How far can my lens step down to?
This is my lens: AF-S VR Zoom-Nikkor 70-300mm f/4.5-5.6G IF-ED
It appears that it should stop down to f/4.5 but when I shoot in aperture priority it only steps down to f/5.6
Why is this?
Added (1). Why is the f/4.5 not effective at 300mm? Also, will it do this even when I shoot on manual and set it to f/4.5 and 300mm?
In many popular zoom lenses the effective aperture changes with focal length.
Your zoom will have maximum aperture of f4.5 at 70mm focal length, but when you zoom towards 300mm focal length the aperture also increases to f5.6
http://aviewfinderdarkly.com.au/2011/07/24/different-types-of-lens-and-their-uses/
For clarity, stopping down is closing the lens aperture to a smaller aperture like f/16 or f/22
What you need to be asking is what is the largest aperture I can use on my 70-300 mm lens.
That would be f/4.5 at 70 mm and f/5.6 at 300 mm.
You have an inexpensive lens (relatively) so the lens aperture changes as you zoom out to a longer focal length.
If you look at the continuous lens aperture lenses, you will see that they are much, much more expensive. Take a look
What you have is a variable aperture zoom lens (not a constant aperture). What it means is that as you change focal length (zoom), the aperture size changes too. At 70mm, the maximum aperture size is f/4.5 while at 300mm, it's f/5.6. This was done to make the lens cheaper.