MatthewRoberts wrote:I found that my portraits gradually get softer at each stop from f4 to f1.4 with f1.4 being the softest.
Can anybody explain why this lens is not very sharp @ f1.4 as as compared to f4?
Chris's explanation is spot on. You will find this lens to be at its best between around f/5.6 through f/11. You should play with the lens to find its sweet spot, whch is where it will help you to yield magical images.
Also, I understand that the f1.4 will allow me to keep the shutter quicker in low light, but then what other other benefits comes with the massive price difference you pay for the extra f-stop?
The benefits are many, but mainly in just a couple of areas. As you've noted, the larger f-stop means that you have the ability to shoot in lower levels of available light. You need to accept that the lens is not at its sharpest wide open, and also that you have a very limited plane where the image is in focus. But when it may provide you the difference between getting the image, or no image at all, that becomes a very significant point.
The design of the lens is such that, with a suitably wide aperture, it provides your images with suitably soft and more attractive bokeh.