Suggested dpi for non-vector image

It's not so much the size of the image, it's more a question of how much memory are you packing. Depending. The optimum color output usually is obtained by printing at no less than a 4:1 ratio of input resolution to output resolution. Using an RGB bitmap at 150ppi and printing at 720dpi, or 75ppi ath 360dpi, or whatever, with a bitmap rendering intent of 'Perceptual' will usually render result as close to what you see is what you get as you're going to get without taking a deep dive into the world of the radical coloristas.

This rule of thumb should work just fine for a mere 42"x42" output.

 

Is the file gonna be full size ??

 

I go with 150 dpi to scale... or 300 if it's a small image.

 

For a "perfect" printed image, I do the same as Tex. 150 dpi at final size on the printed piece. 100 dpi at final size is usually quite acceptable, and 72 dpi is usually OK most of the time. 50 dpi and below it will be obviously pixelated.

That said, the quality of the image plays a large part in this. Pictures that are really sharp and in focus, with bright colors and good contrast will still look pretty good even at fairly low dpi. Pictures that are blurry or out of focus or that are low contrast with low mid-tones will not hold up well to enlargement.

 

If the resolution looks questionable, get yourself a copy of Gigapixel AI and run it through that.

 

PPI is the resolution of the image file before your RIP processes it.

DPI is the resolution of the ink dots your printer outputs.

Two completely different things. A common scenario for a large printed sign might be a 150 ppi image, printed at 720 dpi. Also, note some software incorrectly refers to the ppi of an image as dpi.

 

when in doubt, I print a cropped piece at full size, stick it on a wall, stand back and eyeball it from the distance that it's going to be viewed from. If i'm worried, i make sure the client has seen this too.