Photoshop CS6: Why is ACR set to 240 ppi by default, and not 300 ppi?

Question

Does anybody have any idea as to why ACR is set to 240 ppi by default? Isn't 300 ppi the optimum setting for printing purposes? Is there a good reason, or reasons, why Adobe chose 240 ppi as the default setting?
I'd like to know if it is advisable to change ACR's settings from this default 240 ppi to 300 ppi. I know any perceived difference, when it comes to printing images, may be negligible, but why not, if it is just a matter of simply changing it to 300 ppi permanently?
I am uneasy about changing this, however, until I find out a lot more about it, as I want the best quality, and if there are undesirable consequences by changing ACR to 300 ppi that I am not aware about, and, therefore, why Adobe have it set it to 240 ppi, then I'll leave it at the default setting.

Answer 1

Hi,
Setting this value to 240, 300 or whatever else will not change anything. The TIF or JPEG file generated from the RAW file will just have a metadata field containing this value (actually, there are 2 fields : XResolution and YResolution, usually set to the same value). The rest of the file will remain unchanged. Two files generated from the same RAW one at 240 dpi and the other at 300 dpi will only differ by that field.
When you load the file in PS (or when ACR transmits a raw image to the PS bitmap engine along with its metadata), the dpi value found in the metadata will only affect the default values that will be used in the File | Print dialog, Position and Size section. By default, PS will compute the printed size of the document according to the indication given in the XResolution/YResolution field. If you change the desired dimension or if you "scale to fit", the dpi resolution will change.
The dpi value specified in ACR or in any other software is only an indication. It never affects the image itself. Actually, this dpi value has no meaning until the image is "realized". A dpi value only makes sense when the pixel are transformed into ink dots in a printer or to display pixels on a screen. Until then, this value has no meaning.
Moreover, in Lightroom (which makes printing much more easy than in PS), it is actually recommended to not specify any dpi value. That field should usually be left blank. You specify the print target dimensions and LR will automatically compute the right dpi value.