Help with Aspect Ratio...

I have an image I would like to print that had an original resolution of 3888 x 2592. This is the native camera RAW resolution at high quality (I have a Rebel XS) and I'm not really sure what ratio this is as a 4:3 should be 3456 x 2592. Is this correct? If so should I just re size in photoshop and send to the printer? Thanks.

Your original has an aspect ratio of 2:3. Decent cropping tools, like in PS or Lightroom, have a way to pick an aspect ratio for your cropping tool that matches standard ratios, so you pick the one that matches your print size. It's true that a 4:3 ratio would not be the same -- for example it would print a 12x16 image whereas your original would print a 12x18 image. Don't worry about cropping to a pixel size -- crop to your image dimensions with the composition in view and the pixels for that aspect ratio will be retained as long as you do not "resample" while cropping or resizing to a certain ppi.

In photoshop is that done in the "image size" pane?

No. Image Size can resize to the same proportions, but you need to use the Crop Tool to change the shape. Or Canvas Size, but that's clumsy.

The numbers are right, but do not resize to those dimensions. Resizing just one side would squeeze that side down while the other remains unchanged, which would distort your image. You need to crop 432 pixels off the long side, either at one end or a bit off both ends.

Ok so now I have my image; 3456 x 2592. I have to find the appropriate ppi to obtain a decent printout. here is the size chart for mpix, any suggestions? http://www.mpix.com/Mo​unting.aspx

I don't understand. That is a chart of the print sizes that they offer. Now that you have cropped to 3:4 you are stuck with that ratio. The only 3:4 print I see on the list is 9x12. For that size I would send them the unaltered 2592x3456 which works out to 288 ppi - perfect.

The usual procedure is to keep your image uncropped until you have decided what size print you want and then crop to that ratio.

Mike787 - you really need to use the crop tool in Photoshop. Set the dimension to the size print your want, set the dpi to 300 and draw your crop on your image with the tool, hit enter and you have your properly cropped and sized image for the print size you selected....

You are really making this a lot tougher than it is.