Putting Ink to Paper: From Pixels to Halftone Screens
Did you know that when you take a photo with your mobile device or camera, that memory is captured in pixels? What exactly is a pixel, you may ask? A pixel is the smallest editable unit of a digital image file. These tiny squares are also what affect your photo’s resolution, and how it prints. Resolution on a screen is measured by PPI, or pixels per inch, within the height and width of the file. The more pixels within the linear dimensions, the greater the resolution, the larger it can be printed, and the more continuous the tone. In general, the higher the resolution of a photo, the greater the file size. We commonly use JPG or JPEG compression which, while reducing the file size, may also affect the quality of the photo. Here, you can see a chart of optimum image sizes in pixels for printing with Forever to get the best quality printed photos. When the Forever Print Shop takes these photos and prepares them to print, the PPI is converted to DPI, or dots per inch, which gets used a lot in printing, scanning, and photography. The industry standard is 300 DPI, but can be as low as 240 DPI depending on the size of the printed image. In order to print a continuous tone image on a conventional or digital press, it has to be converted to a halftone, or variable size dots, which create an impression of tone values with one ink color – you can think of these as sub pixels. These halftone screens are measured in LPI, or “lines per inch.” “Lines per inch” is the measurement of how many rows of dots fit in a linear inch (Forever currently prints 180 LPI – the highest quality). So, what we’ve done is taken your millions of little multi-toned square pixels and converted them into millions of little multi-toned dots. As discussed in our previous blog post about “Getting the Best out of your Printed Photos,” printing can only represent a certain area of the Standard Red Green Blue (sRGB) colorspace because of the absence of light in paper. Outputting JPEGs or images for print with Forever in any other colorspace other than sRGB could cause the images to look muddy, grainy, or washed out.
Toners and inks are divided into separate colors: Cyan, Magenta, Yellow, and Black (CMYK). But to produce continuous tone and a full spectrum of tints and gradients, printing has to rely on what is essentially an optical illusion. Early artisans and engravers discovered that by creating tiny, closely drawn cross-hatches they could manipulate the eye’s perception. So instead of seeing individual lines, your eye sees the average of all available colors, and produces the impression of gray. To extrapolate this principle further, making tighter and heavier hatching gets darker gray tones, and looser hatching creates lighter gray tones. In printing photos, the process breaks down the continuous tone image into a grid of regularly spaced cells within which are centered but variably-sized dots. The larger the dots, the darker the perceived gray. When a black and white photo is printed, the image’s negative is transferred (or burned) to a “plate.” Once there, the varying sized dots will determine the shades of black and gray that will print. As you can see in the example, when you get closer to a printed image under a magnified view with what printers call a “loupe,” you can start to see the visible halftone screens.
Looking at the hashing and crosses of black and white photography printing close up can sometimes feel like staring into one of those hidden image picture books - it is fascinating how the dots work together to make the image. When color photography and printing grew in popularity, the same principle of halftoning applied. However, here’s the difference: only one color can be applied to one plate of the four color system of CMYK. These separate colors can create almost any color, and they are all broken down into their own halftone screen, burned to a plate, and then overlayed on each other. Below is an example of how those colors work together to make one continuous image.
Certain things can affect how this image is portrayed, like over-editing or lightening an image. Since there is no white ink, printing relies on the white of the paper to be that balance. As you can see below, when the level of brightness is increased, the white areas lose their dot density and intensity. Sometimes very important details can get lost, causing the photo to look grainy when really it’s the tiny dots left behind from the previous details.
Forever is a one stop shop for all of your memory-keeping needs, and as we help you collect, curate, and celebrate, the Print Shop will only continue to produce sharper and clearer images. Remember, the next time you print a photo project at Forever, use the highest resolution image that you have available for the best results possible. Then when your printed project arrives back from the print shop, grab your magnifying glass and look at all those tiny dots that form the basis of your beautiful photo keepsake.