Hi Kitschenalia,
I had a 'softness' issue a while back. I blamed everything from the lens I was using to the software I was using....
For all it's cheapness, the nifty is optically speaking, superb. It is a bit soft at f1.8 though, as Dean says, try to use it at f2.2 upwards. Outside you should still get pleasing backgrounds from f5.6 provided you're careful and leave a good distance between your subject and any backdrop.
It's good you're using full manual - I'm a firm believer in this being the best way to learn. As for technique, all I can tell you is what I do - some will tell you that this is 'wrong' or whatever but it works fine for me now, but everyone is different and you will find your own way too.
I only use the centre focus point when shooting portraits. For kids I find it nigh on impossible to keep track of them and change the focus points fast enough. I tried this and just missed the shot all the time. Instead I find it much more successful to track my subject with the centre point and reframe. At the right aperture (f5.6 - f8 ish) any movement is negated by the deeper DOF. I always shoot as fast as possible - usually at least 1/125. If there isn't enough light then I have to find ways of improving the light in order to achieve a good exposure. I do this with either my flash equipment or a reflector, or by moving to a place where there is more light. There is no point trying to use a slower shutterspeed or a wider aperture than optimum as the shot will be, on the whole, useless and out of focus if I do. As my 50D seems to be rubbish at high ISO I tend not to stray too far from ISO100. I may go up to ISO400 at a pinch.
As mentioned, some will tell me that this is all wrong (well it might well be :shrug
but it now works for me and I'm quite happy with my shots taken like this. I'm now taking good sharp shots using this technique with my old manual lenses too so it can't be that wrong!
Looking at your shot above, I would suspect its a software/PP issue. I now only shoot in RAW, and I use a combination of Lightroom and CS4 to carry out any PP. I sharpen in CS4 only as I don't seem to get on with Lightroom sharpening. I use the highpass method, Dean uses Lab sharpening - we all have our favourite method. (I'll be trying the Lab mode method next though as Dean swears by it
) If I were you, I would try something along these lines yourself. If you haven't got CS4 then there is a free program called Gimp which is similar. (It doesn't have high pass filters built in but you can download a plugin to do it)
I would google/check the tutorials on TP etc on these methods and give them a try. I would also look at giving your shots a curves contrast tweak which is really easy to do and will improve the look of them no end. (again, google is your friend).
If you take a look at my flickr - the last few shots of my little one (who can move across a room faster than light....) these were all taken with a manual lens in the same way as described above (I used flash here to improve the light) and had minimal PP done to them.
