How do I determine the light output of LED lights for close-up photographs of indoor plants?

Question

is there a method to determine the light output of LED lights needed to achieve a reasonable ISO/shutter/aperture combination to take close-up (not necessarily macro) photos of indoor plants given a fixed distance from light source to subject.

Answer 1

If you use a directional light, and we make some assumptions, we can roughly calculate the shutter speed for the ISO and f stop in your comment.

As a sanity check, here is the calculation for my test setup. It is a stop off, so some of the assumptions aren't correct. This is only a ballpark calculation anyway.

More comments on my test setup. A typical household interior has an EV of 5-7. With one 8W LED spotlight at 18 inches away, and a white reflector on the other side, I measured about EV 9 (based on my test exposure). You can get more powerful spotlights or you could use several. A distance of 18 inches is a little close, you probably want to be further away to get more even lighting. So, I am going to use EV 9 for the remainder of the calculations.

ISO   Aperture  Shutter
800   f8        1/60     With image stabilization, Handheld is OK 
400   f8        1/30     With image stabilization, handheld is marginal
400   f11       1/15     Need a tripod
100   f11       1/4      Need a tripod
200   f16       1/4      Need a tripod

You will need to decide if these are reasonable. See the Wikipedia article on Exposure Value for more information. With a larger or several lights, you should be able to get to EV 10 or 11.

I am assuming a simple setup, if you want a pure white background, you will want separate lights pointing at a white background.

Note that the Color Rendering Index (CRI) of household LEDs isn't very good. If color accuracy is important you should use something else.

I shoot a lot of flowers in my house. I use a speedlight and a small softbox on one side, and a white foamcore reflector on the other side (very close). If I want a dramatic image, I often put another speedlite in the back for a rimlight (need to be careful to shield the lens from this light). I use cheap Yongnuo RF flash triggers instead of sync cords.

Answer 2

Sunlight is exposure value = 15. It's about 100 000 lux.

So, with e.g. about 100 lux you get exposure value = 5, because every doubling of brightness increases exposure value by 1 and every halving of brightness reduces it by 1. 1000 is approximately 10 doublings (well, ok, to be exact, 10 doublings is 1024).

However, LED lights have their output specified in lumens, not in lux. The reason is simple. Lux measures the amount of light going to a square meter of a surface. You can't tell how many luxes one lumen is, because the environment modifies the light by scattering it around.

If you have a 100 lumen source, where the light goes only to a 1 square meter surface, you have 100 lux, or in other words, exposure value = 5. However, to get 100 lumens distributed to only 1 square meter surface needs some directionality. An omnidirectional LED with no light modifiers won't do it.

There's the sunny f/16 rule. With exposure value 15, you use ISO N, shutter speed 1/N and f/16. You can calculate the exposure from that. For example, with exposure value 5, if you use ISO 16N, shutter speed 1/N, f/2 (example: ISO 1600, shutter speed 1/100, f/2) you have the correct exposure.

To calculate how many luxes one lumen equates requires you to know how directional the light is. Lux is lumens per square meter. So, you need to know the full properties of the directionality of the light source.