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Does the dpi number reported by camera in JPG have any meaning?

I am investigating on the size and resolution of the photographs posted on the Internet. I have visited many sites to analyze this information. I have found a 900X500px resolution image that has ~5400 DPI. How is it possible?

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marked as duplicate by mattdm, John Cavan, jrista Dec 14 '12 at 23:59

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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This may be covered by Does the dpi number reported by camera in JPG have any meaning? –  mattdm Dec 14 '12 at 16:13

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up vote 4 down vote accepted

An image has no physical size, it is simply a collection of pixels. A 1280 pixel wide image will be rendered 20cm wide on my old computer monitor, but the same image will be rendered just 9cm wide on my phone.

The DPI value embedded is metadata recording the "intended" physical size. As such it can be set to whatever value the user desires. The value is used by certain software in order to determine the appropriate physical size to render/print the image.

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This DPI information just hints that the image should be printed at a 0.16" (0.42cm) size.

Remember that DPI on an image is just a hint, the resolution is always dependent on the media you are using to render the image (screen, printer).

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The DPI setting (or PPI to be pedantic) only specifies how the pixels should be translated into absolute metrics.

The 5400 PPI for the 900x500 pixel image only means that it represents 0.1667x0.0926 inches (4.23x2.35 mm) in absolute metrics.

When an image is shown in the browser, it totally ignores the absolute metrics. The image is shown by its pixel size, so it's shown as 900 pixel wide, not 0.1667 inches wide.

If you would take that image and use it in a program that did consider the absolute metrics, like a word processor, it would be shown using the absolute metrics.

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It wouldn't surprise me if 4.23x2.35 mm was the physical size of the sensor that recorded the image. –  coneslayer Dec 14 '12 at 16:16
    
@coneslayer: Yes, some cameras use the physical resolution of the chip, some pick some value that gives a resonable print size, and some pick a value that sounds good but isn't really useful for anything at all... –  Guffa Dec 14 '12 at 16:41

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