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While setting up a photo studio for product photography, I often read advice that suggests that one needs extremely strong light sources (1–10 kw total) if shooting without flashlights.

I fully understand why the homogenity and distribution of light is critical, but where is the relevance of the absolute light output? After all, my objects don´t move and I use a tripod, so I could compensate low light with a long shutter. Is there a difference between four softboxes at 50w (i.e. 50w consumption = 200w lightbulb) and four softboxes at 200w that cannot be compensated with slow shutter speed?

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It's a matter of controlling the contrast between lower ambient and higher flash light sources. You have more control over how the lighting looks if you can increase the contrast between ambient and flash. When you're using lower lighting levels with longer shutter speeds, the ability to balance the flash against the ambient is greatly reduced.

For example, turning a background completely white or completely black in a studio setup generally relies on lighting the subject and background independently of one another, and then adjusting those lighting levels differently--lowering the background lighting or raising it on the subject can make a grey backdrop black in the image, and doing the reverse can make it white. If your lighting sources are low-powered enough to be similar to the ambient lighting (long exposure), you can't make much of a difference between how the background is lit and how the subject is lit.

And if you use continuous lighting that is powerful enough to give you that contrast between background and subject, chances are really good it's going to produce a lot of heat and require more power than a flash would.

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  • Good point. Although if you use a 50 W (equivalent) light bulb to light up the room, I assume that the ambient light is even lower. So perhaps the absolute light level doesn't matter, but the relative light level certainly does.
    – j-g-faustus
    May 17 '14 at 19:41
  • Actually, you'd have more contrast if you don't light the room at all. And bulbs that light the room ARE the ambient. :)
    – inkista
    May 19 '14 at 21:51
  • I meant subject rather than room: E.g 50 W bulbs in the softbox should give the same contrast as long as you can reduce the ambient light to 3-4 stops below the light from the softbox. In daytime that might require a windowless room or heavy curtains, but it's doable.
    – j-g-faustus
    May 20 '14 at 14:32
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As far as I know, absolute light power doesn't matter.

It will matter if you want to freeze motion (like when the subject is people or pets), and it will matter if you don't use a tripod, since you can't handhold long shutter speeds.

But for still life subjects shot with a tripod, pretty much any amount of light will do.

There is a lower limit where longer exposures will increase sensor noise, but that's typically down at starlight level. This post suggests 1 minute exposure time as the upper limit before you need noise reduction.

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  • Thank you, j-g-faustus. Can you imagine why so many sources advise to use flash instead of constant light? There must be at least some truth behind that, I´d guess.
    – Zsolt Szilagy
    May 17 '14 at 17:42
  • @zsz85 I guess it's because most studios also shoot people, and if you're at f/16 (for depth of field) and ISO 100 (for low noise), few living things can hold perfectly still for that long. In short, they want to be able to freeze motion.
    – j-g-faustus
    May 17 '14 at 18:13

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