If I have a light that gives me +5 EV, what is the EV for 2 or 3 of the same light at the same distance? I tried working it out and it seems I simply add the EV change of the additional lights. So 2 gives me +10 EV and 3 gives me +15EV, but it seems like an improbably large amount. Anyone have the answer, and ideally the maths behind it? Thanks Dogknees
I would come to the same conclusion that you have. If you have two sources of light in the same place, surely they produce double the amount of light that a single source can provide. No?
It's easier when you think in terms of stops. Each stop (example is f4 to f2.8) is doubling the amount of light. So if you have one light and then add an additional one of the same power at the same distance, you double the light. Where it gets tricky is when you add a third light, you are not doubling the amount of light, you are increasing it by only 50%. If you think of it in those terms it's a lot easier to calculate. You would need 4 lights in total to double the amount of light yet again in terms of stops. 1 light is your baseline (1/250 at f2.8 iso 100) 2 lights is 1 stop more (1/250 at f4 iso 100) 4 lights is 1 more stop (1/250 at f5.6 iso 100) This is why strobes have the ability to control the power of the burst so you can get much more control of lighting as compared to constant lighting.
Thanks for the responses. eddie_dane - What if I start with ambient at say f5.6 @ 2s. If using one light takes me to f5.6 @ 1/120s, What do I get for 2, 3, 4,... of these lights? Thanks dogknees
You'll get 1 extra stop for each doubling of lights. your single light is giving you 8 stops over your ambient so ignore ambient in this case as you've pretty much killed it. 2 lights will now get you f5.6 @ 1/240s or f8 @1/120s 4 lights will now get you f5.6 @ 1/480s or f11 @ 1/120s 8 lights will now get you f5.6 @ 1/960s or f16 @ 1/120s your 3 lights would get you 1.5 stops over the base line so f5.6 @ 1/360s or f6.7 @1/120s so each time you double the number of lights you get 1 extra stop on the camera.
It's also worth noting the difference between constant light vs strobe lighting. Exposures using strobe lighting is controlled by the aperture (f stop). Another important caveat is that some cameras can't sync with strobes over a certain shutter speed. For example, my dSLR can't sync with the strobe over 1/250th of a second. But if you are using constant (or ambient) light, you can also control that with shutter speed. This mostly relevant when you consider your subject. If you are doing a still subject and you have a tripod, a lot of this "adding more light" can be mitigated by the use of a slower shutter speed and a tripod. For example. If you are shooting at 1/120 - f8 - iso 100 and you change your settings to 1/60th - f8 - iso 100, you have essentially doubled the light on your subject. Changing the shutter speed has a dramatic effect on the ambient light in an exposure. It would help to know more specifics about what you are trying to do.
Thanks again. I thought about it after posting and realised what the answer was going to be. Isn't it strange how we think after posting, not before! I'm using some small led lights for macro work so I can get more depth of field. They're far brighter than I'd expected, so the question was kind of academic. Regards dogknees
No worries, stuff occurs to me after a while more often than not. Since LED's are constant, to get more depth of field you will need to close down your aperture (use a higher number). Since aperture has less to do with exposure under constant lighting, you will need to compensate the smaller aperture with a longer exposure time. If you can steady your camera and your subject isn't moving, it should work out great for you.
That's where I'm heading. Need to use a smaller f-stop (bigger number) to get some more depth of field. Already using the tripod and am in the process of building a simple rail to make it easier to adjust the focal distance. The LED torches are getting attached to my "macro-pod". I'm sure you've seen the Gorilla Pod range. Imagine inverting the GP tripod and attaching it by the top to a normal tripod just below the camera. Now attach the LED torches to the ends of the arms and you have a set of 3 lights on the end of "octopus arms" that you can point exactly where you want them. I'm loving this macro stuff. Regards PS I make no claim on the idea of the gorilla-pod macro lighting thing and any one is welcome to use it. I'm sure I'm not the first to do this but I couldn't find anything online.
