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How can I take an 8x10 @300 ppi and enlarge it to either a billboard size or a large standing poster ? To what extent can I enlarge it ?

Akhil Sahgal
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Since you are asking about an 8 x 10 inch photograph, I must assume it’s a picture on paper i.e. a print. This is a logical assumption because few, these days are working with an 8 x 10 view camera. A print created with a DPI (dots per inch) of 300, is suitable for viewing at normal reading distance (14 thru 16 inches (35 thru 40 centimeters). Suppose you copied this print and made a 4X enlargement. The enlarged copy measures 16 x 20 inches. When viewed from normal reading distance, it will appear to be terribly blurred because the DPI will have dropped 300 ÷ 4 = 75 DPI. Thant’s the bad news. The good news is: As a rule of thumb, most people are inclined to view a picture from a distance about equal to the corner to corner measurement (diagonal measure).

Now the diagonal measure of an 8 X 10 is about 13 inches (about reading distance). The diagonal measure of the 16 x20 is about 26 inches (4X larger). Translated: The 16 X 20 is a 4X blowup of the 8 X 10. This enlargement will appear blurry at normal reading distance but; if you step back and view it from 26 inches or more, it will look OK. Same is true if you make an 8X blowup. That’s a print 64 X 80 inch. Such an enlargement will have a DPI that will only be 37 ½ DPI however, it will look OK if the viewing distance is equal or greater than the diagonal measure which is about 100 inches.

Now even better news: If you take the 8 X 10 to a shop that specializes in making posters, their copying and printing equipment will likely provide an enhanced enlargement that will look pretty good. That’s because their printer software will “interpolate”. We are talking about imaging algorithms that look at each pixel (dot), and compare it to adjacent pixels. The software will then insert, between the pixels, a best guess pixel. This software is routine for the shops that make posters. Bottom line, your poster enlargement will likely be acceptable.

No limit to how big a print you can make provided the viewing distance is increased proportionally.

P.S. Reminds me to an Isaac Asimov science fiction story “Buy Jupiter”: Aliens come to earth to bargain for rights to place a giant billboard advertisement on Jupiter. Seems Jupiter is a used as a beacon (lighthouse) directly in the main shipping lanes used by alien space ships.

Alan Marcus
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  • Most 8x10 prints are printed at much higher dpi than 300 dpi (dots per inch). The source files are often 300 ppi (pixels per inch). A six color printer will print at least 6 dots per pixel, so a 300 ppi image would need to have 1800 dpi. – Michael C Sep 08 '17 at 06:39
  • @MichaelClark except in the context of printing, generally DPI=PPI and the two terms are used interchangeably. I learned the term DPI before there were color printers available and still use it to this day. You will see it in e.g. the EXIF specifications. – Mark Ransom Nov 06 '17 at 21:57
  • How is a 13 inch diagonal 4X of a 26 inch diagonal? The total area may be 4X but the linear dimensions are only 2X, and that includes the DPI. – Mark Ransom Nov 06 '17 at 21:58
  • @Mark Ransom - A tip of the hat from Alan Marcus you at right, I erred. – Alan Marcus Nov 06 '17 at 22:38