One of the most basic aspects of understanding digital phtoography is understanding the concept of pixels and resolution. In the past we did not have to understand the science behind how film captured images, but with digital photography we are dealing with details of the process over which we have more control. So, it is important if you are doing any more than casual personal photography to understand exactly what you are dealing with in a digital camera. First we will deal with some pixel basics and then some definitions relevant to working with digital files.
Pixels - Digital Building Blocks
Digital Photography is a binary system, which means it is based on two possible states of an electronic signal, on or off. Off is 0 (zero) black, and on is 1 (one) white. This is called a one-bit image. Obviously limiting, there is no color or shades of gray possible in a one-bit image. But, if we combine 8 bits of information and use the result to define the state of the single pixel, we can expand the possible states of the image from two (black or white) to 256, creating a stepped scale from black to white, or a grayscale image.
A grayscale image is a single channel image with 8 bits of information ranging from 0 to 255, a total of 256 different states or levels of brightness in this case. If we expand this system to 3 channels, each containing 8 bits of information, and let one channel define red values, one green values, and one blue values, we create a 24 bit color image (3 channels times 8 bits per channel), also called "true color". In programs like Photoshop this is referred to as an 8 bit per channel RGB image. The combination of the three channels provides 16,777,216 total color variations, slightly beyond what the human eye can actually perceive in terms of color variations. RGB images represent values of luminance or brightness. Higher numbers are equivalent to more brightness. The combination of values create the hue, saturation, and brightness of an image.
A more complex representation of values is the 16 bit image. This is also referred to as 48 bit color with consideration to the three channels. Raw images opened in Lightroom or Adobe Camera Raw are represented in 16 bit color to allow for maximum flexibility in editing without creating unwanted banding, which is a breaking of smooth transitions between colors. When these files are exported to Photoshop for editing they can be converted to 8 bit images, or maintained as 16 bit images. Final output files, like jpgs are best as 8 bit images.
CMYK images use 4 channels instead of 3, Cyan, Magenta, Yellow, and Black, and they represent the colors of inks used to create images in a printing press. Inkjet color printers basically print in CMYK using inks or dyes. However, images sent to an inkjet printer should remain in RGB as the printer's software handles the conversion. In fact, CMYK images sent to most inkjet printers will be converted to RGB by the print driver before printing. Output for offset printers should also be maintained in RGB as conversions to CMYK for separations requires attention to the printing profile of the output device and is best done by the print house using their software. In other words, photographers rarely have to deal with CMYK files unless they need to use channel information for specific purposes, which is beyond this article in complexity.
DPI - dots per inch.
This is a term used to define output, such as offset printing, and even inkjet printers in that the production of a printed piece is done by putting down dots of ink onto paper. It has been used as interchangeable with pixels per inch which is somewhat confusing. The difference is minor, but the proper use would clarify your meaning.
PPI- pixels per inch
This is a term used to define input, such as a digital file from a camera or scanner. Nothing is produced by an input file until the output device is described. Digital images are simply pixel arrays of a certain set of dimensions. While generally considered an output device, a monitor is defined in ppi as the device uses pixels to display the image.
Resolution is the number of pixels or dots per unit of measure (usually inches) relative to the output device. This number can change depending on the device, but changing the resolution does not change the size of the original array unless the image is interpolated in order to deliberately modify the file. An image array can be characterized as any resolution necessary for the output device and the size of the file will remain the same, but the size of the output will change relative to the output device.
RESIZE or RESAMPLE
To resize is to deliberately modify the size of the pixel array by increasing or decreasing the number of pixels in the array. Without resampling this will change the resolution. Resample means modifying the size without modifying the resolution or modifying the resolution which will affect the size. Note the links on the width, height and resolution with the resample box checked and unchecked to understand.
Interpolation is to change or modify. In digital imaging this means making a modification to the original array of pixels by increasing or decreasing their number. Changing an image's resolution is not interpolation if the array size is maintained.
In this image size box (left) in Photoshop the Resample box is not checked. Note the chain symbol indicating that the width, height and resolution are locked together. This means that a change to any one will automatically mean a change in the others to maintain the relationship. A change in resolution will mean a change in size, or a change in one size will mean a change in the other. The realtionship must be maintained.
This is the same box (right) with the resolution changed from 300 to 72 ppi. Note that the pixel dimensions and the file size are the same but the image size in inches has changed to relate to the change in resolution. No change in the file size or pixel dimension means that no interpolation of the file has been made. Only a change in the characterization of the file relative to its resolution has been made.
This is the same box (left), but this time the Resample box has been checked. The resolution is no longer locked to the width and height. The file size has been reduced considerably and the dimensions of the file have been altered. Notice the pixel dimensions are now 432 x 288. The width and height remain the same in inches but the resolution is lower. This means that the image size will be the same in inches on a 72 ppi output device as the larger file on a 300 ppi output device. The file has been interpolated and the pixel array has been changed. The file size is reduced.
If the 72 ppi image were printed on a 300 ppi device (right) at 300 ppi the image would be reduced in size as fewer pixels would be available to make the print. If printed at 72 ppi the amount of data per inch would be less than the 300 ppi file and the result would be an image with less fine detail. Note the much smaller width and height.
Our digital cameras capture pixel arrays with much larger numbers. What we have to come to understand is that the size of the capture in pixels does not have a specific resolution until we assign one. If you open a file in Lightroom or ACR (Adobe Camera Raw) you will notice that the file information does not have a resolution number, only the size of the pixel array in height and width. In this example the file name is shown along with the size of the pixel array from the camera. This is determined by the camera array size as saved in the metadata of the image.
The resolution of the file is not set until the file is exported to Photoshop as set in the External Editing tab of the LR Preferences (left) or in Adoce Camera Raw. In this case the resolution is set to 360 ppi as the file will be printed to an Epson printer. If a Canon printer were the intended output device the proper resolution would be 300 ppi instead. This has to do with the resolution of the output device's print heads. The print driver manages the relationship for you.
Printers can output to paper at a variety of resolutions depending on the paper being printed on and the desired fineness of the finished image. However, the resolution of the print head does not enter into the decision of the image resolution needed by the printer driver to make the print. A printer may print at 1440 dpi or 1200 dpi, but the file sent to that printer would be properly 360 ppi or 300 ppi accordingly. The rest is managed by the printer driver and is part of the settings in the Lightroom print module. It is not necessary for you to deal with those details unless you are the one operating the printer. When creating a file you would not set the file resolution to match the print head resolution. That is handled by the print driver.
Cropping is the removal of unwanted pixels from an image. Wedding and portrait photographers learned with digital image preparation to crop to the desired aspect ratio of the print without imposing a resolution. This means clearing the resolution box in the crop tool in Photoshop so that interpolation is not imposed. There is a Clear button next to the resolution box for this purpose.
Lightroom does not even allow you to enter a resolution for the image but wisely defines the cropping by the aspect ratio (width to height) of the image. The file resolution is set on export of the image either to an editor or to an output file (tif or jpg).
It is also possible to crop without regard to an aspect ratio, which is what I typically do with a file. You click on the lock symbol which allows you to crop without a specific aspect ratio. You just make the crop what you want it to be without regard to a specific relationship between the width and height. I am interested in what the edges of the file impose on the image in terms of the image content and not how the file might fit into a specific frame.
You might want to fit an image into an 8x10 aspect ratio for example, which would also fit a 16x20 image or other similar dimensions that maintain the ratio. Only if the intended recipient of the file or the output device being used demands imposing a resolution would I impose one. Since the image size can be set at the time of the printing, the actual image resolution is not relevant up to that point. By imposing a resolution you simply define the image size at that resolution.
Lightroom and the software used by printers can do the interpolation of the file better than Photoshop can. Typically a print house will use what is called a rasterized image processor (RIP) to make your image fit to the size you require and the resolution needed by the output device at the same time. Professional photographers outputting files to offset print houses typically impose a resolution of 300 ppi to a file. That is a convention rather than a necessity in the same manner as 72 ppi is conventionally used to describe a file for the web. Doing so only allows you to see the approximate image size in inches as most people are not used to dealing with image dimensions solely in pixels. Clients requesting "high res" images usually expect the image to be designated as 300 ppi. This is also a matter of convenience and satisfies the client's expectations. A file with the appropriate pixel dimensions would always work fine as the resolution can be changed without changing the file size as seen in the original file size illustrations at the top of this article.
When sending files for printing to me or other output services it is not necessary to resize the images to meet your output size. Once the image has been cropped the information in the file is at its best. Resizing the image for a larger print actually may reduce the image quality. At that point just leave the image alone and request the final size you desire. I use Lightroom to print images and it will do the best possible job of making your print the size you request.
Once you get this concept in your head the issues of image resolution should disappear and you only need pay attention to it when exporting an image for a specific purpose.