Photoshop image resolution (DPI)

Question

Firstly where can I find the settings to show the DPI of an image in PhotoshopWhen I export a Jpeg from PS my 300 DPI images convert to 72 DPICould someone please inform me on how to manage thisThanks

davescm · Accepted Answer

HiJust a couple of things to add to Test Screen Name's reply1. Photoshop works in pixels so PPI means nothing within Photoshop for general image manipulation. It is used when sending an image to print so that the pixels can be converted to a real size (cm or inches) for printing on paper. It is also used when previewing at Print Size.2. The ppi value is just stored in the metadata for the image. In other words just a number in the data.3. When you export the metadata is stripped out - so the document has no ppi value.4. When you open an image without a ppi value in the metadata Photoshop assigns a default of 72ppi (which is why it appears to be changing)5. Changing that number does not change the image pixels at all - so you have not lost any image quality.If you do need the ppi number saved with the image then, as advised by Test SCreen Name , use Save As rather than Export.Dave

Norman Sanders · Answer

At the risk of causing confusion to this business of definitions, I would like of offer alternative terms. They refer to dpi, lpi and white balance.

DPI vs LPI

DPI: The number of inkjet printer ink droplets varies throughout the tonal scale with fewer ink dots deposited to create the lighter tones and more of them deposited to create the darker tones. We might refer to this as Frequency Modulation (FM). It is measured in picoliters. That is the only time the term DPI should be used. Sue me.

LPI: Halftone screens used in offset lithography create smaller dots in the lighter tones and larger ones in the darker tones, but the number of dots throughout does not vary. We might refer to this as Amplitude Modulation (AM).

Some halftone history: At one time, before the introduction of computers in the graphic arts, the device used to create halftones consisted of two sheets of glass, each with fine parallel opaque black lines -- perhaps 150lpi -- glued together at right angles to create a grid of small clear windows. When this device, called a glass halftone screen, was positioned in front of high contrast film (not in contact with the film) in a camera and exposed to an image, it broke the tones into an array of various sized dots -- a halftone. Today, glass screens and their successor: plastic contact screens (which are placed in contact with the film) are gone but the term lines-per-inch (lpi) -- a vestige of the glass halftone screen era -- remains to designate the number of dots to the linear inch in the halftone image on the printed page. That is why a 150 dots-to-the-linear-inch on a printed image, for example, is referred to as a "150 line halftone", an example of Amplitude Modulation (AM).

Now, on to White Balance (and I am going to get a lot of heat about this) should be defined as the neutral quality of the highlight values in an image. The neutral quality of tone elsewhere in an image: sidewalk, schnauzer, slate roof should rightly referred to as Gray Balance. These are not arbitrary terms: they are terms that have been used in the graphic arts for about a century. The camera manuals are wrong. There. I said it and I'm glad.

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