How can I keep the original aspect ratio while cropping in Preview?
This question has been asked before but not answered very well.
In Apple's Preview App you can crop an image by dragging to make a selection, then using Cmd-K to crop. The problem is you can only constrain your selection to a square shape (by holding Shift as you drag). There is no simple way to constrain the selection to the original aspect ratio of the image.
Here is a fairly simple workaround:
-Select All (Cmd-A) and copy (Cmd-C). This will put a copy of the entire image in the clipboard.
-Now drag to make a selection of the approximate area you want to crop. Try to match the aspect ratio by eye but it doesn't matter if it's a bit out. Before releasing the drag, note and remember the displayed coordinates of the selection area, you will need them on the next step. In this step we just want the pixel width and/or height of the cropped area.
-Now go to Tools->Adjust Size. Choose "Fit into: Custom pixels". Select "pixels" from the drop-down menu to the right of Width and Height. Ensure "Scale proportionally" and "Resample Image" are both selected. Now enter either the width or height dimension you remembered from the previous step in the appropriate box. The other dimension should adjust automatically. Finally click OK. Your image should resize on the screen but it will NOT be cropped yet. This step adjusts the canvas to the size of your crop using the original aspect ratio.
-Now the clever trick. Paste what you copied in the first step using Cmd-V. Your original image will be pasted onto the resized canvas and you can drag it around to get it positioned exactly right. If you are not entirely happy with the size you can undo (Cmd-Z) and adjust previous steps. In this step you paste the original copied image onto the reduced size canvas and adjust its position.
-Finish off by saving (Cmd-S). This will allow you to edit the cropped image if you need to.
With practice the above method is pretty simple, and means you can crop retaining aspect ratio using Preview alone -- and no need for a calculator and fiddly dragging.
