Hello Phillipandlesley - Welcome to the Sony Forums
If you take a look at page 100 of the manual which is available here:
http://pdf.crse.com/manuals/4287948131.pdf
You will see that this can be accessed by setting the Mode Dial to 'M' and then rolling the Control Dial until 'Bulb' is visible in the bottom-left of the screen. Also, if you hold down the AV button, you can rotate the Control Dial to adjust the aperture before shooting.
Thanks
Simon
