For your Challenge in All About the Glass you took two photos at different focal lengths. Let's compare those photos and explore the effects of focal length.
You just need a big telephoto lens - like 300mm or more - if you are shooting things far away, right? Yes, but it has other uses too.
First, what is focal length anyway? It is not the physical length of the lens. DPReview defines it as "The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is 'in focus'." http://www.dpreview.com/glossary/optical/focal-length
How should we categorise focal lengths?
Any lens wider than about 18mm is Super-wide angle. And there are some specialist lenses in this category like fisheyes.
18mm to 35mm is Wide.
35mm to 85mm is Normal.
85mm to 300mm is Telephoto.
Greater than 300mm is Super-tele.
These numbers are fuzzy, and everyone that tries to define the categories has different boundaries. Some may say that 100mm is normal and Super-tele only starts at 500mm.
One of the key characteristics of focal length is the field of view, or the width of the scene captured. With a Wide lens, you are going to quite an expanse of the vista included. With a Super-tele on the other hand, you are going to be focused in on just a narrow sliver of the scene. Normal lenses give us a view similar to human eyes.
For the set of images above, I changed the focal length from 18mm all the way up to 300mm. 18mm gives a very broad view of the entire Weymouth beachfront, but not much detail and too much sea and sky. At 300mm we can make out the people on the beach quite clearly. For the record, the focal lengths in the set are 18mm, 35mm, 70mm, 100mm, 200mm and 300mm. The church steeple was about 1.2km away from me.
Of course someone else has done a much better job of this than me, so head over to this link and try the simulator from 8mm all the way up to a massive 800mm. http://www.sigma-imaging-uk.com/focal-length-simulator/
The other thing you would have noticed is that Wide lenses tend to make objects behind the subject look smaller, while a Telephoto lens makes objects look much closer than they are - and they compress the space between near and far objects. You would have noticed this effect when watching sport on TV. Obviously they use Super-tele lenses to get close to the action, and you'll see two players seemingly close to each other, when in reality they may be ten or thirty metres apart!
That's all good and theoretical, but how does it help you? One simple rule: if you want to make someone look ugly, take their photo with a Wide or Super-wide lens, as close to their face as possible. Wide lenses accentuate things closer to the lens. So noses and eyebrows become weird and cartoon-like.
A better option when photographing people you like is to you a Telephoto lens. 85mm. 100mm. Even 200mm makes for delightfully flattering portraits. Take a look at this link where the author of the site has taken a series of portraits of a model at different focal lengths for us to compare. stepheneastwood.com/tutorials/lensdistortion/index.htm
Take a photo of your subject at the longest focal length you can, at the widest Aperture you can, and at the closest focus distance your lens will allow.
Every lens has a minimum focus distance - the distance where it just won't focus on anything closer. It could be anything from centimetres to metres, depending on the lens. You have to experiment to find out. And this is a very useful piece of knowledge to have about your lenses.
One method of checking the minimum focus distance is to switch off autofocus (on the side of the lens), then manually focus all the way in one direction. Is that close or far? Manually focus all the way in the opposite direction? Close or far? Once you have the closest thing in focus, rock your body backwards and forwards to get it perfectly in focus. Don't forget to turn autofocus back on when you're done.