Why does my aperture's ability to go lower change?

I'm a new owner of the Nikon D90 (first DSLR as well) and though I have many questions this is one I can't figure out. Some days I'll be able to set my apertures settings as low as F3.5 and others it only goes to F5. Why is this?

You have a consumer grade lens. The aperture isn't controlled by the camera, it. Controlled by the lens and consumer grade lenses usually have variable max apertures that are based on the focal lenght. So your lens is probably an 18-55 f3.5-5.6 (or something along those lines).

The 3.5-5.6 is the variable aperture. So at 18mm you'll be able to get 3.5 but the more you zoom in the smaller that max aperture becomes up to 5.6 around the 55mm range.

The btter lenses have a good max aperture through the entire focal length. Example 24-70 f2.8, 70-200 f4, 70-200 f2.8, 50mm f1.4…

It's all about 'focal length' (i.e: 'zoom') if you have an 18 - 55mm (focal length) lens with a minimum aperture variable of f/ 3.5 - 5 you will be able to use an aperture as wide as 3.5 with a focal length of 18mm (zoomed out) and an aperture no wider than 5 with a focal length of 55mm (zoomed in).

What I'm saying is the focal length constrains the aperture you can use. Don't ask me why though, I have no clue.

Actually this isn't just restricted to consumer grade lenses… Some very good macro primes alter aperture when focusing from infinity to close-up. Nikkor 105mm f/2.8 and Sigma 105mm /70mm/50mm all do this.

My guess if you understood how an apertures f/stop is derived, you would better understand this.

The f/number is a calculation of the focal length of the lens divided by the apertures diameter.

Sample: A 50 mm lens with a 25 mm aperture is called an f/2 lens, 50/25=2

So if the aperture size is the same at 18 mm as it is at 55 mm, the f/stops at those different focal lengths will change as the focal length gets longer. This is basic physics.

Actually if you would just take a class or two in photography, these question will be answered while in class.