How do you know how much power to add to a flash in manual mode?

Hi.

as the title states. How do you?

Do yo simply take a few test shots and guess or do you know a genuine way to calculate teh correct flash power to apply to a given scene to make sure the flash does not overexpose the subject or underexpose?

Thanks

Good question, currently going through the same situation. Interested in the replies as well.

I'm reading the Speedliter's Handbook now (Syl Arena) and he says that he shoots without the flash first to set the ambient exposure, turns the flash on and takes a shot, and if necessary turns the power down by 1/2. Takes another shot and adjusts up or down by another 1/2 and continues to take test shots until he has the exposure dialed in. That's paraphrasing and he also uses EC and FEC as necessary to fine tune the results but that's the gist of his methodology for manual flash use.

Mike

us old timers used to use guide numbers

http://en.wikipedia.or​g/wiki/Guide_number

now i generally just use experience and check the histogram.

I think you do learn to guestimate pretty well over time, but a more direct way is to look at the histogram. If your camera is like mine, the histogram has a verticle grid where the lines are one stop apart. So you can take a test shot and see pretty well how many stops under or over you are, and adjust accordingly. And in terms of flash power, one stop is cutting the power in half or doubling it, so the full stop points are full, 1/2, 1/4, 1/8, etc.

I think Arena's method of taking a no-flash picture to get the ambient the way you want it (which may be essentially black) is a good one. Then adjust flash power to get the final picture. But if the flash and the camera are in manual mode, then EC and FEC won't be involved, so I'm a bit confused about that reference in Mcoomer's post.

Of course the formal, correct way to do this is with a flash meter, where you actually measure the light at the subject and set things accordingly. But that's another level of expense, and some excellent photographers, including David Hobby, don't use them, mainly because they say you can just take a test shot and get the same information.

In my experience, if you learn how to shoot in manual, with no ETTL, you pretty quickly learn to ballpark it, and then adjust based on what the LCD picture and the histogram look like, but mainly the histogram.

Actually, for portraits, with the flash reasonably near the subject, 1/4 or 1/8 power is a reasonable place to start, just as a rule of thumb.

IMHO, the best and fastest way is with a light meter.

The smartass answer is - "Just enough".

JakAHearts wrote in post #15838398
IMHO, the best and fastest way is with a light meter.

The smartass answer is - "Just enough".

+1 for this

Assuming you have a good grasp on the whole ambient/flash dual nature of exposure then yeah you just chimp. Once you do it a few times you can get where you need to be quickly. I still think a light meter is your best friend as you really dial in total exposure and ratios with more precision.

I thought the distance u set on a flash is only for how narrow or how wide you want the flash to output?

So the ideal way is this:

1. do test shots and look at the histogram NOT THE LCD

2. Use a light meter?

I have not used a light meter before but here is how i assume it works. you set the shutter speed and ISO on the light meter, connect all your strobes and flashes together wirelessly and take a meter reading off teh subject using the light meter which triggers your strobes and flashes to fire, giving you the end result of what apparture is needed for correct exposure for this shot when the strobes and flashes have been triggered?

jonneymendoza wrote in post #15837972
Hi.

as the title states. How do you?

Do yo simply take a few test shots and guess or do you know a genuine way to calculate teh correct flash power to apply to a given scene to make sure the flash does not overexpose the subject or underexpose?

Thanks

jonneymendoza wrote in post #15838689
I thought the distance u set on a flash is only for how narrow or how wide you want the flash to output?

So the ideal way is this:

1. do test shots and look at the histogram NOT THE LCD

2. Use a light meter?

I have not used a light meter before but here is how i assume it works. you set the shutter speed and ISO on the light meter, connect all your strobes and flashes together wirelessly and take a meter reading off teh subject using the light meter which triggers your strobes and flashes to fire, giving you the end result of what apparture is needed for correct exposure for this shot when the strobes and flashes have been triggered?

What sort of scenario are you thinking of? One might be using an on-camera flash and another might be using studio flash units. I would typically use very different techniques for the two.

jonneymendoza wrote in post #15838689
I thought the distance u set on a flash is only for how narrow or how wide you want the flash to output?

So the ideal way is this:

1. do test shots and look at the histogram NOT THE LCD

2. Use a light meter?

I have not used a light meter before but here is how i assume it works. you set the shutter speed and ISO on the light meter, connect all your strobes and flashes together wirelessly and take a meter reading off teh subject using the light meter which triggers your strobes and flashes to fire, giving you the end result of what apparture is needed for correct exposure for this shot when the strobes and flashes have been triggered?

The problem with looking at the histogram is that you have two different exposure values when you are using flash. You have the ambient and the flash. If you rely only on the histogram it will keep you from overexposing the scene generally but it may not tell you the balance you want between the two parts of the scene.

I work like this. I select the aperture I want for creative reasons (DOF). I select a shutter speed that will work with that aperture (and ISO) to give me the background I want. Adjusting the shutter speed up or down to raise or lower the background depending on if I want to balance the flash with the scene or underexpose the background. Then add the lights. Using the flashmeter I test fire and see what the flash power reading is compared to the aperture I want to use. Then dial it up or down to match that. The nice thing about a good lightmeter like the sekonic L358 is that it tells you the percentage of exposure coming from your flash so you know where your balance is.

A good light meter will save you a ton of time.

SkipD wrote in post #15838726
What sort of scenario are you thinking of? One might be using an on-camera flash and another might be using studio flash units. I would typically use very different techniques for the two.

off camera flash

Without light meter, knowing that my speedlite will deliver approximately F8 at ISO 200 with the light about 8 feet from the subject I set the camera to those settings and adjust the shutter speed to get the ambient I want. Then do a quick test. If it looks too dark I open the aperture 1 stop and increase the shutter accordingly. If that's too light then I close down 1/3 stop and adjust shutter accordingly. Two test shots and I'm within 1/3 stop.

It would take longer with a light meter.

The key is knowing a baseline exposure and power setting for the speedlite.

I use a lightmeter. After using one for a while you pretty much know your settings. After that it is matter of small change in ss, ISO etc.