Why is DPI changing?

[suci]

I'm not sure what you are talking about Janos, but when I crop, I always have the resolution set at 300ppi. I've used the crop tool to enlarge images for printing to as big as 20x30 inches with res at 300 ppi without any appreciable loss of resolution and as small as 4x6 at 300 ppi, and prints look great.

I am sure there is no problem with what you are doing. As long as you keep in mind that using the crop tool to resize to 20 in X 30 inches your effective resolution is only about 143 ppi. Setting the resolution for the crop tool to 300 simply resamples your image, adding about 157 phantom pixels.
Yes 143 ppi can make a nice 20X 30 since your viewing distance is going to be greater than say for a 4/6 in print. And even 100 ppi can give you a nice 4/6. I'm not saying, that you can't set your resolution to 300 or whatever number you wish. I'm saying it is not necessary or logical to do that since the 300 ppi res. recommended for printing is a minimum, not a maximum number.

[JimL68]

I have an Epson 4800 Pro (wide format printer) and the recommended PPI is 240. I always change the dpi before I resize for print and I print through Photoshop CS4. I get little or no distortion on the photos whether I resize through CS4 or Perfect Resize v3. I have a Canon 5D MK II and I have printed (outsourced) 20x40 and 30x40 on canvas which look very good.

Jim