If you visit the Nikon website you can find if your lenses are compatible with the digital cameras. Most will be.
The change in focal length is a sacrifice on the wide angle end, but a plus on the longest focal length setting.
If you have a 18-200mm focal length lens the digital Nikon will see it as being 27-300mm focal length.
You use a multiplier of 1.5
The aperture specifications on a lens:
If you use no zoom at all....the lens will perform as a f2.8 lens.
If you zoom all the way out the lens will perform as a f4.5 lens.
When you zoom you lose light.
A zoom in-between the two extremes will result in an aperture setting somewhere in-between. But it changes quickly once you apply any zoom.
You can find some lenses that are f2.8 all the way from wide angle to full zoom, but they are very expensive.
To find if the lens can stop down to F16, f32 or whatever, you have to get the specifications for the lens. The Nikon site would be one place to check out the specifications for a lens.
I am wanting to change from 35mm to digital and will stay with Nikon since that's what I have used since '69. I understand that some of my AI lens will work on the D200 body but there is a sacrifice in focal length. I figure the main lens to start would probably be around an 18-200 f2.8 if it's available but I am not clear as to the f stops on these digital lens. If a lens indicates f-2.8-4.5 where did the stops up to and including f32 go? Do I not need them with a digital camera?
Thanks for 101 in digital photography.