Speedlite zoom and Bronica lenses

The guide number table for my Yongnuo 560 II speedlite is based on two variables: the power factor (1/1 to 1/128) and the flash head

zoom setting (14 mm to 105 mm), which refers to the camera lens focal length. My question is this: are these focal lengths absolute or do

they refer to modern digital camera lenses or perhaps to earlier 35mm format lenses. In other words, if I have an 80 mm prime lens on my

Bronica SQ Ai, should I use the 80 mm flash zoom column on the table to determine a guide number or should I use the 50 mm column

since a 50 mm "normal" lens on a 35 mm camera corresponds to an 80 mm "normal" lens on my Bronica MF?

<p>The zoom settings on most hotshoe flashes, YN 560 included, do (or should) coincide with the angle of view for a standard 35mm full frame or film camera's view. So 80mm on the flash would cover, when directly over the lens, the angle of view of an 80mm lens on a full frame 35mm dSLR or SLR. If you move the flash off camera or to the side of the lens you're changing where the light reaches.</p>

Thanks Dan. That confirms what I understood.

<blockquote>

<p>"The guide number table....."</p>

</blockquote>

<p>Ignore it! Guide numbers as given by any flash manufacturer are pure fantasy. Most of them are <em>at least</em> one stop overestimated. You need to either use a flashmeter or calibrate the flash against real exposures. Failing that, if you divide all the guide numbers by 1.5 (or halve the ISO speed) you won't be far off getting the right exposure.</p>

<p>I arrived at the above conclusion after testing several different makes, ages and models of flashgun. None of them come anywhere near to the maker's given GN, as verified by the readings from 3 different flashmeters and by practical exposures using a DSLR camera. But if you prefer underexposed pictures..... then follow the GN table.</p>

Until I can get a flash meter, I'm stuck with guide numbers, but I plan to take some test shots to determine a correction

factor for the mfg's GN values so that I can make my own table. According to tests on Speedlights.net, the Yongnuo 560

numbers are about 1 stop too high at ISO 100 and 35 mm.

 

I have no idea which flash meters to consider. For available light, I'm still using a prehistoric Gossen LunaPro.