August 7, 2021
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Creating a digital file for digital printing is very much like creating a digital file that is going to be displayed on a monitor as a web page or a PDF or JPG – with a few very important differences. In order to help ensure the highest quality of your printed materials, this article offers a useful set of practices for you to follow, from how to set your color space, to bleeds, to resolution. Part 2 deals explains how to save your digital file at the best resolution for optimal digital printing.
We also have Tip Sheets available on color conversion, file resolution, bleeds, and how to set up your digital files in various programs. You can find them all on the Printing Tips page of our website.
Resolutions. They’re not just for New Years anymore. The resolution of your digital files refers to the number of pixels per inch your file contains. The resolution is variously referred to as DPI (dots per inch) or PPI (pixels per inch).
Sharp, clear images are an important marketing factor. The best final file type to send to your digital printer is a 300ppi PDF. If you want to send a native file (e.g. Illustrator, InDesign) you certainly can, but you will need to include all fonts and linked images. That’s so much work when a PDF does the job all in one package. (Farther on we’ll talk about more guidelines for your digital file output.)
Not only your final document, but also all the images in your document should be at 300ppi at the final size – or the size at which the digital file will be printed.
That last part is important. For instance, say you have a photo that is 300ppi at 2X2 inches. You place the photo in your layout program and then decide to make it twice as big. Now the resolution of that photo is 150ppi – not high enough for good quality printing. The final size of the image is the size at which it will be printed, and best practice says it should be at 300 ppi at that size. (Although we can fudge that a little and get good printing out of, say, a 250 ppi image if necessary.)
A good plan, then, is to know what size your final images are going to be. Then if you are doing image correction ( to improve color, contrast, sharpening, etc.) in Photoshop or another program, save those files at 300 PPI at the final size. Generally, the best format for saving images that will be placed in a layout program (e.g. InDesign) are PSD, Tiff, or JPEG (JPG) – if you use JPEG, save at the highest quality. (Note, while PNGs are excellent files for online use, don’t use them for printing.) If you are creating bitmap art or gradations, create them at 300ppi at the final size.
But what if your photo (or any bitmap graphic) simply doesn’t have enough resolution? Then you can change the image size and raise the resolution so that it will – to a point (more on that below). In Photoshop, this is done in the Image Size dialog. Let’s say you need to take your 2X2 photo and make it 4X4. Open the Image Size dialog box and pit the new height and widthdimensions in. Be sure that the Resample Image option is checked. Also, at the bottom of that dialog box, is a dropdown menu that gives you the option to choose the method that Photoshop uses to resample the image. For resampling photos, choose Bicubic Automatic. Click OK and you have resized your photo.
OK, what’s that bicubic part about? It sounds like something your dentist wants to yank out. Here’s what’s happening in Photoshop when you resize. Photoshop can’t actually make more image than you have. So what it does instead is to interpolate new pixels into the existing pixels, so that you’ll end up with a larger image at the desired resolution. Interpolate is a fancy way of saying that Photoshop does its best to imitate the existing pixels in your photo in order to size it up. And it has a few methods of doing this. Bicubic is the best for resampling photos.
But there are limits to how much resizing you can do. Remember the “to a point” comment above? The problem with making small photos a lot bigger is that Photoshop – or any other image program – cannot add detail to your original photo. It can only take its best calculated guess at what pixels to reproduce, in effect reducing detail. So while bicubic interpolation is a brilliant way of upsizing a photo, the resulting photo will have slightly less detail than your original. Push that too far and you will notice a lack of quality.
How far is too far? The reason we can’t give you a definite answer is that that depends on the amount of detail in the photo. It it’s a photo with a lot of sharp detail you will noticeably lose sharpness and detail quicker than if you resize a photo of a lighthouse in dense fog.
We’re digital printing experts serving the Portland, Oregon area and the U.S. For more information, check out our website at: https://rhinodigital.com. Or better yet, give us a call at: 503-233-2477. We’ll be happy to answer all your questions and help you with tips and information on getting your printing projects done in the most economical and timely manner possible.