which lens for bird and flower photography?

[Galam]

Hi,

I am interested in photographing birds and flowers. Currently, I have the "Nikkor 28-200mm f/3.5-5.6G ED-IF" and "Nikkor 55-200mm f/4.0-5.6 AF G ED AF-S DX" lens for my Nikon D70s, but I found them to be not sharp enough.

Maybe this is due to my lack of photographic skills since I am pretty new to SLR. I shoot most of my photos on small aperture (large f-number ). This is because I read somewhere says that a large f-number will give me less depth of field which is great for isolating the subject from its surroundings. But the small aperture cause me to not able to use fast shutter speed.

What aperature do most people use for flower photography? Do I need a faster lens?

Thanks!