Many people believe that if you have a zoom lens the best way to focus is to zoom into the subject, focus manually and then zoom back out again out to get your composition. The purpose is to magnify on where they want to achieve sharp focus. There was a time, perhaps over 30 years ago, when this was a good idea. Back then zoom lenses did not "migrate" so when you zoomed in and focused manually and then zoomed back out again, the focus did not move. Unfortunately, today this is no longer the case.
Let's say you are photographing a person about 9 feet away. If you zoom into their eyes and focus manually and then zoom back out to your composition, their eyes will no longer be in focus. The glass elements have moved. To prevent migration, zoom lenses have to be much larger and are much more expensive to manufacture. With advancements in auto focus however, manufacturers decided to reduce this cost by taking away this capability. The general consensus is that the auto-focus system would make up for this problem very quickly which is fundamentally true.
To demonstrate the point take a look at the images below. In the first column we did the following: 1. zoomed in on the shutter button 2. Autofocused on the shutter button 3. Turned to Manual focus 4. Zoomed back out 5. Took the picture. In the second column we simply put the focus point* on the shutter button and let the camera auto focus and took the picture. You can see clearly in the magnified image of the shutter button that the first method left the image out of focus while the second one is sharp.
The bottom line is to focus at the focal length (or zoom factor) that you want your final composition to be at whether you are using manual or auto focus.
* Focus Point - in many cameras (usually prosumer and above) there is an ability to focus very precisely on a small portion of the image. These portions are referred to ask "focus points". These points are highlighted within the view finder and the photographer has the ability to move them to where they want the camera to precisely focus.
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