Unfortunately, there is no getting away from the fact that the quality of the lens you are using has a significant impact on its ability to resolve detail. This can mean a higher price tag, but there is no doubt that it is worth investing in the best quality glass you can afford. This is particularly important if using one of the top-end DSLR sensors where resolution is sufficiently high to render the tiniest of detail. Unfortunately, this also means they can also capture the smallest of movements in the equipment, so if you have recently upgraded your camera you may need to get accustomed to shooting with a tripod or at a faster shutter speed than previously to eradicate any blur.
Most photographers are aware that use of a teleconverter will impair sharpness, but may be surprised by the difference a filter can make. Again, it is worth investing in a good one to minimise any negative effect. Whatever lens or filter you are using, don’t forget to keep them clean! A layer of dust, smears or fingerprints on the outer optics will not be conducive to optimum image clarity!