DPI explanations in photoshop

Hello,

I created an image in photoshop...

Once i changed the PPI (which supposedly is same as DPI) the image itself got much bigger.

For example, first im working at 75 PPI and then i maximise it to 300PPI. Is this a normal thing?

Also, why is it sometimes i lose some quality - is it best of to work after i set the image size (PPI).

Is the image size thing normal after changing the PPI/DPI?


DPI = PPI - right?

Yes they are the same, what you are realizing is a resampling of the image. If you uncheck resample image, and you have a 100dpi (for simplicity) 3x3 image, and then adjust the dpi without resampling, you'll get a 300 dpi 1x1 image, the "technically correct" result. However resampling normally works very well and you can, depending on the innitial resolution get decent results.

The reason it gets bigger (with resampling) is because most monitors are 72dpi, so a 1x1 @ 72dpi will show up as a 1x1 inch squar on a normal monitor, but at 144dpi it will be two inches, because each real pixel will now span twice (and since we are dealing with area), actualy 4 (a 2x2 square) pixels on your monitor.

so overall, if i change the dpi/ppi (are they both the same?)...

The image height and width will get bigger and is NORMAL....

I need to change an image from 75dpi/ppi to 300dpi/ppi.


Thanks

Depends on whether you are re-sampling the image for screen or print... big difference...

If you do not resample, the only thing you will be changing is how large the image will be displayed (by DPI-aware software) or printed.

Allow me to use Quark Xpress (publishing software, just in case you wonder!) as an example...

If I take a screenshot, I have a 72dpi (screen resolution) 1024x768 pixel image. If I import this image into Quark, it will fill the screen, and be considerably larger than an A4 page... at 361x270mm (1024/72 = 14.2 inches!). But it will print at lower-than-acceptable quality, because it's blown up so much.

Changing the resolution to 300dpi in photoshop - but NOT resampling - will bring it in to Quark at the size you can safely print it (at print resolution, typically 300dpi) without losing quality - 86x65mm (1024/300 = 3.4 inches)

Re-sampling from 72dpi to 300dpi will add more information to the image (otherwise termed "interpolation") to bring it up to the 361x270mm size at print resolution... but whether you interpolate ("blow up") a 72dpi image in Quark or Photoshop, you're essentially asking the software to add information to the image to make it look bigger. And different methods of re-sampling (available in photoshop) will achieve different results.

So, re-sampling = adding information to the picture, whereas changing the DPI (resolution) = telling the output device how large to show/print the image.

Any clearer?

Anyway, it is always best to work at the higher resolution that you will require (e.g. 300dpi, for printing) and then down-sample the image to 72dpi if you need it to be smaller (e.g. for on-screen viewing) - rather than the other way around.

Cheers,
Dave