[Brightcolours]

I'm not sure the difference in DOF is going to be of major significance. I was perusing the Sigma 85/1.4 test on TDP once and there's an excellent rollover comparison tool. I couldn't honestly call the difference between f/1.2 and f/1.8 shots noticeable, let alone between f/1.2 and f/1.4 shots (before you ask, that comparison also included a series of samples from Canon 85/1.2L II). I always felt that f/1.2 mattered more on film where you couldn't just ramp up the ISO, and now Canon has such lenses mostly for bragging rights (like the 50mm f/1.0 back in the day).

http://www.the-digital-picture.com/Compa...rture.aspx

The Canon 85mm f1.2 L USM has (very) noticable DOF difference to the Sigma 85mm f1.4, even in above comparison. The Sigma 85mm f1.4 gives more or less comparable DOF results at f1.4 to the Canon 85mm f1.8 USM at f1.8.

 

Scroll down to the up close wall corner sample here, to see the difference with f1.2 to f1.8 on the 85mm f1.2, and the difference between the 85mm f1.2 at f1.2 to the Sigma 85mm f1.4 at f1.4:

http://cannonfastreviews.com/canon-85mm-...m-f1-4-ex/

 

Whether the difference is important to you as photographer, that is personal. But the difference is clear and there....

For more light, one should not bother. As most of the extra light gets lost at micro lens level anyway. Same is true for f1.4 vs f1.8, by the way.