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?
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.
The formula to work out required dpi is as follows:
Lines Per Inch (LPI) x 1.6 = DPI
- 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.