Minimum focus distance
How close can you get?
Lens design is a complicated balance between many elements. The minimum distance at which you can focus is one of them.
Why does it matter?
Sometimes people equate the focal length with the ability of the lens to magnify. They say a 300mm lens shows you things much larger. Which is sort of true. The problem is that what the focal length does is to reduce the field of view fitting a narrower part of your vision into your frame. So if somewhere 5 meters away there is someone reading a newspaper, using a 300mm lens you may be able to capture text on the newspaper better than with your naked eye. But the same lens will not be able to help you to magnify everything. It may not be able to focus to something that is only 1m away.
The distance at which the lens works is one of the parameters that lens manufacturers optimize. They realize that most of the time, people will want to picture other people and given the field of view they can make a good guess at how close people will want to get to their subjects. This allows them to make the lenses, simpler, cheaper and lighter.
For most normal lenses, as a rule of thumb the minimum focus distance is about 10x the focal length. Of course this is a gross generalization and does not always hold. The tricky part are zoom lenses that cover a great range. The old Nikon 28-200 D for example can only focus to about 2m, where as the newer one (G) focuses to less than half a meter.
These pages are for Amateur Photographers and not really for seasoned photographers and professionals. I have no affiliation or commercial interest with any brand/make. I write from my own experience. I ended up using mainly Nikon, so I am more familiar with this brand than others. See price for notes on pricing as well as photography related links.