If you use Photoshop’s "Save for Web…" feature (File > Save for Web… -or- ALT+SHIFT+CTRL+S), then the DPI change to 72 is made for you. Note: your source file’s DPI is not affected.
So then when people are talking about how a jpg is set are they referring to the quality setting?
Which brings us to the next question: PS’s JPG quality setting (1-100) is based on a percentage system? If so, is that 100 based from 72DPI or (in my case) the 300 DPI of the original file?
Thanks,
Cid
😀
The quality settigns of JPEGs has nothing to do with the resolution of the file itself. Rather, it is a (rough) measure of the amount of degradation in the file
Jpeg quality and ppi are two totally different things.
When a file is saved in the jpeg format, the data is compressed. The greater the compression the smaller the file and the lower the quality. Try using the Save For Web dialogue in Photoshop in the 4-up view and you’ll see how you can reach a compromise between file size and quality.
DPI (DOTS per inch) is used in determining PRINT quality. Image files are measured in PPI (PIXELS per square inch).
When saving a file for the web it really doesn’t matter what the resolution is set to. Most monitors display at 96ppi, therefore a 640 pixel by 480 pixel file will display on screen at approximately 5×6.5 inches. If that same file was saved at 72ppi it would still be 640x480pixels and would still display at the same size. If it was saved at 300ppi, the 640×480 pixels would still occupy 5×6.5" on a screen displaying at 96ppi.
Hope this helps.
Chris.
Then what would be the reason for saving at a higher ppi rate? Printing??
When you print an image you need far more detail (information) in the file than is required for a screen display.
300 ppi is widely considered the norm for producing high quality prints. In reality the printer – be it a desktop inkjet or a laboratory machine printing onto conventional photographic paper – will take the image information and interpolate it up or down to match its inherent DPI (dots per inch) setup.
when you send a file to the printer you need to specify the size of print required so the printer driver can carry out that calculation. So you could send a 240ppi file set to 10×8" or a 300ppi file set to 10×8" or a 400ppi file set to 10×8" and they would all print out to the same size. The file sizes in these examples would get progressively larger – and I doubt you would notice any difference in the print quality.
Dropping the PPI too low, however, will affect print quality. You wouldn’t get a sharp smooth-toned 10×8" off of a 72ppi file for instance.
For convenience we tend to talk of 72ppi as being for web use and 300 ppi for print. It allows us, at a glance, to have an idea how large the image will be. But remember: it is the PIXEL size of the image which is important.
Chris.
You’re welcome Cid
Chris.
