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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?

  • Out of interest, why aren't you using a tripod? – ElendilTheTall Nov 20 '14 at 13:00
  • 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. – Ken Nov 20 '14 at 13:46
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    It does have the advantage of drastically improving sharpness though... – ElendilTheTall Nov 20 '14 at 13:52
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    How long is a piece of string? – ElendilTheTall Nov 20 '14 at 16:03
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    With continuous lighting at 1/100s and ISO 100, yes, you are being unrealistic. – Robin Nov 26 '14 at 20:02
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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).

  • That kinds seems obvious now that you mention it. – Ken Nov 20 '14 at 19:09
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You may find useful this wikipedia page

You can calculate the exposure value with the given formula:

 EV = log2(N^2 / t) 

setting:

N = the f-number (aperture) you will use 
t = the exposure time you will use

they both depends 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 50mm lens, you should use 1/50 of sec))

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

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

edit:

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

  • 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? – Ken Nov 20 '14 at 18:28
  • 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. – Chosmos Nov 20 '14 at 18:56

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