Hi Richard - Welcome to PSG!
To elaborate on what Sam said, the colloquial use of the phrase, "Zoom in" almost always means that in Photoshop (or any other image editor), you first crop away unwanted material, and then you magnify the desired area. Photoshop calls only the second step "zooming in", not the overall process.
Unfortunately, when one crops away the unwanted areas of the image, you are left with fewer pixels, ie, less information, so that when you "blow them up" / "magnify" / etc. them to make them the same size as the original, they will always look more blurry than one would hope for.
If you start out with many more pixels than you need, and have a very sharp image filling those pixels (because you didn't shake when you took the picture, the subject didn't move, the camera was focused perfectly, the quality of the optics in the camera is high, etc. etc.), one can often get away with this process of cropping and then "zooming in". But if one of these factors isn't there, you're going to wind up with a soft image. Maybe it's acceptable, maybe not.
I hope this explains the process and its pitfalls a bit.
Tom M
PS - Unfortunately, all the TV crime & forensics shows have given the public the impression that there is magic software out there that won't blur an image when one "zooms in". That's nothing but pure science fiction -- it doesn't exist, but lots of people think that it does, LOL.
