Photoshop image resolution (DPI)

[photofan731]

Firstly where can I find the settings to show the DPI of an image in Photoshop

When I export a Jpeg from PS my 300 DPI images convert to 72 DPI

Could someone please inform me on how to manage this

Thanks

[Test Screen Name]

1. To see resolution, which it is now fashionable to call ppi, use Image > Image Size.

2. Export is meant for web use not print, so it ignores resolution. This was certainly clearer when it was called Save For Web. I wonder why they changed the name.

3. So, use Save As not Export.

[davescm · Accepted solution]

Hi

Just a couple of things to add to Test Screen Name's reply

1. 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

[photofan731]

Thanks davescm for explaining this

[Theresa J]

DPI stands for ‘dots per inch’ and refers to the resolution of the print device. Image resolution is measured in pixels, or ppi. Image size is measured in total number of pixels not resolution. The resolution is only relevant when printing. A 72ppi resolution image will print larger than a 300ppi resolution of an image that contains the exact number of total pixels.

This is confusing at first and misunderstood by many. I recorded this video to try and make sense of it all. Maybe this will help.

Photoshop Image Size and Resolution - YouTube

[photofan731]

Thanks very much for reply

I found your youtube very helpful

Cheers

[Norman Sanders]

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.

[davescm]

No argument from me Norman. Great post .

Dave

[Norman Sanders]

Sometimes it pays to be 90, Dave. I used glass screens and contact screens.

[Derek Cross]

I go back to arch-lights and dichromated colloids!

[Norman Sanders]

Way to go!

[D Fosse]

Yeah, great post!

The next person who refers to image resolution as dpi gets to write Norman's entire post - and Dave's! - up on the blackboard 100 times. We have a big blackboard here, and we'll be watching