature are broken down into individual discrete steps of tone. Most output devices can only display 8 bits or 256 steps per color channel, which science claims, is more than enough for humans to perceive as continuous tonal gradations. Resolution conditional upon output is commonly called output, or print resolution. If there are enough samples, both spatially and tonally, we perceive it as faithful representation of the original scene. 256 colors each of red, green and blue – a 24 bit image – may not seem like a lot, but it is actually a huge number because 256 x 256 x 256 equals more than 16 million individual colors.
Capturing the image data
But before that happens, its important to understand how the image was captured in the first place and the correct terminology for that is SPI or samples per inch, a measurement of image resolution that was set at the time of image capture by say, a scanner. This device takes a sampling of portions of the image measuring the value at a specific place in a scanned image. The more samples that are taken per inch, the closer the scan is to the original image because there is more information available about how the image should look. A scanner samples 8-bits if information per color – red, green and blue – for a 24-bit color scan. Thus, the digital image, composed of scanned “samples”, provides information about how to display that image. Scanned images are saved as .tif files. Your camera samples somewhat differently, and depending on how you adjusted the settings, will save an image as a 24-bit .jpeg file or a 48-bit Camera Raw file.
With a digital camera, a shutter opens to allow focused light to shine via the lens onto a sensor made out of silicon, for a precise amount of time. The sensor is made up of a grid of tiny photosites, much like an array of miniature solar panels, that are sensitive to light. Each photosite resides on a piece of hardware known as a CCD or CMOS chip, and is called a pixel, short for “picture element”. There are millions of these individual pixels in the sensor of a DSLR camera recording the incoming light. The sensor has no concept of color; it simply sorts through the information it is given, the way a bank teller might separate coins without regard to their total value. The camera’s computer compiles an image from the sensor’s data resulting in a mix that potentially includes more than 16 million possible colors.
During this process an electrical charge is accumulated over the length of the exposure which is then transferred and converted to an analog voltage that is amplified and then sent to an Analog to Digital Converter where it is digitized (turned into a number). Each pixel’s electric charge is then written to the camera’s memory card as a series of ones and zeros that correspond to its primary color values. If the camera delivers RAW images, this string of ones and zeros will be left uncompressed. However, if the camera delivers JPEG images, the camera’s computer will compress the string of numbers and throw out redundant values, greatly reducing file size and often compromising quality.
The capture resolution is the amount and type of information stored for that image. Later, through resizing and resampling, the stored information of that image can change so that the image resolution in pixels per inch (PPI) is different from the original capture resolution. More in Part 2.