Okay, so you've got these images in InDesign showing a whopping 1321 PPI, which, on paper, should be more than enough.
But you're noticing they get a bit fuzzy when you zoom in on the PDF. That's a classic head-scratcher, isn't it?
InDesign's 'effective PPI' tells you how many pixels are packed into each inch within the InDesign layout. It's calculated based on the original image's resolution and how much you've scaled it down. So, a high effective PPI in InDesign is a good sign, but it doesn't guarantee a perfectly sharp PDF at all zoom levels.
What's happening when you zoom in on the PDF is that Acrobat is trying to display those pixels at a much higher magnification than they were originally meant for. Think of it like blowing up a small photo on your phone screen eventually, you'll start seeing individual pixels, and the image will look blurry.
Now, here's where it gets a little tricky: Acrobat's display resolution isn't always a perfect match for the PDF's actual resolution. It's trying to render the image based on your screen's PPI and the zoom level you've selected. So, what you're seeing is a combination of the image's inherent resolution, the PDF's rendering, and your screen's display.
Even though the image looks blurry when you zoom in, you're right to consider how it'll look to a typical reader. Most people won't be zooming in that close. If it looks good at 100% zoom, or even a little bit more, you're probably fine.
When you're looking at images for printing, the big question is, "How many pixels per inch do I really need?" It's not a one-size-fits-all thing. If you're going for high-end, glossy prints, like in a fancy magazine, you generally want 300 pixels per inch, or PPI. That's because those presses use really fine screens, and you want all the details to come through sharp.
But, if it's just a regular print job, like a brochure or a report, you can often get away with 200 to 300 PPI. And if it's newsprint, you can even go lower, like 150 to 200 PPI, because the paper itself is coarser and doesn't hold as much detail.
Now, here's the thing about those offset printing presses, the ones that use plates and ink. They use something called halftone screening, which is basically how they create the illusion of different shades of gray or colors. And the number of lines per inch, or LPI, that the press uses is really important. A good rule of thumb is that your image's PPI should be roughly 1.5 to 2 times the LPI. So, if the press is using 150 LPI, you'd want around 300 PPI for your image. But, you can often push it a little lower if the paper isn't super smooth.
Even with grayscale images, you need to think about this. Shadows and subtle changes in tone really benefit from that higher 300 PPI. But, if you're printing on rough paper, like uncoated stock, you might not notice much difference between 200 and 300 PPI.
Digital printing, like laser or inkjet, is a bit different. They don't use halftone screens in the same way. Instead, they use these clever algorithms to create the image. So, you don't need nearly as many pixels per inch. A 1200 DPI printer, which is a measure of how many dots it can put down, doesn't mean you need 1200 PPI. Most of the time, 200 to 300 PPI is perfectly fine for digital prints. For things like brochures and booklets, you might want to aim for 240 to 300 PPI. But, if it's just a quick printout, like a proof or a mockup, even 150 PPI can look okay.
Of course, file size is something to consider. If you're printing a huge document with lots of images, those high-resolution files can really add up. But, with modern computers and storage, it's not as big of a deal as it used to be. Still, if you're dealing with a massive project, it might be worth thinking about whether you really need all those pixels.
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