what is a telephoto lens?

A telephoto lens is a lens that has a focal length longer than that of a normal lens, or a lens that approximates the optical qualities of the human eye. A normal lens has a focal length of between 40mm and 58mm on a full frame camera so any lens with a focal length longer than 60mm can be considered a telephoto lens. The longer the focal length, the more magnification there is.
The most popular telephoto zoom lenses have a focal range of 70mm to 200mm. Any lens with a focal length longer than about 300mm can be considered a super telephoto.

The main uses for a telephoto lens

The main effect of telephoto lenses is that, like a telescope, they magnify distant objects. Distant football players, small birds sitting in trees, and other similar subjects are all much easier to capture with a telephoto lens.

It’s very easy to get a shallow depth of field with a telephoto lens; it’s one of the reasons they are so popular with portrait photographers. The 70mm-105mm range is very very popular for them.

pros of the telephoto lens

Telephoto lenses are great for getting close to your subject without having to be physically close. You can’t walk into the middle of a football game or wander right up to a shy bird to take a photo so instead, with a telephoto, you can sit back and shoot from a distance.

cons of the telephoto lens

While telephotos do everything they’re meant to do wonderfully, they do have a few downsides. They tend to be big and heavy. It’s just a side-effect of having a long focal length.

Telephoto lenses also need faster shutter speeds to get sharp exposures. Even with a tripod, if you use a shutter speed lower than about 1/200th of a second you risk having camera shake in your images, so night photography with a tele lens is almost impossible.