Standard Image Resolutions
High quality publications usually print at 133 or 150 Line Screen(LS). Double this to determine the PPI. This is why most stock photography is provided at 300 PPI(DPI).
Very high quality publications may print at 175 or 200 LS and will therefore need a 350 or 400 PPI image to produce the optimal quality.
Newsprint is commonly printed at 85 LS and so a 170 PPI resolution would be most appropriate.
These are all just general rules. The printer determines the press resolution (Line Screen) and it is the job of the pre-press shop to prepare the graphic elements accordingly.
A safe bet is to always supply an image at a higher resolution than you think it might require.
Reseting the PPI
If you buy a high resolution image and it shows up extremely large or extremely small when placed in your layout you simply need to reset the resolution or resize the image within your layout.
If you set the resolution (PPI) to half the amount the image will become twice as large, if you reset it to double the size the image will show up on your page layout at half the size.
Resolution (PPI) is only a designation, the real issue is the pixels wide and the pixels high.
pixels / PPI = inches
3000(pixels) / 300(PPI) = 10 inches
6000(pixels) / 300(PPI) = 20 inches
5000 / 250 = 20 inches
6000 / 10(Billboard PPI) = 600 inches or 50 feet
Determining PPI by Viewing Distance
To determine an optimal PPI (resolution in Photoshop) for a photo, the calculation is:
7000 / viewing distance in inches = PPI
This is for optimal results; using half that number will give good quality results. Any lower and you will begin to sacrifice image quality.
A quick chart:
Under 24 inches = 300 PPI
24" to 36" = 200 PPI
36" to 60" = 120 PPI
5' to 10' = 60 PPI
10' to 20' = 30PPI
Billboard = 10 to 20 PPI
Megapixels and Megabytes
Megapixels is the common term used for digital cameras while megabytes is often used by professional photographers who started using digital images by way of high resolution scanners.
Both refer to the uncompressed size of an image. Megapixels simply measures the pixels wide by the pixels high while megabytes measures the pixels wide by pixels high by the number of colors.
Generally megapixels x 3 = megabytes.
Megabytes = pixels wide x pixels high x 3 (RGB)
Megapixels = pixels wide x pixels high
To maximize the use of an image, a non-compressed (shot as RAW or TIFF) 6 megapixel digital image may be interpolated up 150% and saved for use as stock photography. This would result in a 40 megabyte RGB file.
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