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Old Jan 3, 2005, 7:05 AM   #21
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Alady, the USM drive in either the Canon 100-300 or 75-300 will do very little to alter the image quality of a photo. All the USM does is allow the camera to focus an image faster giving you a better chance of capturing rapidly changing scenes. For slow or stationary subjects it should make little or no difference to your current non-USM lens.

However, the 100-300 and 75-300 might offer better optical quality such as sharperimages, less vignetting/distortionsand better contrast. But this is seperate to USM, the USM will only help with how quickly your camera can focus on the subject.
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Old Jan 4, 2005, 11:57 AM   #22
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I did not see mentioned the 200 IS 2.8. If you can afford it with a 1.4 tele converter you have a pretty versatle setup. The 2.8 lense is a very quick lense for focus and will allow faster shutter speeds to overcome some of the mistakes us newbys make.

With the 1.4 converter, you get nearly 300mm and if not mistaken, still faster than the F4 lenses.

On the downside, it is a more costly setup, but; IMO perhaps better in the longer run and more versatle.

Good luck with your descision.

PS: looking at the other pics you others folks linked is very humbling. Those shots are really great.
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Old Jan 5, 2005, 7:43 AM   #23
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Golfer brings up another good point :idea:

1 - The f/2.8 helps to speed up the focusing because the lens is brighter (regardless of ultrasonic or not)!
2 - This is especially true in darker environment or on overcast days
3 - F/2.8 also provides more out-of-focus information for the camera to work with especially on a 20D or above, ie @ 300mm the variable aperture zoom is @ f/5.6 (2 stop down)!

"With the AF system, a faster lens (wider diameter) gives the equivalent of the longer base length, since the left and right edges are farther apart. That's why, all other things being equal, a faster lens gives better AF performance -- the phase differences in the waves are more dramatic and easier for the CPU to detect."
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