Quantaray 28-200MM F3.8-5.6 Lens for Canon AF "set fixed length setting brother" Questions

...set at 1/125 of a second and the f:stop is f:3.5. When I zoom out to 200mm, the shutter speed is still 1/125 of a second, but the f:stop is now f:5.6 and I have decreased exposure (darkened the ...