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I have a large-ish lightbox similar to this.

I would like to use it with a handheld camera. How do I go about calculateing how much light I need?

I would like to use cheap continuous lights. Is there a formula or even guidelines for estimating how many lumens are required?

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  • \$\begingroup\$ Out of interest, why aren't you using a tripod? \$\endgroup\$ Nov 20, 2014 at 13:00
  • \$\begingroup\$ I am using a tripod currently, and its working fairly well. But I've not got much room so I am constantly setting it up and taking it down so it's not in the way. Also I find that it takes a little bit of fiddling to adjust the camera position. I figure if I can get a handheld setup I could get through things much quicker. \$\endgroup\$
    – Ken
    Nov 20, 2014 at 13:46
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    \$\begingroup\$ It does have the advantage of drastically improving sharpness though... \$\endgroup\$ Nov 20, 2014 at 13:52
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    \$\begingroup\$ How long is a piece of string? \$\endgroup\$ Nov 20, 2014 at 16:03
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    \$\begingroup\$ With continuous lighting at 1/100s and ISO 100, yes, you are being unrealistic. \$\endgroup\$
    – Robin
    Nov 26, 2014 at 20:02

2 Answers 2

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It's not trivial to calculate from scratch the amount of light required (as you have no idea how much is absorbed, reflected etc. and it will vary according to how the lights are positioned).

What you can do, is find out what shutter speed your camera meter is suggesting currently and work it out from there.

You'll want to aim for 1/2f where f is the focal length. So if your lens is 50mm, your target shutter speed is 1/100s. If your camera is reporting 1/25s in A mode, then you need 4x as much light to reliably shoot handheld. So if you're using a 25W bulb now, you'd need a 100W bulb (of the same type).

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  • \$\begingroup\$ That kinds seems obvious now that you mention it. \$\endgroup\$
    – Ken
    Nov 20, 2014 at 19:09
  • \$\begingroup\$ When this was written the lamps were probably incandescent, so the light output went up faster than the wattage. Now that they are LEDs the linear relationship probably holds but the spectrum is not as nice. \$\endgroup\$ yesterday
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You may find useful this wikipedia page

You can calculate the exposure value with the given formula:

$$\begin{align} \mathrm{EV} &= \log_2{N^2\over t} \\ &= 2\log_2{N}-\log_2{t} \end{align}$$

where:

  • \$N\$ is the f-number (aperture) you will use; and
  • \$t\$ is the exposure time you will use.

They both depend on your camera: search a middle aperture for your lens and a time you will likely use (a rule of thumb is that the slower time you should use freehand is the inverse of your focal length (i.e. if you have a 50 mm lens, you should use 1/50 of sec))

When you have an EV number, check against the table in page I linked and see if you can convert it in a bulb :-)

Apart from that, it's interesting to measure EV value in itself; frankly I think using a support is a better policy because it also allows you to study your composition more at ease. Consider using a monopod if a tripod is too big.

Also, the above formula is for ISO 100 sensibility; should you use a different ISO setting, you need to correct the EV value accordingly. In the linked page there are the necessary details.

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  • \$\begingroup\$ OK so if I'm understanding this correctly, according to the article quoted, if I want an EV of 10, I need 2560lux coming from the scene. So that puts a lower bound on the amount of light needed? So I need 2560lumens/sqm going INTO the lightbox as a minimum? \$\endgroup\$
    – Ken
    Nov 20, 2014 at 18:28
  • \$\begingroup\$ So it seems, even if I must confess that it looks a bit too much, at first sight. You can adopt the more pragmatic approach described by Matt Grum and compare the results, it could be interesting if you post here your findings. \$\endgroup\$
    – Chosmos
    Nov 20, 2014 at 18:56

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