Go outside and take pictures.
The zeiss 50 f2 is superior in closer photos..... the planar 50 is better when the subject is further away.... same goes for the 135 sonnar... At first I thought something was faulty on my sonnar
2.5-2.8 is where this lens really shines for portraits.. f2 is still a bit soft
I have done similar testing between the 50mm 1.8 and zeiss 50mm 1.4.. in just at home point n shoot images, they were similar.
When it came to actual real life work, I noticed a difference. The zeiss renders scenes different and has better micro detail and color. The zeiss is a more natural lens.. colors a bit cooler (it also depends on the coating, as ze is different than the mmj, mmg, hft, aeg, and aej.. all have their own unique look
The 50mm 1.8 is a very good lens, but the planar 50 is an amazing lens.
F5.6 jpeg right out of the camera (+7 sharpness, neutral everything else)
Canon 6D, Canon L 24-105, Zeiss Distagon 28mm 2.8, Planar 50mm 1.4, Planar 85mm 1.4, Sonnar 135mm 2.8 & Zeiss Mutar 2x, Canon 50mm 1.8
http://www.edwinraffphotography.com/