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As I'm beginning my journey into controlled lighting I took my Sony HVL-F43AM outside on a sunny afternoon, put a model under a canopy (shade), and wanted to see if I could get enough light out of the off-camera flash to get a blown-out background. [Clarification: My intention was to let the direct sunlight blow out background, use the shade to take the sunlight out as a key light on the subject so I could instead use my flash as key, and then use a combination of reflectors and ambient to fill. I was shooting around 1/200 and initially had my flash at full power firing through an umbrella, but I didn't think about the flash's distance: I left it something like 15' away from subject!]

Turns out that even in shade my flash at full power couldn't make a dent in the ambient light, even with no modifier. (I did get a measurable amount of fill using a 4' x 8' sheet of white coroplast as a sun reflector.)

Now this was with a Guide Number (GN) 43 flash. I'm trying to determine whether I should step up to something like the Strobist favorite Lumopro LP180 speedlight, which has a 110 GN. I had been assuming that I would have to jump to monolights, but according to Paul Buff's Expected Output even the stronger monolights are in the 100-GN range shooting through a diffuser like a softbox.

Am I thinking about this correctly? E.g., can I compete with sunlight using flashes? And is a 100-GN speedlight in the same class of output through a diffuser as a 600 Ws monolight?

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    To blow out the background just increase the exposure, especially by using a slower shutter speed. You need flash power when you want to do the opposite: kill the ambient. photo.stackexchange.com/q/34392/15871 – Michael C Aug 14 '14 at 5:07
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    That "Guide number 110" on the Lumipro is in feet, not meters. Granted, that's at a fixed and fairly wide angle of coverage, but it's actually about a half-stop less powerful (GN 33m vs GN 43m) than your Sony flash. – user28116 Aug 14 '14 at 8:46
  • @user28116: Good catch! I didn't think to check the distance units on the GN! – feetwet Aug 14 '14 at 14:15
  • What aperture and iso were you using? Because iso 100+f/16 is a big ask of any speedlight, and even more from 15' away (inverse square). – inkista Aug 14 '14 at 18:18
  • Not sure, but I assume it's not even worth checking: The point is that at 15' (or whatever the distance was) the speedlight at full power couldn't measurably affect the exposure given all the ambient pouring in from sunlight reflections off the surroundings. I guess I was too distracted to remember the inverse square law and try to minimize that distance. – feetwet Aug 14 '14 at 20:28
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Compete with sunlight? Sure, easily, especially in shade. My 600EX-RT through a small soft box is still sufficient to compete with sunlight and give primary lighting from the flash if I want it to. The key is to a) be close enough and b) shoot fast enough.

A flash is not sustained light, it is very intense light for a very short time. The longer your exposure, the more time ambient light has to accumulate on the sensor. Shorter exposures give your flash a higher degree of influence. When you hit the HSS point for your camera, you will lose a fair bit of power, but it may still work out to be worth it for the extra shutter speed you can get to reduce the ambient further as well.

Adding additional flashes to the setup makes it even easier to accomplish, allowing shooting from further away or will slower shutter speeds (though still pretty fast).

I'm not sure what you mean by blowing the background. That normally means it would be all white, which doesn't makes any sense in this case. To blow the background to white, you simply would over-expose the background and wouldn't want flash. If you meant you want the background to be black, that also doesn't make any sense as you would have to be many (6+ at least) stops brighter than the ambient light for the time exposed, which is an insane amount of light, far, far beyond trying to simply compete with sunlight.

From your update, being 15 feet away is certainly a big part of the problem. That's forever away during outdoor shots. You basically want the flash as close as you can get without being a) in frame or b) casting undesirably hard shadows. Light power falls off exponentially over distance, so the closer you can get, the more power you have.

Also note that if you can't completely blow the background without the flash, you really aren't going to be able to do it with the flash because the flash is adding light to the foreground which means that exposure of the background will go down, making it less blown.

  • Thanks: I hadn't considered the distance factor! Just updated original question with clarification of my setup and intention. – feetwet Aug 14 '14 at 14:17
  • +1 for the last paragraph, which is where the OP's misunderstanding lies. Flash adds light to the foreground, which means you expose less, which means the background gets darker. If you want the background blown out, you want less light on the foreground. This is not a job for a flash. – Wayne Dec 19 '14 at 15:50
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Since you are letting the ambient light blow out the background, your flash doesn't come into play. I would start by adjusting my settings to get the ambient light the way you want first, no flash at all. Larger aperture and slower shutter speed will accomplish this, as long as you keep it fast enough to get a sharp portrait in the end.

Once this is done, introduce the flash to light the model. Then you just need to focus on the artificial lighting and ignore the ambient.

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