Ppi

[i_am_beam]

Ok this might be really dumb, but I have to ask.

In Photoshop, I've set the image size to 8x10 with the intention of ordering prints. Then I looked and saw PPI (pixels per inch). So I figured that must just effect the resolution since the image size is already set. I cranked the PPI all the way up to 400 and the picture got really big. I zoomed out and everything is fine. Right?

What I'm asking is what is the relationship between Pixels Per Inch and Image Size? Also, does it matter if I set the Image Size to 8x10 or 4x5, it's all proportional correct?

Any help/advice/input would be appreciated.

[dcclark]

Assuming that you have the "resample" box UNchecked (so that the image is not physically changed by your settings), then setting PPI in Photoshop has no actual effect on the pixels which make up the image. It is just a software setting which determines how the image is shown onscreen.

The same goes for setting the image's dimensions -- it affects how the image is shown (so that you can preview the size of a printed image, for example, and see how big some details will be).

From a practical standpoint, when printing an image, your PPI is determined by the number of pixels in your image (10 MP = 10,000,000 pixels, for example), and the dimensions (8x10, 5x7, etc.) at which you print. The PPI is essentially how many pixels have to be squeezed into each inch of printed page. When you send a jpeg to any print shop, they will not care about your PPI or image dimension settings -- they will want the image and the dimensions you want, and they will fit the pixels into that sized area.

The relationship among number of pixels, image dimensions, and PPI is similar to the relation among shutter speed, aperture, and ISO: if you change one, then (to keep the same image / the same exposure) the others must change in some predictable way. Hope that helps!