The crop tool is on the command bar above the image, and the image ratio control is located at the bottom under the image. In order to pick the image ratio the cursor must be moved completely across the image.
Suggestion: the crop tool and its associated image ratio control should be available anywhere, for example by right-click.
I used to find this to be quite annoying too; as my main need was to change aspect ratio to “Unconstrained” … but then I found that I can hold down the Ctrl-key when moving one of the crop “handles”, and the aspect ratio will switch automatically to “Unconstrained”.
HtH - John M
I often want to change the aspect ratio to a specific one, such as 1 : 1 or 16 : 9
I’m new here so I’ve been doing some experimenting. Hitting R brings up the crop tool without having to go across the image with the cursor - you probably already know, and I find the shift key lets me go unconstrained.
Hi Bebop
Under Windows, there is no “R” keyboard shortcut. (recadrer in french)
Is there a quick access to constrained built-in aspect ratios ?
Oh that’s a shame, sorry, I am on a Mac. I did it accidentally as that is the Lightroom shortcut.
I couldn’t find anything to choose the different aspect ratios - I tried a few combinations 
I’ve just had a play, and you can set up a 1:1 crop as a preset.
Edit: ah, but then you would have to go into the crop tool to move it to exactly where you want, so it is not ideal!
Hi Barb,
What’s good with the forum is that we can learn that a solution already exist to a request from another user.
Being with Mac, I will now use « R » as shortcut for the crop tool.
Thanks.