No question is a Goondoo question. First of all congrats on your Sigma purchase. Everybody has to start from somewhere.
The F numbers you are seeing on your lens refers to the F-stop number. The F-stop number represents the Aperture Size. Aperture is the hole where light passes through from your lens to the sensor in the camera. The bigger the number of the F-stop (e.g F/5.6 or F/8.0), the smaller the aperture size. The smaller the F-stop number (e.g F/1.8 or F/2.8), the bigger the aperture size.
When greater F-stop number is in use, both near and far objects will appear sharp and in-focus, this is also known as a deeper depth-of-field. When smaller F-stop number is used, only the subject will be sharp and typically the background will appear to be blurred. This effect is also known as Bokeh or can be said that the picture has a shallow depth of field.
What is the negative consequence of the greater F-stop number? The greater F-stop number will cause less light to reach the sensor as the Aperture size is small. The user will then have to decrease the shutter speed to allow more light to reach the sensor. Slower shutter speed will cause blurriness due to hand shake. Your Sigma's OS (optical stabilization) must do its work or your picture will be blurred.
When you set your focal length to the minimum, 18 mm, the minimum F-stop number is F/3.5 and at your maximum focal length, 200 mm, the minimum F-stop number will be F/6.3. What many people will do to get a well-exposed picture at 200 mm, is to jack up the ISO (let say to 400 or even 800) and hopefully you can get a decent shutter speed to get a blur-free picture.
A lens that have F/2.8 or F/1.8 or lower than that is often called a Fast Lens. Why? Because they can operate at a much faster shutter speed without jacking up the ISO. Jacking up the ISO in most cases is undesirable as higher ISO will give more noise.
I hope I don't confuse you too much. If you are still unclear, do come for hands on demo during the Club 1000D's 3rd outing.
CheerZ!
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