Tip: A Free Resolution Chart and a Guide to Image Resolution
I recently did a presentation about inkjet printing and made available a “Resolution Chart” PDF which I created over 10 years ago. It’s nothing fancy, but I’ve found that the chart often helps people to better understand how file sizes change as PPI or file dimensions increase or decrease. It can also help to quickly determine the file size you need when ordering scans, or when making your own scans.
A cropped section of the Resolution Chart.
HOW TO GET THE CHART
A link to the resolution chart, along with 12 inkjet tips (one a week for 12 weeks), will be sent to you by subscribing to my Inkjet & Imaging Tips Newsletter, which is a free newsletter sent periodically with tips, information about gallery shows and workshops, as well as info about imaging related products and offers that I believe are valuable to readers. It’s not a newsgroup, so you won’t be sent messages by others–only I post to it, and it arrives in your in-box like most e-mail newsletters. The box to subscribe is below, or you can enter your e-mail in the form on the right-hand side of this website.
When you confirm your subscription, you’ll be directed to a landing page with two links-just copy and paste each link into any browser to see the 10 tips and to download the chart.
HOW TO USE THE CHART
After downloading the chart, you will see a series of numbers. Along the Y-axis (along the left side) are common film and image sizes (File Dimensions). Along the X-axis (across the top) are various PPI (pixels per inch) numbers, as well as some RES numbers. RES30, RES40 stands for Pixels Per Millimeter, and the term is often used by companies who make continuous-tone transparencies and negatives. (Just multiply the ppm (or RES number) times 25.4 to get the equivalent PPI).
To determine file size for a specific dimension and resolution (PPI), just choose a dimension, such as 11×14 inches, and follow the line across from 100-2032PPI to see how the file size changes (this assumes an 8-bit RGB file in TIFF format with no compression or extra layers). A grayscale file would be one-third the size since it has one instead of three channels. A CMYK file would be four times the grayscale file’s size because it has 4 channels.
HOW DO I KNOW WHAT RESOLUTION TO USE?
The question of what PPI at what size is always a question that comes up. I always say “test, test, and then test again!” to see what works for your images. I print most of my work around 300PPI at final size to inkjet printers and continuous tone photo machines (like those found at Pro Labs, or drugstores). However, 180-200PPI or even lower has been fine for me in most cases, especially when making larger prints. Your file’s image quality, plus the paper, printer and final output size all contribute to the final quality of your prints.
It’s quite amazing how relatively small files can make outstanding prints, especially if they are not over or under-sharpened or have artifacts (common with JPG files that have been compressed, or with lower-quality cameras). It’s also amazing to me how two different papers output on the same printer can show a very different level of visual sharpness.
DETERMINING FILE SIZE IN AN IMAGING PROGRAM
You can check just about any file size quickly in a variety of imaging programs. Below, I show the File>New dialog box for Adobe Photoshop. Just enter the dimensions, PPI and Color Mode (for example, Grayscale, RGB, etc) and you will see your file size appear at the top of the box.
If you are an educator and would like to make copies of this chart for your students, please contact me, and I will review your request. I’ve seen many of my students truly understand for the first time the concept of resolution after seeing how it works in a visual form.
All the best!
Andrew Darlow
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