Curious as to what the practical difference would be between 50/1.7(8) and 50/1.4. That's only about 1/3rd of a stop, not enough to make a noticeable difference in shutter speeds, and the DOF calculator for an object 5' away (or 1.5m, which would be the closest portrait distance) shows the DOF for f1.4 at .05m and for f1.7 at .06m - the difference of just .01m / 10mm or less than 1/2". So, unless you're after a particular lens for it's particular character, what makes 1.4 more desirable than 1.7/1.8 ? I would think that the impact won't be as noticeable as going from 2.8 to 1.8, or from 1.8 to 1.2. Now a disclaimer - I don't own a 1.4 lens, the only one I ever came across was MD 50/1.4 and by most user reviews it's not as sharp as 50/1.7 so I passed.