Well, as I'm sure all of you A850/A900 owners know, there's an A700 inside your FF body AND contrary to reports I've read, I can use all DC Sigma lenses on my A850 as well as others such as Tamron 17-50 F/2.8.
Now as you'll know, setting the format to APS-C means you can take images of 11MP using the viewfinders inner rectangle for the cropped lens in use, thus (when a WA lens is being used) the vignette you see in the viewfinder does not appear on the final image as it's in the area between the APS-C rectangle and the edge of the viewfinder used for FF lenses as shown below for the standard G focus screen:
So, how do you get your FF lenses to get this 1.5x boost? Simple . . turn on the APS-C option for cropped lenses when using a FF lens. Take a test shot in normal and APS-C modes, and you'll see the 1.5x crop factor applied to the image (albeit @ 11MP) but I'm sure it's the cheapest teleconverter you'll ever have that allows the lens to perform at it's maximum speed and without any loss of IQ.
How good is that? I have to 'assume' that this is something that has been known about since the introduction of the A850/A900, but having played around with the settings, it was an unexpected 'discovery'. What your images will look like if you already have a 1.4x or 2x TC already fitted I can only imagine.
Anyway, my coat is on the hook, so I'll be off now.