The only time it will alter the resolution is when you have resampled checked. When it is unchecked then it determines the size of the image when printed.
Use this formula
width in inches x ppi = number of pixels wide
height x ppi = # of pixels high
When you know two of the three variables the above formula will determine the third variable.
Therefore your image that is X inches x X inches @ 120 ppi, you can reduce the width and height to increase the ppi to 300 ppi without changing the number of pixels.
Are you saying that I can only increase to 300 ppi (or dpi? same thing??) by reducing the width and height of the image. But it wouldn’t work if I had to enlarge an image?
Excuse my ignorance, I’m not too up on all this …
You can upsample if you have to, and if the original is of good quality you can usually get away with it. But it’s generally not recommended, simply because Photoshop has to invent pixels to do it.
In the image size dialog, choose "Bicubic smoother" (bottom). See Silkrooster’s formula above for the actual settings.
BTW ppi is correct, dpi refers to printing.
I only choose 300ppi as that is the value you choose in the original post. The smaller the image is when being printed the higher the ppi is if you do not have resampled checked. Resampling allows photoshop to alter the actual pixels of the image.
If resample is checked then photoshop can either downsample (remove pixels) or upsample (add pixels). The problem with upsampling is photoshop does not know what the new pixels should be so it guesses. This can cause blurring or aritifacting. The best picture to have it one that is taken at the highest resolution you need. You are better off taking a picture that is higher in resolution than what you need and crop your image or down sample than to have an image that is to low of a resolution and try to upsample.
Think of resampling unchecked like scaling the image. You can scale the image up or down and only the ppi is changed.
Because there are no values in the above formula you can change the width and height to any value and the ppi will adjust accordingly.
The opposite it true as well you can change the ppi to any value and the width and height will adjust accordingly.
