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Old Aug 24, 2006, 10:08 PM   #1
Join Date: Aug 2006
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I'm had some time to play with the various lenses I have for my DL. I've noticed that auto-focus with the kit lens seems faster than with my Sigma 24-135mm or with an older 80-200mm that I tried.

Has anyone noticed poorer AF performance when using non-DA lenses as well, or is it just my imagination. Also what is the best way to clean the contacts on the lenses/camera body. I'm sure mine are clean, but it might be good to know just in case.
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Old Aug 25, 2006, 2:25 AM   #2
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Hi intence.

Different lenses just focus at different speeds. Some factors are:

1. Max aperture. Faster lenses allow more light to pass to the focus sensors so they generally work better.

2. Contrast and sharpness. Some lenses are more contrasty, some are sharper. This might be due to the coatings, the glass composition, or other optical factors that I'm not aware of (I'm no optics expert, I just know what works for me). AF uses maximum contrast in the area of the focus sensor to determine focus.

3. Mechanical factors. Smoothness and ease of rotation of the focusing mechanism and the amount of rotation it takes to go from minimum to maximum focusing distances, and the weight of the lens elements that are moved to focus. The more effort and/or distance of travel, the slower focusing will be.

4. Scene brightness. AF will always be slower as the amount of light drops. Less light means less contrast, and as I stated before, contrast is what the AF system uses to determine focus.

5. Ease of hand-holding -- If the camera is constanly moving a lot, the focus sensor isn't able to lock on to something to determine focus. Longer teles, which are not only heavier, but also essentially magnify the image, are harder to hold steady, so there are situations where this hampers AF.

6. JFM (Just Freakin Magic) Some lenses just focus faster for no apparent reason. This factor explains a wide range of phenomenon for me in life.

Many users have found that CRV3 and Lithium batteries, which produce more voltage more consistantly throughout their lives than NIMH rechargeables boost AF speeds across the board. Being ultimately cheap, I use NIMHs and am rarely bothered by AF speed, though faster would be nice if it didn't cost more on a shot to shot basis.

I wouldn't sweat the differences in focusing speed -- just make a note about the characteristics of the particular lens and make whatever adjustments that you need to so that lens' particular characteristics don't hamper your shooting style.

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