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I am designing a magazine where we have spreads for photography. We have around 275 ppi for those. I'm not sure how a 275ppi would look on print. Does it matter a lot for a picture as big as a double US letter size spread? How hard and fast is the 300dpi rule? What is the least one could risk?

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Not strictly at all. There is in fact almost no difference between 275 PPI and 300 PPI. The biggest risk is that you get moire on your screens, which is more or less a hit or miss thing.

Also you should try out stuff on actual paper to see how much the PPI affects things. You can easily get almost no difference between a 250 and 300 PPI image. YMMV.

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The formula to work out required dpi is as follows:

Lines Per Inch (LPI) x 1.6 = DPI

So...

  • For a screen with 85 Lines Per Inch.... 85 x 1.6 = 136dpi (common newspaper lpi)
  • For a screen with 150 Lines Per Inch.... 150 x 1.6 = 240dpi (common offset lpi)
  • For a screen with 175 Lines Per Inch ..... 175 x 1.6 = 280dpi (common magazine lpi)
  • For a screen with 300 Lines Per Inch ..... 300 x 1.6 = 480dpi (high-end "gallery quality")

I most commonly use 240ppi as a bare minimum resolution for most offset work, since a 150LPI screen is the most common frequency in general offset.

There are, of course, occasions where a higher LPI frequency is used and more PPI is beneficial.

There's really no such thing as "too much" ppi. This is generally why 300ppi is used as a standard - it covers the broadest range of possible screen frequencies and is an easy round number to remember.

  • Altough, the formula ia more like 1.6-2.2 x LPI, which in turn is not much info as, A) you can affect the LPI, B) some devices do not have LPI's and C) even if your machine has higher LPI does not mean the image needs to be using tha absolute maximum quality... – joojaa Apr 17 '17 at 7:56
  • ... and humidity in the press room can vary by ±20%... and "Todd" the pressman can be having a bad day... there are always particular variables for a particular case. It's just not possible to think in black and white terms for questions like this. I gave a general rule of thumb.. is it perfect for every instance, no... however I also alluded to that in the answer :) – Scott Apr 17 '17 at 8:37
  • Agreed +1, 300 dpi is more of a rule of thumb than anything. – joojaa Apr 17 '17 at 8:51
  • It's interesting that no one mentions WHY resolution of a digital image (which is not measured in dpi, but is ppi) should be considered in terms of halftone screen frequency. Something you might want to look up (TL for here). Also, I would disagree with "there's never too much dpi." You can choke a RIP. You can wind up with soft focus with higher dpi than necessary for the screen frequency. You can produce files that are unmanageable if they have to be transmitted through email or other methods with limits in the megabytes. – user8356 Apr 17 '17 at 18:10
  • If your concern is print production, you can't also be concerned with file size and email. That is just contradictory thinking in my opinion. There are plenty of electronic delivery methods which do not make file size a concern. Quality should be the primary issue where print production is concerned. – Scott Apr 17 '17 at 18:24

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