The Canon EF 300mm f/4 L USM Lens is regarded as one of the sharpest lenses in the Canon L Series.
The 300 f/4 L is relatively light, well built and has excellent bokeh (background blur quality). I like the convenience of having the lens hood built into the front of the lens (sliding in and locking out).
One of the features the Canon EF 300mm f/4 L USM Lens is most noted for is sharpness. This lens is sharp wide open and very sharp when stopped down slightly.
The 300 f/4 L is a partially weather sealed lens. A lens mount gasket is not present, but the switches and focusing ring have moderate dust and moisture resistance. A front filter should be used for sealing purposes.
Separating the subject from the background is easy with this lens. Put a little distance between the 300 f/4 L's subject and the background, dial in a wide aperture setting and watch the subject emerge from beautiful, smoothly blurred background.
The Canon EF 300mm f/4 L USM Lens is a good lens for nature and wildlife photography. When used on a 1.6x FOVCF body, the 300 f/4 L is entering the long focal length lens zone - Roughly the equivalent of 10x binoculars. When paired with the Canon Extender EF 1.4x on a 1.6x body, this lens can even venture into the bird photography arena.
After all of the positive comments, you ask why I no longer own this lens? It takes a very bright day (or high ISO setting) to get a shutter speed high enough to handhold this lens on a 1.6x field of view crop factor DSLR. For this reason, the Canon EF 300mm f/4 L USM Lens did not have enough use to me to justify keeping it. I am much happier with my Canon EF 300mm f/4.0 L IS USM Lens.
There has been much talk about the Canon EF 300mm f/4 L USM Lens being far superior to the Canon EF 300mm f/4.0 L IS USM Lens. I disagree. I highly recommend the Canon EF 300mm f/4.0 L IS USM Lens over this lens - IS (Image Stabilization) is well worth any small image quality difference that exists in my opinion.
Another Canon L Series Lens that often competes with the Canon EF 300mm f/4.0 L IS USM Lens is the Canon EF 100-400mm f/4.5-5.6 L IS USM Lens. Be sure to read my review before you make your decision to buy this lens.
Also note, the Canon EF 300mm f/4 L USM Lens has been discontinued by Canon. It occasionally shows up on the used market, but finding a good copy will take some work.