right way to prepare image for outside printing

[Blazk]

Hello,
I was submitting online some images for outside printing through.
I got a warning that the resolution of few of the images is below the minimum requirement, which is 700x1050 for 4x6 prints.
I checked the size of one of the images. The resolution was ok before cropping, but after I applied cropping, the resolution went down. Also, under "Doc. Size", for some reason Resolution shows 72pix , which is supposed to be for Web. I understand, that I have to change this number for something around 300 ??
Now, how to increase the Resolution ?

THANKS! [upset]

 

[MindBender]

The crop tool allows you to change the resolution at the same time as the dimensions. My guess is that somehow the resolution box got changed and went unoticed.

 

[Erik]

Resolution is not really the problem as it can bechanged easily in Image>ImageSize. Uncheck the recalculate pixels thing and your pixels won't be altered.
For outside printing a 4x6 inch, and given the size you mention, it should be 750/4= 175 175x6 1050. 175 dpi is very little for professional printing. 266 is much more realistic. This gives you a 1064x1596 size.
Scan for this size, then set to 266dpi as said above (300 is also good, but tends to soften a bit)

Simple calculation: fixed size, and you say som many pixels per inch gives you that size in inches. That size in inches is needed at that numbver of pixels per inch tells you how many pixels are needed. Simple as that.