As you change focal length there will be two main things that change in your image, firstly the background will appear to be closer to the subject as the focal length gets bigger. Second, you will notice that the image will become stretched or squished at the extreme focal lengths, at a very high focal length your image will appear squished and at a very low focal length your image will appear stretched.
18 mm 
24 mm 
35 mm 
55 mm 
70 mm 
100 mm 
200 mm 
300 mm
Matt Chamberlain - Utah Photographer 