I am about to buy a camera. Can somebody enlighten me about how much faster is a USM lens in focussing time compared to a similar Non-USM lens. There is a substantial difference in price. Is there some site where focussing times may be published.. Thanks

-- Ashok Grover (groverak@home.net), March 02, 2001

Answers

Focus time is such a variable item that it isn't important to know what the times are exactly. Contrast of subject, how much light is hitting the subject, speed of lens and overall capabilities of the body all contribute to focusing speed. IF you're buying a new camera, you should have the best focus times. All the new bodies focus pretty quickly.

The difference between USM and non USM is more than speed. USM is more reliable (less hunting), plus adds a full time manual ring. That allows you to focus with the camera, but touch up the focus if you wish with that ring. That being said, I use 2 non USM lenses (24mm f/2.8 and 50mm f/1.8) for my wedding photography and have no trouble. But they are fast lenses that allow quite a bit of light in the viewfinder which helps focusing speed.

IMO, you should go to a store and hold an Elan 7 (Canon's newest body) with both a USM and a non USM lens and see what they can do. Then hold a Nikon N65 or N80 with lenses that are comparable in price and range and see what happens. You should see a huge difference.

-- Colin Miller (ckmiller@pond.net), March 02, 2001.

Not all USM lenses allow full-time manual focus. Some use a geared mechanism which can be damaged if the lens if focused manually while in AF mode.

-- Damon Taibi (phlegmat@hotmail.com), March 03, 2001.

If the lens sports a ring-type USM that drives an internal or rear lens group it will really rip. For example, the EF28-105 3.5-4.5 USM, EF28-135 3.5-5.6 USM and EF100-300 4.5-5.6 USM are extremely fast at focusing.

The cheaper Canon lenses offered with either a Micro-USM or a Micro Motor are both very slow (the typical kit lenses). The Micro-USM is no match for a ring-type USM drive. The only advantage of the Micro-USM is its pianissimo dynamics. Unfortunately, the Mirco-USM lacks FT-M (full time manual focus override) and is slow due the use of a drive train to move large front lens elements during focusing. Examples of the these Mirco-USM and Micro Motor lenses are the EF80-200 4.5-5.6, EF28-80 3.5-5.6 and the EF75-300 4.5-5.6.

-- puppyface (doggieface@aol.com), March 04, 2001.

You've got some reasonable advice above. What you will find is that the difference in speed comes more from the overall design (how much heavy glass is being moved), the subject being shot (is ther contrast), and the body / lens combination (max aperture vs AF sensor sensitivity)

As examples: - my 85F1.2L is the slowwwwwest lens I've ever owned (great big piece of glass to move) one any body - the EOS3 makes quick work compared to the A2e, ElanIIe, or the D30 with any given lens

So - DO try them out... and have fun

Mike www.arttech.on.ca

-- Mike Milton (mike@arttech.on.ca), March 04, 2001.