This I s a subject that comes up often here, but usually brings out a lot of divergent views. I thought I would add my two cents worth in this new thread.
Questions come up in a lot of different guises, thus:
- How are speed light guide numbers related to flash Energy
- How does my hot shoe flash energy compare to this or that strobe?
- How is flash exposure related to ambient exposure?
- Do I have enough power with my hot she flash to balance bright sunny ambient.
I have recently written a couple of posts where I have reported the precise energy of my Yongnuo YN600EX-RT flash energy to be 76 Watt seconds(Joules). This, of course, covers the Canon 600 EX-RT flash. So we know the energy of these most important hot shoe flashes, to start.
Now I want to write down the exposure comparisons between these flashes and bright ambient light.
We all know that the ambient exposure value is given by
EV = Log2[exposure time* Fnumber^2], or
Fnumber^2 = t*(2^EV)
On the other hand, for flash exposure, it's all in the guide number thus, with
Fnumber^2 =( g^2/R^2)*(P/P0), g is the guide number in ft, say, and R is the flash to object distance. Evidently se have equivalent exposure when we set these two expressions for aperture to be identical:
R^2 =[ (g^2)*(2^-EV)*(P/P0)]/t ; P/P0, ratio of flash energy to nominal single flash max energy
Quite obviously, we select the shutter time, t, to be the fastest flash sync time available for the given camera. In my case, t = 1/250 seconds. And on a bright sunny day, the ambient EV is 16. Using this and approximating 1/250 ~ 2^8, we easily get
R^2 =(g^2)*(P/P0)*2^-8, with g given as 137.8 feet for a 50mm lens FOV;ISO100, it follows that
R =8.61 feet*Sqrt(P/P0),
where we note that if multiple speed lights are used, (P/P) can equal N, the number of flashes used. So that if 4 speed lights are used in tandem, the flashes can be mounted up to 17 feet from the lit scene.
A couple of final points. First we note that, exposed in this way, the scene is over exposed by one stop, since either the ambient or flash were nominally sufficient. This is easily remedied by stopping down the aperture or lowering the ISO etc. We note, too, that on account of their RF wireless detachability, these lights can trivially be deployed in such bright balancing environments.
Regarding use with mods, I have a 5' reflective umbrella, that has a 2 stop throughput loss, even this can be managed using a 4 flash mount, 4 flashes just balance the 2 stops. Such umbrella works well at 8.6 feet.
All for now.
Questions come up in a lot of different guises, thus:
- How are speed light guide numbers related to flash Energy
- How does my hot shoe flash energy compare to this or that strobe?
- How is flash exposure related to ambient exposure?
- Do I have enough power with my hot she flash to balance bright sunny ambient.
I have recently written a couple of posts where I have reported the precise energy of my Yongnuo YN600EX-RT flash energy to be 76 Watt seconds(Joules). This, of course, covers the Canon 600 EX-RT flash. So we know the energy of these most important hot shoe flashes, to start.
Now I want to write down the exposure comparisons between these flashes and bright ambient light.
We all know that the ambient exposure value is given by
EV = Log2[exposure time* Fnumber^2], or
Fnumber^2 = t*(2^EV)
On the other hand, for flash exposure, it's all in the guide number thus, with
Fnumber^2 =( g^2/R^2)*(P/P0), g is the guide number in ft, say, and R is the flash to object distance. Evidently se have equivalent exposure when we set these two expressions for aperture to be identical:
R^2 =[ (g^2)*(2^-EV)*(P/P0)]/t ; P/P0, ratio of flash energy to nominal single flash max energy
Quite obviously, we select the shutter time, t, to be the fastest flash sync time available for the given camera. In my case, t = 1/250 seconds. And on a bright sunny day, the ambient EV is 16. Using this and approximating 1/250 ~ 2^8, we easily get
R^2 =(g^2)*(P/P0)*2^-8, with g given as 137.8 feet for a 50mm lens FOV;ISO100, it follows that
R =8.61 feet*Sqrt(P/P0),
where we note that if multiple speed lights are used, (P/P) can equal N, the number of flashes used. So that if 4 speed lights are used in tandem, the flashes can be mounted up to 17 feet from the lit scene.
A couple of final points. First we note that, exposed in this way, the scene is over exposed by one stop, since either the ambient or flash were nominally sufficient. This is easily remedied by stopping down the aperture or lowering the ISO etc. We note, too, that on account of their RF wireless detachability, these lights can trivially be deployed in such bright balancing environments.
Regarding use with mods, I have a 5' reflective umbrella, that has a 2 stop throughput loss, even this can be managed using a 4 flash mount, 4 flashes just balance the 2 stops. Such umbrella works well at 8.6 feet.
All for now.
