คำตอบที่ 2
ส่วนผมได้ USM กับ IS มาฝากครับ
USM คือระบบมอเตอร์focus ที่ออกแบบมาให้ทำงานได้ไวและเงียบ
IS คือระบบรองรับการสั่นสะเทือน ช่วยเรื่องภาพให้ถือถ่ายได้ที่ speed ต่ำกว่าปกติสัก 2-3 stop
USM= Ultrasonic motor
One bit of Canon terminology that is troubling is USM, a technology for autofocusing that stands for "Ultrasonic Motor." USM was a term they first
started using in 1987 when they developed a new fast and quiet motor, which they dubbed a "ring ultrasonic motor" -- the motor uses tiny vibrations to generate rotation. As the term "ring" implies, the autofocusing mechanism sits in a ring around the optics of the camera, and most importantly, it
allows you to do manual focusing even when in auto focus mode. Steve Weixel put together a nicely photographed dissection of his broken ring ultrasonic motor. Here's a photo of the ring motor itself:
IS =Image Stabilizer
As already mentioned, this is the first Canon IS SLR lens, and according to Canon the IS allows you to handhold the lens at shutter speeds that are two
stops slower than otherwise possible. In accordance with the classic "1/focal length" formula a camera with a 300 mm lens, for example, must have a shutter speed of at least 1/300 sec, but if you use a 75-300 IS at 300 mm
Canon claims that the camera can have a shutter speed of about 1/75 sec. In my opinion it is difficult to say precisely how well the IS works in all situations, but Canon's claim seems to be fairly realistic. However, IS is no replacement for proper technique, so if you shake the camera the
photograph can be blurred even if it was taken with IS, and a photograph that is taken at a slow shutter speed without IS can be fairly sharp if you
hold the camera still enough. The IS of the 75-300 neutralizes both vertical and horizontal camera shake, so unlike some more expensive IS lenses it has no special IS mode that allows you to pan with the IS turned on. Also, unlike the IS of some "L" series telephoto lenses it does not work properly
if the camera is mounted on a tripod, but it works fine with a monopod, and then the shutter speed can be at least 2-3 times slower than would otherwise be required without IS.
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