Speedlite GN with 6x6 MF

[Benoît99]

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?

 

[Pioneer]

I use the 35mm equivalents of the lenses I am using and it usually works out relatively well. For example my 75 is the rough equivalent of a 50mm lens on 35mm, and my 150mm is roughly equivalent to 90mm, or 80 if that is what shows on your flash zoom. That seems to work out relatively well for me but it probably depends on your type of shot. In most cases I am taking pictures of people when I use my flash so I am more interested in the performance in the center, or close to the center, of the frame. There may be some fall off on the edges that I am not accounting for with my method.

 

[MFstooges]

If I'm not mistaken that speedlite's GN is 34, that means f11 is about right. Subject at 10 feet will be properly exposed and 55mm lens framing will be lit evenly.