A telephoto lens with a fixed aperture of f8 takes quite a bit of light.  Unfortunately we saw a family of cheetahs early in the morning on a cloudy day and my pictures were underexposed.  Unlike lions and leopards, cheetahs do most of their hunting during the day, and they specialize on the diminutive Thompson's gazelle.  Speed and agility are a must for running down and tripping a Tommy, and a mother with cubs must hunt successfully virtually every day to provide enough food for herself and her growing offspring.  Not only does she risk having lions steal her meal or kill her cubs, but vans full of camera-toting tourists undoubtedly foil many of her hunts.  It must be a tough life.  Cheetahs don't put on body fat, so they always look lean and hungry.  They are imperiled due to habitat losses and a lack of genetic diversity.  Scientists estimate that they came close to extinction about 10,000 years ago.