Canon Lenses Usm Is
What are the differences between these two Canon lenses?

Canon EF 300mm 75-lll and Canon EF 300mm USM III-75 Which is better for portraits, sports, and wildlife? Why do we need these lenses to a Canon Rebel XTi What does USM do? Are these better lens with USM?

The EF 75-300mm f/4-5.6 IS USM III and III are CHEAP entry level Canon zoom lenses. The quality and optical performance is mediocre not considered great by any stretch of the imagination. But they have their place in the Canon lens line up. As some have said before with the USM III to get an ultrasonic motor that will help to focus faster (marginally). It is a great way to get 300mm zoom especially when used to the flat light (daylight when you have enough light around.) You can make pretty decent sports and portrait photography, as long as there is not much light (enough to go 1 / 500 or faster speeds shutter). But once the light begins to fail f/4-f/5.6 opening begins to hit home. You can get very good performance of the lens, if you mount the camera on a tripod (will be mounted by the body not the lens … so there is no influence of target). If you have a limited budget $ 200 and you have to have a xx-300mm zoom then its a good buy. But beyond it and not expect to do wonders in low light. There is a reason why the 70-300mm IS USM costs $ 550 and why the cost of 70-200mm f/4L about the same too. The package definitely f/4L superior "L" optical quality and see the difference, plus a constant opening lens. While 70-300mm IS USM has an image stabilization 3-stop which is very useful and has much superior optics compared with 70-300mm III. With objectives are more or less get what you pay for.