I often get asked a particular question, and to tell you the truth, it confuses me.
People always want to ask what power my lights were set at when I took the picture and I also see that people like me reporting this information on their BTS diagrams.
But it confuses me because unless you’re going to take a similar photo with the same modifier at the same distance, in the same room, then the input power of my lights isn’t really relevant. Move the lights a foot, introduce a different modifier, add a white wall or a shiny floor and all bets are off. Then there is to color of your subjects hair, clothing or skin tone, all of which will make a big difference.
The only number that I ever want to record is the meter reading of the light on the subject, because no matter where I shoot or which modifier I have with me, I will have a fighting chance at repeating the results.
When using TTL I would record the flash exposure compensation for each light would be key.
If you’re new to lighting I would suggest shooting a photo with a hair light and bracketing the exposure from -3 to +3 in 1/3 stop increments relative to your main light. So if you camera is set to f8, adjust your exposure of your main light so it meter at f8 and then set your hair light up so it meters at 2.8 and then shoot a photo, increase the hair light power 1/3 stop, take a photo, then 1/3 stop bright and take a photo and so on until you’re hair light would meter at f16. Then look at the series of images and decide for yourself what you think looks good. For me its going to be somewhere between -1 (f5.6) and -3 (f2.8). Rarely have I gone higher and that was only for affect or because the persons hair was dark and matte.
The point is that you want to recognize when you look at a test photo which way you need to adjust the light to get the look or feeling you want. Repeating the same experiment for fill lights (stay below your main lights exposure or it won’t be fill), edge lights and background lights will help you learn what to do.
Now if you’re wondering how many watt seconds your lights should be in general, then that’s a different discussion. But in basic terms you need about 500ws for a giant softbox and 50-100 watt seconds for a small one or a beauty dish. Gels on the other hand are going to block a lot of light, so if that’s what you’re into, you’ll need a little more power.
Now if your concern is more about battery power, recycle time and possible shots, then I apologize for getting it all wrong.
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