Sigma zoom lens aperture trouble?

I just bought a lens which looks like this
&imgrefurl=http://www.inetgiant.co.uk/AdDetails/Nikon-D200-with-Sigma-zoom-lens-18-20mm-1-3-5-5-6-3DC/2462994&usg=__CdX3LvshxIM07PImfWyfvDct2pc=&h=400&w=400&sz=14&hl=en&start=25&zoom=1&tbnid=Lian6i8p56hATM:&tbnh=129&tbnw=129&ei=cJhlTcuzDtTH4gaDrpz8CA&prev=/images%3Fq%3Dsigma%2Bzoom%2Blens%26um%3D1%26hl%3Den%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1092%26bih%3D874%26tbs%3Disch:10%2C484&um=1&itbs=1&iact=hc&vpx=120&vpy=517&dur=215&hovh=225&hovw=225&tx=149&ty=118&oei=9pdlTZyuE8a1hAex_On6BQ&page=2&ndsp=28&ved=1t:429, r:6, s:25&biw=1092&bih=874

however, i don't understand how to change aperture! I realise the lines on the side must have something to do with it and i can see the f stop numbers on the side.

thanks for reading!

Added (1). The lens is not electronically connected to my camera

Usually the aperture is automatic and changes with the zoom. There should also be an arperture setting on your camera, detailed in the manual.

That seems to be an electronic aperture lens (like those used by Canon and some Nikon lenses) so the aperture in controlled using the camera dials.

Set the camera to "manual" mode or "aperture priority" mode and you'll be able to control the aperture.

The lines on the side are for "depth of field" reference.

edit

What do you mean the lens is not "electronically connected to the camera"? If that's a digital camera then it has to be electronically connected unless you bought the wrong lens for your camera.

What camera do you have?

\Is the lens a Sigma 18-200mm? (bit difficult to tell from image, but that's what the text said).

Some modern DX lenses don't have aperture rings (like the Nikon G series) because the aperture is controlled by the camera via the CPU contacts in the lens.

You can't mount these types of lenses onto old 35mm SLR's, because although they physically fit, there's no way to control the aperture.

That is an electronic only lens. Essentially it's similar to a G-type lens which requires electronic signals from the camera body to control the aperture. The lines on the side of the length give you an estimate to your depth of field and act as a hypefocal calulation aid.

Please tell us what camera you are trying to use this lens with?