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I know the technical aspects of controlling flash power: increase or decrease flash exposure compensation, and if that doesn't work, increase or decrease shutter speed, and use more flash power for longer focal lengths. I understand this.

I wanted to check if there's any guideline to find roughly the right range of flash powers for a given scene, rather than painstakingly trying out a couple of dozen shutter speeds from 0.8s to 30s to find the one I like. Is there a rough analog of the Sunny 16 rule, but for flash power?

For example, I tried taking a photo of my garden without flash:

enter image description here

And with flash, which overpowered the scene, resulting in a bad photo:

enter image description here

By trying various shutter speeds from 0.8s to 30s, I found that 8s looks best, filling the dark foreground without making it obvious that a flash was used:

enter image description here

Are there any guidelines to make this process less painstaking than taking a dozen or two photos each time?

Note that I don't have off-camera flash, or multiple flashes, or umbrellas, diffusers or other accessories. Neither do I have the ability to change the angle of the flash on my camera (Sony NEX-5R). The only variable I have control over is the ratio of flash to ambient light.

  • Do you have the ability to choose between manually controlled flash power or flash power controlled automatically by the camera? Do you have the ability to set flash exposure compensation? – Michael C Jul 29 '15 at 23:01
  • I can set flash exposure compensation from -3 to +3. That's the only control I see. If I find myself needing a greater range of flash exposure compensation, I use the shutter speed to simulate that. For example, if FEC of -3 is still too bright, I note the shutter speed it used, and double it. Is there a better way of doing it? – Vaddadi Kartick Jul 30 '15 at 0:20
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    Hi --- I found the two tutorials on cambridgeincolour.com/tutorials.htm about flash being really useful and easy to follow. Maybe they can help; especially cambridgeincolour.com/tutorials/camera-flash-2.htm seems to fit your problem nicely. – Rmano Aug 1 '15 at 12:09
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Yes, the "guide line" you seek is called Guide Number. It applies to direct flash, and Manual flash mode. Your flash user manual probably contains a Guide Number chart. For example if at some flash head zoom value, and ISO 100, maybe the Guide Number is say GN 100. It might be feet or meters, GN in feet is simply 3.28 times GN in meters (3.28 feet in a meter). I am assuming feet here.

This GN is distance multiplied by distance, so the meaning of GN 100 is that correct flash exposure is 10 feet at f/10, or 5 feet at f/20 or 20 feet at f/5. See http://www.scantips.com/lights/flashbasics1c.html for all you want to know about guide number. It also has a calculator that shows the "correct" distance span range of the flash exposure.

But note that flash suffers the Inverse Square Law, varying with distance, specifically meaning flash exposure is only "correct" at some ONE distance. All other distances will be different.

And note that if using TTL, the system meters the flash and tries to set it to do correct flash exposure, at the one distance (central area in your picture).

Your picture does not want a "correct" flash exposure, you think it is too bright for the nighttime situation you want to create.

So your answer is, your plan is good. If it is too bright, just turn it down, set it like you want it. Flash compensation is how we control what the camera TTL metering does.

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The duration of the flash is much shorter than any of the shutter speeds you are using, so the shutter speed doesn't directly affect the amount of light from the flash that winds up in the scene. What does seem to be happening here is that the camera is treating the flash as fill light and automatically adjusting the flash power to balance with the amount of ambient light, which is affected by your selected shutter speed. If you don't have a way to manually control the flash power directly then pretty much all of the standard "rules of thumb" go out the window - your camera will alter the power to counteract most of what you do. At least it will until you close the aperture and reduce the ISO to the point where the flash is at full power so the camera can't add any more to compensate for allowing less ambient light into the frame.

In addition to flash exposure compensation, you can also dial in regular exposure compensation. This will reduce the amount of total light (ambient light + flash light) that the camera thinks it needs to get a well exposed shot.

  • I don't understand how the amount of ambient light is affected by the shutter speed. I'm in shutter speed priority mode, so the aperture and/or ISO should vary accordingly to cancel out the effect of the shutter speed, and so capture the same amount of ambient light. – Vaddadi Kartick Jul 30 '15 at 12:17
  • The longer the shutter is open, the more light is allowed into the camera. When shooting in low light such as your are doing, it is usually best to use manual mode. Not being familiar withe the NEX-5. I don't know how the camera handles long shutter speeds in shutter priority mode, but at a certain point the ISO can not be reduced any further and the aperture can not be narrowed any further and at that point leaving the shutter open even longer will increase the amount of ambient light in the scene. – Michael C Jul 30 '15 at 23:27
  • Also, if the aperture is narrowed or the ISO is reduced to compensate for a longer shutter speed, then the amount of influence the full power flash has on the scene is also reduced. This means the camera needs to let the ISO or aperture stay at a slightly higher exposure value to let the ambient light AND the flash make up the difference. – Michael C Jul 30 '15 at 23:30
  • Not to even mention that the wording of your question leads one to believe the shutter speed is the only variable wrt Tv, Av, and ISO as it would be were you in manual mode. – Michael C Jul 30 '15 at 23:33
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    You also cede control when you use the built-in flash that cannot be taken out of TTL, vs. an external flash that can be switched into M mode. – inkista Aug 1 '15 at 2:38

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