Hi David,
Most of the lenses that are for sale these days are pretty good. Even the low-end consumer lenses that most people put on entry-level cameras should be able to do a decent job if used properly. Before getting too concerned about your lenses, there are a few things you should consider:
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Do you half depress the shutter release before taking the picture? It helps to pre-focus the shot before taking it.
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Is your camera set up to release the shutter only if the subject is in focus? Your camera likely has such a setting. Check it.
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Is the camera's focus point directly on the subject? Most cameras today offer several focusing modes, some of which select the focus point automatically. I don't recommend those. If your camera should pick a subject different than what you intend, your subject may be blurry.
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Are you using enough shutter speed? If your subject is moving, or you are using long telephoto, the use of a slower shutter speed will likely leave you with a blurry photo. These days, many people use the camera's automatic exposure modes, and these can select settings that may not serve you well. I recommend shooting manual exposure. It is not difficult to learn to do that.
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Are you shooting an optimal aperture? As others have noted, most lenses have a "sweet spot". I recommend f/8. As long as the light is not bad, that should serve you pretty well. Using too wide an aperture could limit the depth of field in your pictures. Using a really tight aperture such as f/16 could limit your options on shutter speed....and if there's dirt on your camera's sensor, you will see it in your images big-time at f/16.
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Are you holding the camera properly and using image stablization properly? If the camera is not held steady, it won't matter if you have everything else perfect. If you are on a tripod, turn off image stablization, as it will create unwanted motion.
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Are you sharpening your photos properly in postprocessing? Postprocessing is more than half of getting good pictures. If you don't properly sharpen the image in post, even a perfectly good image can look soft.
Needless to say, there are a lot of things that can affect picture sharpness.
If after doing some tests under controlled conditions, you are still convinced that your pictures are not sharp enough, it may be that either your camera body or lens is malfunctioning. Autofocus problems happen. I had such an issue on my Nikon D7000 a year and a half ago, and I got a lot of soft pictures on an expensive charter. Talk about maddening!!! I ended up sending both the camera and the lens back to Nikon for service. Fortunately, the lens was still under warranty, but the camera body was not. The camera repair cost me about $250....but it fixed the problem. It was cheaper than getting a new camera or lens.
Hope that helps!
/Kevin
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