How to unlock aspect ratio of Render Output

[Mr.Bee]

When I manually change the aspect ratio (custom) in Render Output the width and height ratio are locked. The Chaosgroup help docs tell me the ratio can be unlocked but I can't see how to actually do so. I can't find a lock/unlock button for example.
I'm trying to render a scene with a wide panoramic proportion while retaining the existing height aspect.

Your help and advice would be most appreciated.

Thanks.

[GD3DESIGN]

After selecting 'Custom' from dropdown menu, you can then manually type in your desired aspect ratio on the next line down at 'Aspect Width/Height'....2:1....or whatever pano you're looking for. Then enter either your pixel width or height above and your aspect ratio will be retained.

[Mr.Bee]

Thank you GD3Design. However the result is a heavily cropped image top and bottom on a wide background. What I'm attempting is to retain the full image top to bottom while extending the background image left and right. Probably need to zoom out but II want to retain the current camera angle.

[GD3DESIGN]

If I understand what you're trying to do correctly, you need to turn on 'safe frames' and frame up your shot after setting up your aspect ratio. If you are using a 2:1, safe frame will grey out the top and bottom of your viewport...kind of like a print preview of your render. Then you can just pan/zoom to get your shot lined up to fit.