You’ll need this procedure primarily if you want paper copies of your photos. Digital cameras – apart from DSLR’s and a few expensive compact machines – usually use a 4:3 size ratio, while traditional paper photos are of 3:2.
Of course nowadays it is easy to order 4:3 copies as most providers have migrated to the digital ratio, but if you still prefer 3:2, also more comfortable for the viewer, you have an easy way to go in Photoshop.
Open the picture in Photoshop you want to crop. Our example has a 4:3 size ratio, common for digital cameras and computer monitors. We want to crop it to a traditional ratio of 3:2. Of course, this means discarding certain parts near the edges of the photo.
Select the Rectangular Marquee Tool on the toolbox.
You can see the options for the selected tool on the top of the window. Under Style, select Fixed Aspect Ratio, and set Width to the size ratio of the greater dimension of the picture, in this particular case, 3. Similarly, set Height to the ratio of the smaller dimension, 2. Naturally, for photos with a portrait orientation, use a ratio of 2:3.
Drag a rectangular marquee on the picture from a corner. The size ratio for the marquee stays at the set value (3:2). If you have reached the desired size, click the inside of the selection and drag the mouse to set the exact location of the marquee box. You can do the same by pressing the arrow keys (for steps of 1 pixel) or Shift+arrow keys (for steps of 5 pixels).
When the selection box is in its proper place, click Image/Crop to crop the excess parts. Here you are, at 3:2 instead of 4:3.