Poor vision and iMac resolution HELP

Hi

I've been considering a iMac 27" mainly for image editing with Photoshop. However, I will not be able to use it a the native resolution as the test/tools in PS will be too small for me. (I have some complicated vision problems that would make the native resolution far too difficult to work with for a long period). So, how is the iMAC screen at lower resolutions and will setting it as such affect my checking focus accuracy of images?

Any other advice you could give on choosing/using an iMac screen would be appreciated.