A question I get asked frequently in my classes is, how do I know what the optimal resolution for my images is? So, I decided to write a brief post about it.
Resolution is expressed in however number of pixels (per inch) an image has. Therefore we say that an image is 300 ppi (pixels per inch). Commercial presses however do not use the pixels per inch model, but rather rely on LINES per inch. To give you an example, most glossy magazines (e.g. People, Newsweek, Entertainment, Sports Illustrated) will print at 133 or 150 lpi (lines per inch). Some will print at higher resolutions like National Geographic that prints at 200 lpi. Newspapers will typically print at something like 100 LPI (or even lower).
The formula for getting the resolution of your images is: Lines per inch (LPI) X 2.
In the example above, you multiply 150 (LPI) x 2 = 300 PPI images for a glossy magazine. However, in 1996 I took a seminar taught by David Blatner where he explained that more than often you can get away with lower resolutions by using this formula instead: LPI x 1.5 = PPI. In the previous example this would mean that you could getaway with a resolution of 225 ppi for images and still be okay.
How InDesign handles resolution
Ensuring that images are the right resolution when you save them out of Photoshop, is only half the battle. I say this because InDesign is a bit weird in the way it handles the resolution of linked images. As a matter of fact, when you place an image, InDesign will show you two sets of resolutions for the same image. There’s the Actual PPI which refers to the native resolution of the image and then there’s the Effective PPI. This last one refers to the resolution AFTER the image has been scaled, up or down. In the example below, the image has been scaled down to 44% in the layout causing the resolution to increase to a total of 534 ppi. Yikes! BTW, this great image was provided by Paco Rocha, who happens to be an amazing photographer out of Spain and a fellow V2B instructor.
So, how do you handle this multiple personality thing going on with image resolution in InDesign? Well, the answer will depend on the document’s output. Generally, you don’t have to worry too much for digital content. The reason is that InDesign will export the images to the appropriate format and resolution based on document setup and export settings you’ve chosen. That said, it’s a whole different ball game for print production as you’re responsible for correct output of the print-ready PDF you’re going to be handing off to your client. Here are a few techniques that can help.
1. Setup a custom Preflight profile for checking image resolution.
2. Edit the final images in Photoshop to correct size and resolution. (Make sure you have backed-up all the originals in case something goes “fatal”). My approach to resizing in Photoshop is a bit different than others. Here’s how I do it:
Note the scale percentage in InDesign,
3. Alternately, you can use a commercial plugin to the heavy lifting for you. For example, there’s LinkOptimizer from Zevrix that makes the task of scaling all images to size a snap. If you’d like to know more about InDesign plugins and extensions, I created an online course for Video2brain all about the subject. Of course you’d have to know Spanish as the course is Spanish only. But I know many of my readers are bilingual.
4. Save and package your document.
5. Export to a valid PDF. One that meets the printer’s requirements. How do you know what the PDF requirements are? Ask the printer. He’s your partner in this as he has an vested interest in you getting the job right. After all, anything you do wrong, he’ll have to fix.
But, what happens if you decide to go with higher or lower than required resolutions? Well the results can be explained with a simple metaphor. If the doctor prescribes two teaspoons of medicine a day, to cure your pesky cold, what happens if you decide to take 6 teaspoons instead of the two? I’m willing to bet that nothing good will come out of it. Well, the same applies to images. Resolutions that are too high can result in blurry images where as too low, causes jaggies. So, stick with the optimal resolution for your images.