It is because you are using a zoom lens. It will go farther towards zero when you are at 75mm....the minimum f-stop at 300mm or somewhere above 75mm is f5 so the lens is telling your camera that f5 is as far as it should go to get a usable photo at that zoom length. If you put it at say 150mm, your f-stop should be able to go lower than at 300mm but not as low as if it were at 75mm....Hope this makes sense, tried to keep it in a nutshell.
@isabel32 As Shades of grey explained it: ) your 75mm-300mm will have a F stop of lets say f 3.5 - f 5 meaning minium F stop at each end of the range is F 3.5 @75mm or F5 @300, and something in between as you zoom in or out.
You can get Pro glass that has a high F stops say a F2.8 70mm-200mm this will have an effective F stop of 2.8 @ 70mm or 200mm.
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You can get Pro glass that has a high F stops say a F2.8 70mm-200mm this will have an effective F stop of 2.8 @ 70mm or 200mm.
:O