imac 27 second monitor caueses resolution problem???

i have imac 27" , mini display port/ vga adaptor. have been running 2nd 22" monitor dual screen setup. never had problem

bought 46" 1080p tv pc input (vga) yesterday. decided hook imac send movies , not. when plug in imac resolution drops , cant seem go native 2560x1440 resolution? wants stay 1600x1200 or something. when plugged tv. stand alone or second monitor , no problems

normal?

 

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