Strobe Guide Number - Is it enough power?

I've been trying to figure out how Guide numbers work.

Let's say I want to light an object that is 100 ft away from the strobe on a bright sunny day. The sunlight is coming from behind the strobe and I want to use the strobe to brighten the subject by 1 stop.

Using the sunny 16 rule, I'm at:
Iso 100, Aperture f/16, Shutter Speed 1/100

I want to underexpose ambient by 1 stop
Iso 100, Aperture f/16, Shutter Speed 1/200

If I assume no ambient light, then at f/16, I would need a massive strobe guide number of 1500 to properly expose the subject. Now I'm stuck in the calculation. How do I figure out what guide number would I need from the strobe to properly light the subject in combination with the ambient?

Think about it this way:
One light source is providing proper exposure at F16, ISO 100, 1/100 (Lets say its a big fresnel spot on a movie set. The director says He wants to double the light on the subject. So you get another large fresnel spot with the same light output and place it immediately next to the first one. Twice the light.

You're doing the same thing with your example.

Now, because the constant light can be halved by increasing the shutter speed, you are getting and extra EV of output from the strobe because it is shutter speed independent. (presuming x sync is 1/200 or higher.

Try it with a strobe and room light.

I am not sure where you're trying to go (or why) with all this Guide Number calculations but here is a website with a GN calculator that may be helpful: http://www.scantips.co​m/lights/flashbasics1c​.html

A Sekonic light meter will quickly tell you the % of contribution of the flash in the total exposure. In the lower part of the display is a scale that shows the f stops of difference between the ambient and flash meter readings.

In my recreation of the picture in the Manual you can see that my meter is showing an ambient reading of f/3.2 and the flash is f/5.0, contributing 70% to the total metered exposure which is f/5.6. The difference between the ambient and flash is +1 1/3 stops. Much easier.

100 feet is tooo far. May I know what you want to shoot at that distance?

nixland wrote in post #18241801
100 feet is tooo far.

Yup.

remote on the flash and place it close to the subject.

The subject and distance aren't exactly the important part of the question. It was more the method.

From dmward's post, I think the way I should be thinking about it is:
1. I'm properly exposed at Iso 100, Aperture f/16, Shutter Speed 1/100
1a. If I needed to provide this lighting from a strobe at 100ft, then I would need a strobe capable of a guide number of 1500
2. If I increase the shutter speed to 1/200, I'm now 1 stop underexposed.
2a. This is the equivalent of having the photo properly exposed at f/11, but setting the camera at f/16
2b. putting this information into the site noted above, means the sunlight is providing the equivalent of a guide number of 1100.
3. To achieve a guide number of 1500, with one light at 1100, I need to add another light with a guide number of around 1000 to achieve an overall guide number of 1500 a^2+b^2=c^2

If you want to illuminate a huge target at 100 feet with a flash source, you're going to need one heck of a powerful flash unit. On the other hand, if you're illuminating a small target at 100 feet, you might be able to get away with a much smaller flash source and a device that focuses the light into a very narrow beam.

Rather than fool around with calculations, some experimentation would probably show you more. Using a handheld light meter that can measure light from a flash unit and a helper would be the fastest and most accurate way to experiment.

s1a1om wrote in post #18242149
The subject and distance aren't exactly the important part of the question. It was more the method.

From dmward's post, I think the way I should be thinking about it is:
1. I'm properly exposed at Iso 100, Aperture f/16, Shutter Speed 1/100
1a. If I needed to provide this lighting from a strobe at 100ft, then I would need a strobe capable of a guide number of 1500
2. If I increase the shutter speed to 1/200, I'm now 1 stop underexposed.
2a. This is the equivalent of having the photo properly exposed at f/11, but setting the camera at f/16
2b. putting this information into the site noted above, means the sunlight is providing the equivalent of a guide number of 1100.
3. To achieve a guide number of 1500, with one light at 1100, I need to add another light with a guide number of around 1000 to achieve an overall guide number of 1500 a^2+b^2=c^2

Almost...
100' * f/16 = GN1600
Increasing shutter speed to 1/200 effectively underexposes ambient by -1EV (1/200 f/16 vs. Sunny 16) , while the flash gives sufficient subject illumination.
Ignore the sun 'guide number'... you already know its correct exposure via Sunny 16 and you know you are underexposing by -1EV
Yes, the flash is 'properly exposing' by itself ('1 part' of light), so any contribution by ambient at 1/200 is effectively putting an additional '1/2 part' of light, so the subject is getting 1.5 parts of light...if you wanted to compensate that you could choose an aperture -0.5EV smaller, but I would not bother.

If you have GN1100 flash, that is 100/ f/11 and you want f/16, so you want to double the light!...you need a second GN1100 flash unit!!

SkipD wrote in post #18242160
Rather than fool around with calculations, some experimentation would probably show you more. Using a handheld light meter that can measure light from a flash unit and a helper would be the fastest and most accurate way to experiment.

As an engineer, it's in my nature to try and understand the math behind everything. It's a bit of a curse. I come home from work and play with Solidworks (3d modeling) and Octave (computational analysis) for fun.

Wilt wrote in post #18242273
Almost...
100' * f/16 = GN1600
Increasing shutter speed to 1/200 effectively underexposes ambient by -1EV (1/200 f/16 vs. Sunny 16) , while the flash gives sufficient subject illumination.
Ignore the sun 'guide number'... you already know its correct exposure via Sunny 16 and you know you are underexposing by -1EV
Yes, the flash is 'properly exposing' by itself ('1 part' of light), so any contribution by ambient at 1/200 is effectively putting an additional '1/2 part' of light, so the subject is getting 1.5 parts of light...if you wanted to compensate that you could choose an aperture -0.5EV smaller, but I would not bother.

If you have GN1100 flash, that is 100/ f/11 and you want f/16, so you want to double the light!...you need a second GN1100 flash unit!!

It took me a few attempts at reading that to figure out what you were saying, but I think the gist is that you wouldn't bother accounting for the affect of ambient on the subject and would be ok with overexposing the subject by 50%. Either that, or decreasing the aperture 50% and letting the background be slightly underexposed. Either way, I think we are reaching similar conclusions.

The only thing I question with your response is needing a second GN1100 flash. My reading suggests that the total guide number of a series of flashes is the sum of the squares of the guide numbers. That is:
Guide_Number_total^2 = Guide_Number_1^2 + Guide_Number_2^2

That would mean I'd need 4 GN1100 flashes. Talk about some serious $$$$.

s1a1om wrote in post #18242588
That would mean I'd need 4 GN1100 flashes. Talk about some serious $$$$.

I've heard that some manufacturers of flash equipment tend to over-rate their guide numbers. In the end, you'd have to experiment for sure.

When flash and ambient are within 4 stops or so of each other then they have an accumulative effect on the over all exposure. So the total exposure value will be greater than any one of the individual exposures. Usually when more than 4 stops or so apart, then the greater one will be contributing 100% of the exposure. When they are both contributing 50% to the overall exposure then they will be of equal exposure but the total exposure will be one stop greater than the two individual exposures regardless of Guide Numbers or sunny 16 rules.

Any thinking, in this scenario, is flawed if you want to disregard the ambient exposure as having no effect on the overall exposure. Exposure is exposure whether it comes from ambient or flash. You have only lowered the ambient exposure by one stop from the final exposure you want. So you need to add a flash exposure of F/11 to arrive at an overall exposure of f/16 when you combine the flash exposure and ambient exposure which are both contributing 50%. An ambient exposure of f/11 when combined with a flash exposure of f/11 results in a final exposure of f/16.

In the 2 images below I have recreated your scenario. So I took an ambient meter reading in my office and at 2000 iso, 1/100 shutter I showed f/4 (funny 4 rule) as my aperture and final exposure. This is displayed in the first image. This corresponds to your 100 iso, 1/100 shutter and f/16 (sunny 16 rule).

I then lowered my ambient exposure by 1 stop by increasing my shutter speed by 1 stop to 1/200. I then added flash till I achieved the f/4 exposure we had in the first ambient only exposure. You can see in the second image that the flash is contributing 50% of the overall exposure while ambient is contributing the other 50%. It just so happens that there is only one little dot on the bottom scale that represents both the ambient exposure as well as the flash exposure because they are both the same at f/2.8.

To answer what I think was the question, you need one flash with a guide number of 1100 or 2 flash units each with a Guide Number of 800 or 4 flash units each with a Guide Number of 560 or 8 flash units each with a Guide Number of 400 or 16 flash units each with a Guide Number of 280 or 32 flash units each with a Guide Number of 200 or 64 flash units each with a Guide Number of 140 to give you an f/11 flash exposure to combine with your ambient exposure of f/11 to give you a total exposure of f/16.

That's all simple math but once again it is so much easier to just use a meter and move the flash closer.

Much cheaper also to use one or two AB1600 at a few feet than 64 Canon 600EX-RT's, set to 1/2 power at a 200mm zoom setting, at 100 feet. You could use 1/1 power at a 50mm zoom setting but you'll get faster recycle time at 1/2 power.

Your looking at it wrong. Gen are usually used for comparisons not shooting. Ev i.e. Exposure Values are a better thing to learn A light will give different amount of ev at different distances from the subject. Let's say you put your light at full power at 1.4 meters then you move it back to 2.8 meters the power of the light is still at full power but your on getting half the power of the light than at full the closer position. Learn by reading a book on basic lighting.

Blackey Cole wrote in post #18242846
Your looking at it wrong. Gen are usually used for comparisons not shooting. Ev i.e. Exposure Values are a better thing to learn A light will give different amount of ev at different distances from the subject. Let's say you put your light at full power at 1.4 meters then you move it back to 2.8 meters the power of the light is still at full power but your on getting half the power of the light than at full the closer position. Learn by reading a book on basic lighting.

You of course meant "quarter the power of the light".

Forget all this G/N stuff. If you shooting something like portraits with Ambient light ad strobes, you need to start with 600ws for minimum. I would want 1200ws but then it gets heavy. High efficiency reflectors will help but sometimes you want different look and having the power is nice, particularly when using double diffused softbox/octobox etc.

bobbyz wrote in post #18243113
Forget all this G/N stuff. If you shooting something like portraits with Ambient light ad strobes, you need to start with 600ws for minimum. I would want 1200ws but then it gets heavy. High efficiency reflectors will help but sometimes you want different look and having the power is nice, particularly when using double diffused softbox/octobox etc.

True. Ive ended up with 3 1000ws strobes. The rest are sub 1000. Having the WS available makes outdoor shoots "better" or rather they provide more flexibility in modifier and look.