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I know this may seem like a stupid question, but I wanted to know how I could literally weaken the light of my flash. I take photos of a prayer group at night, and in general, it is very dark, but people also move quite a bit, so that a fast shutter speed is required. So I need to resort to using a flash, even though it runs the risk of disturbing the people praying. So I was looking for a diffuser, such as an octabox for the flash, but I'd like to hear from you guys, is there some way of making the flash weaker in order not to disturb people so much? At home I put a piece of paper in front of the flash – but is there some more professional way of doing this? Thanks.

This is an approximate human translation of the original question submitted in Portuguese. See the edit history for the original.

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Ambient exposure is controlled by iso, aperture, and shutter speed. But flash exposure is controlled by iso, aperture, flash power, and subject-to-flash distance. If you want to lower the flash power, increase your ISO, open up the aperture, or get closer to your subject.

If you balance your flash to be only fill against the ambient (i.e., most of the exposure of the overall image will not be coming from the flash, but from the available ambient light), you can use a high ISO setting and a lower shutter speed, and let the ambient do the heavy lifting, and the power output required from the flash won't be so high.

Also, if you bounce the flash, you could use Neil van Niekerk's "BFT" (Black Foamie Thing). This is a sheet of black craft foam that you use a flag to block any direct light coming from the flash head from hitting your subject. It ensures that the only flash illumination that shows up in the image is bounced light, which has softer shadows, so your image looks better. But on top of that, it also keeps direct flash from inadvertently blasting anybody in the face, and can help make flash use less disruptive.

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If a scene needs a certain amount of light, you need to provide that light. Using a diffuser or bouncing means that your flash will not just provide the light for the scene in question but also additional light in other directions. So it's not really useful for reducing the impact of the flash.

Instead of making your flash less effective (which improves the quality of the photograph but increases the impact on onlookers) you want to make it more effective. Focus it on the actual scene in question: if it is a zoom flash, make sure that it uses the zoom level you need. If you have the option to move closer to your subjects, do so (this will make the background darker in comparison, though). If you want to minimize disturbance, turn off red-eye preflashes (of course with the obvious consequence of being more likely to get red-eye).

Anything you can do to get a better light yield with your camera will decrease the strength of the flash: open up aperture as much as possible for your subject, increase ISO as much as image quality permits. Do not use exposure times shorter than flash sync time to avoid HSS (high speed flashes) which are considerably less efficient regarding the light yield. Sometimes it is possible to switch a flash to "Auto" or "Thyristor" mode instead of "TTL" ("through the lens"). That makes the flash strength less precise and not based on the exact image framing but foregoes the TTL preflash. When it does work, the result is a reduction in light the subjects receive for the same result.

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Ok...Depends.

There are praying events of 50,000 people praying and some other of only a couple of them. There are outdoor events and indoor events.

On big events and in outdoor events, you are almost "limited" to use higher ISO and a faster lens.

Do NOT use a diffuser on the flash, it is a bad idea. Instead of seeing a tiny burst of flash people will see a BIG ONE. This is not what you intended.

My recommendations are:

Turn off any TTL

TTL fires twice that will be more distracting than just the one you really need.

Turn off any red-eye reduction. This fires a really distracting burst of flashes.

Use little power

Use just the power you need to reduce your shutter speed or ISO a bit. Do not try to freeze a praying person. If a small power is enough to reduce a high ISO with too much noise, or reducing a walking person blur a bit, that is good.

Bounce the light upwards

This can be tricky. It only works if you have a high enough ceiling and is somehow light gray-ish or white-ish.

Get a faster lens, a decent high ISO camera and a tripod

This is the best solution.

If you want to capture the ambient of the situation, you want to document the event as people feel it, moody and dark... do not use a flash. Some motion blur can actually be a good thing.

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@inkista answer is sound, basic photography principles. The exposure triangle gives you control over how much extra light you mean, but you could be trading off quality. You could also purchase a faster lens which will let in more light and offer better results at night.

I'd like to add a further suggestion. In wildlife night photography, we use infrared lighting to illuminate the scenes. It's not cheap but if you want to do professional night photography with reduced or no visible lighting then infrared lights are the way to go. With smart LUT's you can even get some color back into them.

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