The lens is wide open..f/5.6 is as fast as it gets at 300mm. This image is noticeable less focused and contrasty than at f/8.0. There is also noticeable light falloff...the picture is darker than f8.0.
Nikon D50
,Sigma 70-300mm f/4-5.6 APO Super Macro 1/1500s f/5.6 at 300.0mm
full exif