Q: Cinema HD Display 23" to 27" Imac 3,5 i7 with Mini DisplayPort auf DVI Adapter

Hi Dirk Michael,

Welcome to Apple Support Communities.

It sounds like there is an issue with the image quality on a Cinema Display connected to your iMac. Try calibrating your display as shown in the first article below, also use the second article to verify you're using a compatible video adapter.

OS X Mavericks: Calibrate your display

http://support.apple.com/kb/PH13768

Get help choosing Mac video adapters - Apple Store (U.S.)

http://store.apple.com/us/mac/mac-accessories/cables/cable-chooser#!

I hope this helps.

-Jason