Aperture Clarification

[Lewtron_Stellae]

Hi All, 

I wonder if someone can please help, I'm fairly new to astrophotography and confused about aperture. Basically I'm looking at a Canon EF-S 55-250mm lens. It has the specs
f4-f5.6 does this mean that the lowest point of aperture possible is F5.6 or will it go lower. What I find confusing is that I have a lens that's the EF-S 18-55mm lens which has the specs of f/3.5/5.6 however My camera does allow me to set the aperture at f/8 The reason I ask cause it is advised to use a low aperture of f/8 to capture images of the moon.

Any help and guidance would be greatly appreciated!

[fireshipjohn]

It's a reference to the widest aperture available over the range of the zoom, typically more at the lower end of the zoom range, so in your example at 55mm it would be f4 and at 250mm it would be f5.6. 

All the smaller apertures are available at all zoom lengths, so f8 is available across the range of the zoom.

[Lewtron_Stellae]

Thank you so much, I find this really helpful.