martinslade said:
I was wondering if anyone can help with advice on this. I need to shoot mountain bikes in bright light at 1/30th sec to get some background blur and want to pan + fill flash to get some detail in the subject. The bright back light from trees/sky is causing small apertures to keep slow shutter so do I need extra flash power to get the right balanced effect? My flash will be mounted off camera 3-4m from subject and I will be low shooting up about 2m out. My flash is rated 36 and I will probably be at 100 iso. Any help much appreciated... thanks...
Are you trying to overpower full sun or is there some other ambient light source? (I assume full sun)
What camera body are you using? (Minimum ISO?)
How did you get to a guide number of 36? (I assume 600EX-RT at 35mm zoom?)
Are you fixed to 1/30s? (Could you use a faster shutter speed?)
Given your constraints, I don't see how you can make it work.
According to my (self-made) exposure calculator...given ISO-100, 1/30s, you will need to shoot at f/29 to get a proper ambient exposure in full sun.
(GN) / (f-stop) = range
(36)/(29) = 1.2 m
At f/29 there's a lot of diffraction so for a sharper aperture you'll need to use an ND filter. With a 3-stop ND filter you can open your aperture wider to f/10. Note, ND filters do have a negative impact on image quality and viewfinder experience and AF performance. The big problem though is that your flash still only illuminates properly up to 1.2m.
In order to counter this one could gang together multiple Speedlites, (e.g. http://www.bhphotovideo.com/c/product/897247-REG/westcott_2223_bruce_dorn_triple_threat.html) to boost your flash power. I reckon that you will need 5-6 speedlites to get sufficient light to properly illuminate the subject at a distance of 4m.
I assume that mountain bikes are moving relatively fast so maybe 1/125s can still work? If it can, then trading slow shutter for larger aperture size allows you to:
a) gain flash range or
b) reduce recycle times or
c) gel your flash
So, sorry I don't have a magic bullet to solve the problem, but there are a few strategies at your disposal. Think of it this way, if it was gonna be easy then anyone could do it and the end result would be less valuable.
P.S. See what Canon EoL Tyler Stableford does here: https://www.youtube.com/watch?v=KaALe0w992E