Reading the user manual helps to understand the flash function of the 6008. With regard to "Preflash, Manual Aperture Control'
"Preflash (manual aperture control) First apply the prefl ash custom function (“SF PrE“) to switch 31, then switch this on. See “Custom functions“. The preflash mode is based on the spot sensor incorporated in the refl ex mirror. Therefore position the subject to be metered in the center of the focusing screen for optimum exposure. Pressing the shutter release will set the working aperture and initiate a release cycle without actual exposure or fi lm advance. The light balance displays the exposure value determined. Vary the aperture or the flash output and repeat the preflash until the light is balanced, that is, until only the large central line is visible. If the reading is away from correct exposure by no more than 1 EV (light balance should not be blinking!), proper exposure may be determined without renewed metering, by simply varying the f-stop. If the difference exceeds 1 EV, be sure to repeat the reading, since the metering range has been exceeded and subsequent adjustment is impossible. Finally, use switch 31 to terminate the custom-function setting. To ensure proper metering, be sure to avoid stray light on the focusing screen. Note: Contrary to TTL fl ash metering, the prefl ash mode is restricted to a fi lm-speed range of ISO 25/15° - 800/27° and is not available in any of the automatic exposure modes. In this case, the display will read “no Auto“."