The Sigma 18-35mm f/1.8 DC HSM Lens retails for  $944, Metabones T Speed Booster Ultra 0.71x Adapter for $1390 and the Hoya 72mm Circular Polarising Filter $136.00, a total of $2470.

This lens combination with speedbooster and circular polarising filter has only been used a handful of times and is in brand new absolutely mint condition and will sell for a bargain at $1900 with the original packing boxes.

This is an awesome lens and is well respected amongst video shooters, especially when mated with micro 4/3rd cameras such as the Panasonic GH4 and GH5.  

It is a professional-grade lens with a constant f/1.8 aperture zoom lens for DX/APS-C formats.   It has a focal range equivalent to 27mm-52.5mm in 35mm format and is fantastic for low-light situations.  This lens was designed so that it will offer the same control over depth of field as an F2.8 zoom does on full frame and when combined with an APS-C sensor will offer effectively the same light-gathering capability as an F2.8 lens on a full frame camera.

The Speed Booster effectively shortens the focal length of the lens and with a crop factor of 1.28x so coupled to a Sigma 18-35 makes the lens behave as a 13-25mm lens with a slightly wider angle of view than you'd get by mounting that same lens on an APS-C camera.  Youtube reviews will reinforce that this adapter is a perfect match for this lens.

The Sigma lens glass is protected by the HOYA 72mm circular polarising filter which on any camera system will help colors to be more saturated and bright blue skies can take on a deeper huea.  Circular polarising filters also help reduce flare and the chance of reflections.

https://www.digitalcamerawarehouse.com.au/sigma-18-35mm-fa18-dc-hsm-lens
https://www.digitalcamerawarehouse.com.au/metabones-t-speed-booster-ultr...
https://www.digitalcamerawarehouse.com.au/72mm-hoya-72mm-circular-polari...

Reviews:
https://www.starkinsider.com/2017/10/best-lens-panasonic-gh5-video-shoot...
https://www.dpreview.com/articles/5547568336/metabones-speed-booster-ult...