I bought a Canon 50D and then a year later I wanted a Canon 5D and when I tried to use my wide angel lens I bought for the 50D on my 5D it wouldn't fit right. The lens mounted on the new camera alright, but there was a big dark ring when I looked through the view finder. I was sickened with the thought that I will have to purchase a new wide angle lens for the camera I had just bought for landscape photography. My thought was a canon lens will mount to a canon camera not knowing that higher end cameras have different mounts. I never really given much thought to all the letter and numbers associated with a Canon EF 100 – 400 f/4.5 – 5.6 L IS USM Telephoto Zoom Lens. So let me break it down for you.
I prefer Canon Camera myself so when I buy Canon Lenses there for EOS cameras which is Electro Optical System. But the EF on the Canon Lens stands for Electro Focus. Canon Lenses have the motor built into the lens. The EF lenses are compatible with all there EOS cameras. And there is also an EF-S line of Canon Lenses and they don't fit all EOS Cameras. The EF-S lenses have a smaller image circle that protrude farther toward the camera and will not fit Canons full frame digital bodies like the 5D or 1DS. When you see 100 – 400 is the focal length of the lens and is measured in millimeters. The f/4.5 to 5.6 refers to the F stop. This is the aperture size of the lens the smaller the number the faster the lens is. And what they mean by a faster lens is the aperture is wider and takes less light for taking shots in low light situations.
The L which is just after that stands for luxury some people clam it stands for low Dispersion UD glass. I've read in several articles where these are the main two answers. But no matter what it really stands for the L signifies this is a professional lens. No matter what sporting event you go to or what event is taking place that is being photographed by a photographer making a living with his or her camera and lens. If there shooting with a canon camera their using an L series lens. This is identified by the red stripe that goes around the front end of the camera lens. The IS means the lens is equipped with an image stabilizer, and the USM stands for Ultra Sonic Motor. A telephoto lens will magnify images at a much farther distance and make them look closer than what they are. A zoom lens will bring a subject in closer as similar to a point and shoot camera with a zoom.
When somebody refers to a lens as a first party or a third party lens what this means is a first party lens is made by the camera manufacture and probably is the best fit for your camera but is usually more expensive than a third party lens. A third party lens, you guessed it, is made by a lens manufacturer that make lenses for all camera makers. These lenses are most likely compatible with your camera and will do an excellent job for you and are usually a couple hundred dollars cheaper than the first party lenses. And I myself love saving a buck, but the next lens I buy for my Canon 5D will be a first party lens.