The pic looks alright to me, perhaps could use a bit of USM...and maybe fill in flash to bring out the details in the cat's face. About the softness part...depends on whether u r talking abt just the cat's face or the whole picture in general. AFAIK, lack of DOF does not cause softness, the subject (in this example the cat's face) can be sharp but the rest of the picture thrown out of focus due to the wide aperture used. Softness is usually the result of handshake when handholding the lens at low shutter speeds, or when the subject is not in focus.
The term 'fstop' refers to the aperture value being used. For SLR cameras, the aperture blades are usually found on the lens, and you can control the value of the aperture (i.e control how big or how small the opening in the lens which allows light in) using the camera body (in aperture priority or Manual mode). Aperture and shutter are two different things altogether. Shutter blades are found in the camera body in front of the film plane or CCD sensor and activated only when you depress the shutter button to allow the image to be recorded on the film plane/CCD sensor. This is only applicable to SLR type of cameras. For consumer digital cameras w/ fixed lenses, I believe the aperture and shutter mechanism is combined together.
Hope this helps...do correct me if I am wrong.
And thus I had to exploit the poor little cat that wanders outside my house.
