I have a Yonguo 560-II speedlight and a Luna Pro F meter for use with a Bronica SQ Ai. I'm having a problem figuring out how to use the speedlight zoom function. I know the mfgr's guide numbers are optimistic (probably by about one stop) but using the Luna Pro F, I cannot find any real correspondence with the GN table supplied with the speedlight. At 10 feet, ISO 100, full power flash, zoom 35mm, the meter indicates f/8, which would be a GN of 80. According to the table, the GN would be 127.9 (probably in fact more like 115), which would be f/11 at 10 feet. This is where I get confused. I thought I should get a GN based on the flash meter that corresponds to the 35mm column (after adjustment) in the speedlight table, but mine is considerably lower. In some situations, using the flash meter is not convenient and using guide numbers would be handy. How do I do that, using an 80mm or 150mm lens on my Bronica 6x6? Which zoom factors do I use for the 80mm and 150mm MF lenses?