I have a nikon d50 and I am a little new to photography. My question is. When using any of my lenses, Nikkor 18-55, Nikkor 28-80, or Tamron 28-105 I set my f stop to the highest setting on the lens then adjust through camera. When I am at the lowest zoom setting the my f stop will be at the lowest if that is what I set it at, but when I move to the highest zoom then the camera will not let me use the lowest f stop setting. I am looking for a very blurred back ground on my portraits. ex. My 28-105 lens has f stop from 4 to 22. When zoom is set to 28 the f stop can be set to 4. But when zoomed to 105 the f stop will only go to 5.8. Can someone explain this and can it be changed in camera? If so how because I can't find it. Or at least explain why the camera and lenses do this. Thank you.