Throughout most of the early part of my career (as a graphic artist and art director), I mainly produced small format print products, ranging from print books to brochures, from announcements to stationery and business card packages. So the rules for file preparation, especially regarding photo resolution, have become second nature to me.
In short, the main rule is to include all images at 266 to 300 ppi (pixels per inch, which is similar to dpi, or dots per inch) resolution at the final printed size.
What this really means is that images should be twice the halftone line screen the offset printer will use. In my experience, 150 lpi halftone screens, reflecting the number of rows of halftone dots per inch, are fine for this math problem.
The purpose of this simple rule is to make sure that all images are of sufficient resolution that the reader’s eye will not see any pixelation (i.e., the square image elements that make up a computer-imaged photo on a display screen).
But here’s the rub. What about large format print graphics? I designed a roll-up banner stand a few years ago, so I had to go back online and reread the rules for large format print design. I knew that huge graphics would require a lot of storage space for the Photoshop file and/or the InDesign file, so I wanted to see what was really needed for high-quality imaging.
“Perceived” Resolution of the Photo
Again, the key phrase here is “what is really needed.” The viewer’s eye is forgiving. It needs to see images rendered at 300 ppi (or at least 266 ppi) to avoid noticing the square pixels of an image. But this is only because of the distance involved (0 to 3 feet). If you’re reading a print book or magazine, your eyes are this close to the reading material.
This is also true for some large format print banners you might design for a trade show, or a table throw to lay across your convention table. But an image that will be seen from, say, across the room (6 feet or more) can be of a lower resolution, and yet your eye will still see the images as being continuous tone (no visible halftone dots or pixels).
To further clarify this point, let’s go in the other direction. Presumably, most of the images you see on the internet are 72 ppi. This is the resolution that is perceived as continuous tone on a computer monitor based on the size of the pixels that make up the screen. If, however, you take the 72 ppi image and put it in InDesign, even at 100 percent size, the image will look like a checkerboard. The pixels will be visible and distracting. The images will look grainy or blurry. Moreover, they will be even worse if you place the photo in InDesign and then enlarge it (for example, doubling the size of an image cuts its resolution in half; therefore, a 72 ppi image enlarged by 200 percent would be 36 ppi in resolution).
How this translates into large format printing is as follows. If you are designing a roll-up banner stand that will be viewed from 3 to 6 feet away, you can include images that are 150 ppi rather than 300 ppi. Your eyes won’t know the difference, and your final art files won’t be unnecessarily large.
Interpolation
There’s a word for doing what I just said you shouldn’t do. Enlarging a low-resolution image to make it the right size for printing is called “interpolation.” While it is possible to do, it is ill advised because as the computer software enlarges an image, it actually creates picture information to place between existing pixels. This image information is fabricated. It is not part of what the camera captured, so there will be degradation of the overall quality of the image. And this will be visible.
That said, I personally have had some success in enlarging images slightly by making this enlargement process in very small steps. (For instance, I once enlarged an image 103 percent repeatedly without visible pixelation until it reached the desired size.) You may want to research this work-around online. But it’s still ill advised, and it doesn’t always work. I got lucky.
Reducing the Size of Images
Reductions in size are another matter. Go for it. If you have an 8” x 10” image and you’re making it smaller (perhaps for a photo montage on a fabric banner stand or table throw), your image resolution will go up. More specifically, if you reduce a 150 ppi 8” x 10” image to a 4” x 5” image, it will have a resolution of 300 ppi (which is twice a printer’s 150-line halftone screen). So you’re golden. (When you’re making the photo smaller, you’re actually removing picture data rather than adding it—or interpolating, or making up picture information.)
Vector Type Layers
Let’s say your banner-stand image will include type, a gradation of a color, and a photo. How can you best prepare your art files? We’ve already discussed the photo, which is bitmapped. But you could conceivably also render the type at a high enough resolution to make the edges of the letterforms appear smooth. However, there’s a better way. In Adobe Photoshop, and other software, you can put the text of your banner on a separate vector layer.
Vector images are defined with mathematical formulae. They are not a grid of dots (like the bitmapped photos discussed above). Therefore, you can enlarge (and print) vector type at any size, and the edges will be smooth. (Actually, vector type is only turned into a bitmap at the final printing stage, by the software RIP, which stands for “raster image processor.” And this transition from vector to raster type is done at the highest possible resolution of the prepress or printing equipment you’re using.) Similar in its effect to Photoshop’s vector type layer, Illustrator has a “create outlines” function, as does InDesign. In all three cases you’re creating an infinitely enlargeable vector image instead of a specific size of text in a raster image format.
In addition, you would be well advised to also use vector images for any line drawings and logos that you want to include on your large format print product.
Gradations
Now, finally, gradations. I once learned a secret about gradations (colors that lighten gradually from a solid hue at one end–like the bottom of the banner-stand art–to white–let’s say at the top of the banner-stand art). You can create a gradation mathematically (and automatically) that will gradually darken or lighten from one end to the other, or you can create a gradation (to the exact size) as a separate art element in Photoshop format. (You would then import it into your InDesign banner file as you would a photo or type.)
In my experience, if you create the gradation in Photoshop, you will often get a smoother transition from white to the solid color. This is because “banding” can occur in some mathematically produced gradations, depending on the physical distance from one side of the gradation (let’s say solid blue) to the other (let’s say white) and the resolution of the output device. The banding in question is a visible and abrupt change from one shade to the next adjacent shade. That is, the gradation is no longer smooth. It has one or more bands disrupting the even flow.
Unfortunately, what you see on-screen might not be what you get when you print, depending on the physical length of the color transition within the gradient.
As a work-around, I have found that creating the gradation (like a piece of bitmapped art) in Photoshop can mitigate this. I have also found that adding “noise” to the gradation in Photoshop can reduce banding.
Just a thought. You might want to check this out online.
Color Space
Please remember that your monitor creates color with light, within the RGB (red-green-blue) color space, and yet your printer (both offset and digital) produces colors with ink, within the CMYK (cyan-magenta-yellow-black) color space. Therefore, you should always convert everything in your Photoshop, Illustrator, and InDesign files to CMYK prior to creating final art files for your commercial printing supplier.
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on Wednesday, September 2nd, 2020 at 6:53 pm and is filed under Large-Format Printing, Prepress.
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