The only benefit of the DO is it's weight. The lens is very light. It is super sharp, rivaling the 4002.8 but the bokeh is funky. Out of focus specular highlights have rings in them. If you are careful it isn't noticeable. If I need light weight I use the DO. The 2.8 is a super sharp lens, the only one that is better is the 200.
400DO
200f2 - The BEST beauty lens
I do have a 50f1 (and a 50f1,2 and a 50f0.95 for Canon rangefinder cameras). This lens, although super fast, is not very sharp until you hit f2.0-f2.8. The softness is very interesting though as it is super sharp in the center but sharpness falls off as you move off center. Not bad as a portrait lens but way too short in focal length.
I love the 135 soft focus for portraits. It has a ring that allows you to dial in the softness from totally sharp to quite soft in three increments. It produces softness by inducing spherical abberation. The result is a sharp image with a soft image on top of it. It is unique effect most similar to using Zeiss Softar filters. Very different form diffusion filters.
135 Soft Focus
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78 Z1R, '08 ZX10R, 2010 BC Roadster S/T, 6M, Stillen G2 CAI, Invidia CBE, Berk test pipes, Painted headlights (180˚Custom), JDM rear fog, B&M short shifter, Stillen front plate mount, stubby antenna, setrab oil cooler, uprev. 337rwhp 254ft/lb corrected!