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I have a question I hope someone can shine a light on for me.

I usually shoot with flash only or ambient only. I need to do a small shoot on location where I must balance ambient with flash. Its a fairly bright area, a sports hall. The ambient reading is ISO 100, f/5.6 and 1/30.

The only lights I have are a Godox SK400II with a lowest setting of 1/16th and a Godox 600 with a lowest setting of 1/32.

How can I calculate if I can use these strobes? I want to know in advance if it can be done or if I need a light that can dial in a lower power?

I think there must be a way that I can calculate this first, but really don't know how to.

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  • Will you be shooting sports, or some other type of event in this location?
    – Michael C
    Oct 19, 2022 at 3:48
  • There are many variables involved that you haven't told us much about. With lights and power, distance is always a prime concern that is just as critical as power output. Whether your current lights are too bright or not, even at their lowest power settings, depends entirely on how far from your subject you can/will place them. In simplest terms, doubling the distance has the same effect as halving the flash's power. Modifiers (diffusers, soft boxes, etc.) will also reduce power to one degree or another.
    – Michael C
    Oct 19, 2022 at 3:56

2 Answers 2

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You can use the guide numbers to calculate usable distance(s) and power settings.

The guide number for the SK400ii is 65m at ISO 100. Simply divide the GN by the f# to get the distance for correct exposure at full power. E.g. 65÷f/5.6= 11.6m. If you require a different distance you can divide the GN by that instead... E.g. 65m÷6m=f/11; f/11 is two stops more than f/5.6, so set the strobe to 1/4 power @ 6m.

You can also apply the inverse square law... i.e. double the distance = 1/4 the power/exposure, or 1/2 the distance = 4x the power/exposure (2 stops).

If you need even less power you can get ND filters for them, and if you need more power you can combine them together.

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  • thank you, let me rephrase the question than, if i do trial and error, how can i change my settings for stil getting a good balance, but with smaller aperture and higher iso?
    – Marco Veringa
    Oct 18, 2022 at 13:59
  • @MarcoVeringa, if you increase ISO to compensate for using a smaller aperture then nothing changes. What you might want to do is change the shutter speed which affects the ambient exposure but not the flash (within sync speed). You could also use high speed sync with those strobes, but that does affect/reduce flash power; at least initially.
    – Steven Kersting
    Oct 18, 2022 at 22:39
  • Many thanks for this to all, ill try it out!
    – Marco Veringa
    Oct 19, 2022 at 1:52
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There really is no way to accurately calculate flash exposure for the "sports hall" situation you describe. Too many variables will come into play.

The size of the room, size and number of ambient light sources, flash distance to subject, flash angle to the subjects, flash angle to the camera, color and placement of reflective walls and furnishings, etc., will all vary the amount of light reaching the camera.

Are you planning to shoot from a tripod with a fixed location? If not, as you walk around, you will constantly be changing these variables.

I would suggest using a flash meter, or simply relying on trial and error.

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    I agree with the Light Meter suggestion. I do stock with a mixture of natural plus flash and without a light meter, it becomes lengthy as it boils down to trial and error.
    – Abdul N Quraishi
    Oct 18, 2022 at 23:00

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