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I sometimes encounter a situation where using a flash would greatly enhance the pictures because of the low ambient light, but I don't use the flash as it is mighty distracting in a low-light cafe/bar environment.

Would using a CTO gel help make my flash "fit-in" more with the ambient light and create less of a distraction?

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It will still be quite noticeable. Even at very low power a flash is hard to miss. A flash set at 1/128 power is seven stops dimmer than the same flash at full power, yet it is hard for a person to tell the difference just by observing the flash fire. This is probably due to the fact that the irises of the eyes of people in a dark location are enlarged and allow much more of the light from a flash to pass through to the retina, much like a larger aperture allows more light to pass through a lens to a camera's sensor. A CTO gel doesn't reduce the power of a flash anywhere near that level of seven stops, so it isn't going to reduce the perceived amount of light by a person whose vision is acclimated to a dim environment.

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    I thought that maybe the blueish light of a flash without a gel is causing the distraction as a cafe mostly has a yellowish light atmosphere. But this doesn't matter much apparently? – Saaru Lindestøkke Mar 7 '14 at 0:44
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    Yellow, green, or purple the flash is going to be so much brighter than the ambient light in a typical cafe/bar at night that it will be perceived by eyes acclimated to the dim environment as white light just as light that is heavily red, blue, or green will appear white in a photo if all three color channels are at least bright enough to be fully saturated in all three channels. It doesn't matter if there is five or ten times as much red as blue or green in the light, if there is enough of the weakest color to be fully saturated it will look white to our eyes. – Michael C Mar 7 '14 at 1:46
  • Have you ever looked directly at a gelled flash while it is being fired in a low ambient light environment? – Michael C Mar 7 '14 at 1:47
  • I have not. That's why I asked the question. But the answer seems pretty clear, thank you :) – Saaru Lindestøkke Mar 7 '14 at 2:28
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    On the other hand, if you survive the beatings your pictures will probably be nicer. (It would be the right thing to do for a planned shoot in the environment with appropriate permissions, say for a lifestyle advertising piece, but annoying for candids.) – user2719 Mar 7 '14 at 5:17
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The rapid change in light level caused by the flash will be detected by rod cells in the eye. These cells are responsible for detecting movement and do no see in colour so changing the colour of the flash will not decrease the change of it being noticed.

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