Balancing ambient light with manual flash -- question about manual flash settings.

[jwbryson1]

Neil van Niekerk says in his book, Off Camera Flash, that when balancing ambient light with manual flash there are several approaches that can be used. One such approach is if you already have the correct exposure for the ambient light and just need fill flash for the subject. In this case, he suggests metering the flash 2 to 3 stops under the ambient light to use the flash for fill.

So, let's say you have metered for the ambient light and you wish to use manual flash for fill light. If you are familiar with using manual flash, you know that you can set the flash fire power at 1/1, 1/2, 1/4....1/64 down to 1/128. These are the flashes fire power.

The question is this -- Assuming we do not have a light meter, how do you know based on the fire power of the speedlight where you are in terms of STOPS below the ambient exposure? Stated another way, is there a "standard" way of determining what fire power is a stop below the ambient exposure? For example, if you have determined that the proper exposure based on ambient light is f/4, ISO 100, and you need the fill light 2 stops BELOW that, where do you start in terms of power settings to get 2 or 3 stops below? Is it trial and error? Do you try 1/16 and look at the LCD screen and make adjustments from there? Is there a more simple way?

Does anybody even know what I'm talking about? If you have the book, turn to page 48 and look under the left column entitled "Two Broad Scenarios" "If we Just Need Fill Flash" and you will see his discussion.

Thanks for looking. :thumbup:

 

[tirediron]

Guide numbers are your friend! But... considering how little a good, used flash meter can be had for, why wouldn't you have one?

 

[Ysarex]

Proper flash exposure is a function of flash power and flash to subject distance (for a given ISO).

Tirediron is correct; you want a guide number for your flash. Let's say you flash has a guide number of 160 for ISO 100. That's all you'd need to know along with the distance from the flash to the subject. The guide number divided by distance = f/stop.

I had a student a couple years ago who managed to get and internship with a local Pro portrait photographer. She was going to try her first outdoor fill-flash shoot and he kindly lent her one of his portable strobes, battery and stand. Then to help her manage exposure he tied a string to the light stand and put knots in the string for each full f/stop. All she had to do was pull the string from the stand to her subjects face and count the knots.

Let's try a walk through. You've got the ISO at 200 and your flash is the one noted above with a GN of 160 for ISO 100. You determine the ambient light exposure for your scene is 1/250 sec at f/11 (ISO 200) and you can't raise the shutter speed because 1/250 is your sync speed. You put the flash 15 feet from your subject on a light stand.

160/15 = 10.6 or let's round that to f/11. But you ISO is 200 so the correct flash exposure would be f/16. You want the flash 2 stops less than the ambient light so you really want to flash exposure around f/5.6. That's 3 stops difference from the f/16 exposure based on the flash power over distance so you turn the flash down to 1/8 power.

Joe

 

[tirediron]

... Then to help her manage exposure he tied a string to the light stand and put knots in the string for each full f/stop. All she had to do was pull the string from the stand to her subjects face and count the knots...

That's f**king brilliant!

 

[fjrabon]

one issue that I think is clouding your thinking is that the setting depends on what your equivalent exposure settings are.

What I mean is this, if your exposed your background, and you got f/2.8, SS 1/120 ISO 100 (we will call this EXPOSURE A for reference later). Now, we know that f/4, SS 1/60, ISO 100 (exposure B) would produce an exactly equivalently exposed background, since all we did was slow the shutter by a stop and close the f/stop by a stop, canceling one another out.

HOWEVER, how flash would imapct these two photos is drastically different. Let's say you just used trial and error, and realized that in exposure A, 1/16 power on your flash gave you what you were looking for as a fill light for your subject. However, in exposure B, 1/16 power wouldn't be enough. Flash doesn't care about shutter speed. Because in exposure B, your ISO is the same, and your aperture a stop darker, you'd need flash power 1/8 to properly fill your subject in exposure B. Two equivalent exposures of ambient will react to flash differently, because shutter speed below the sync speed doesn't affect the exposure due to flash.

The solution is what Ysarex laid out. you have to use guide numbers and work the math backwards from there. Which means that you must have the f/stop transitions memorized and are very good at doing math in your head. Most people find this is easier done by either a light meter or a histogram and LCD.

 

[Derrel]

This is a fantastic scenario in which to use the OLD-school method of artificially elevating the ISO on the flash to cause deliberate under-exposure. A Vivitar 285 HV, set to an AUTO-Thyristor flash mode, but with the ISO artificially BOOSTED two stops, will provide two stops' less fill-flash.

Exposure is often set for and figured for ISO 100 in MANY bright-daylight flash situations because of obvious flash synch limitations of focal plane shutters; if the FLASH is commanded to fire at ISO 400, then the flash output is two f/stops (or two EV values) BELOW the ambient. Give this a trial and in ONE afternoon, you can get a feel for how this works in the real world. INFLATE the ISO setting on the flash in order to make it put out less light, for gentle fill-in lighting of the shadows.

 

[jwbryson1]

I'm working with 2 YN560 II speedlights with a GN of 58 at 100 ISO and 105mm, and a single SB-700 with a GN of 92' at 35mm.

Looking at the examples above, a GN of 58 seems pretty weak! :er: I thought that YN 560 II was a fairly powerful speedlight.

How much is the GN affected when using a bounce umbrella?

 

[fjrabon]

58 is a pretty decent guide number. speedlights as a whole don't have a whole lot of power.

edit: and how much the reflective umbrella takes from the guide number depends on how far your flash is from the umbrella.

 

[Derrel]

I'm working with 2 YN560 II speedlights with a GN of 58 at 100 ISO and 105mm, and a single SB-700 with a GN of 92' at 35mm.

Looking at the examples above, a GN of 58 seems pretty weak! :er: I thought that YN 560 II was a fairly powerful speedlight.

How much is the GN affected when using a bounce umbrella?

With the flash zoomed to a tight, concentrated beam-spread setting for a 105mm lens angle of view, a GN of 58, at ISO 100, if that GN is in FEET, is pretty WEAK...

At 10 feet, that would mean an exposure of f/5.8 at ISO 100, with the flash zoomed to a tight 105mm lens angle of view beam...

Is it possible that the Yongy's GN is spec'd in Metres??? Because at 105mm zoom, at ISO 100, a GN of 58 in FEET is...gutless!!!!

 

[fjrabon]

I'm working with 2 YN560 II speedlights with a GN of 58 at 100 ISO and 105mm, and a single SB-700 with a GN of 92' at 35mm.

Looking at the examples above, a GN of 58 seems pretty weak! :er: I thought that YN 560 II was a fairly powerful speedlight.

How much is the GN affected when using a bounce umbrella?

With the flash zoomed to a tight, concentrated beam-spread setting for a 105mm lens angle of view, a GN of 58, at ISO 100, if that GN is in FEET, is pretty WEAK...

At 10 feet, that would mean an exposure of f/5.8 at ISO 100, with the flash zoomed to a tight 105mm lens angle of view beam...

Is it possible that the Yongy's GN is spec'd in Metres??? Because at 105mm zoom, at ISO 100, a GN of 58 in FEET is...gutless!!!!

It's almost certainly in meters. At 35mm, ISO 100, the YN560 is more powerful than the Nikon SB910

Speedlight Feature Comparison Side By Side - Strobist Data i-TTL E-TTL | Speedlights.net

 

[Derrel]

I REALLY,REALLY,REALLY wish the flash manufacturers would specify their flash Guide Numbers PROPERLY, every time. Not stating GN's in Meters, not stating GN in both Metres and in Feet, stating GN's ONLY in Feet...the lack of proper specifications is a huge issue when shopping on-line, and so on. Imagine if the car makers told us the Chevy Silverado was spec'd at 154. What? Miles per hour? Cubic feet of cargo space? Or that we were buying airline tickets in Pesos, and the cost of the tickets would be 15 million units...or that at 5,000 RPM Engine X generated 349. 349 what? Horsepower? Foot pounds of torque? Simply stating a Guide Number, with absolutely NO reference to feet or to meters is effin' stupid...and yet, I see a lot of listings that way on Amazon...

 

[fjrabon]

I REALLY,REALLY,REALLY wish the flash manufacturers would specify their flash Guide Numbers PROPERLY, every time. Not stating GN's in Meters, not stating GN in both Metres and in Feet, stating GN's ONLY in Feet...the lack of proper specifications is a huge issue when shopping on-line, and so on. Imagine if the car makers told us the Chevy Silverado was spec'd at 154. What? Miles per hour? Cubic feet of cargo space? Or that we were buying airline tickets in Pesos, and the cost of the tickets would be 15 million units...or that at 5,000 RPM Engine X generated 349. 349 what? Horsepower? Foot pounds of torque? Simply stating a Guide Number, with absolutely NO reference to feet or to meters is effin' stupid...and yet, I see a lot of listings that way on Amazon...

Yeah, they also will specify their guide numbers at all sorts of varying lengths. sometimes it's at the flash's longest length, sometimes it's at 35mm. Researching speedlights is one of the more frustrating experiences ever. There have been times when I was literally on the phone with a B&H associate, where eventually they just grabbed one off the shelf and metered the thing their self because nobody could figure out exactly how powerful the flash was.

 

[jwbryson1]

My bad. I looked on Amazon and didn't notice the meters reference. :meh:

Its GN is 58 Meters at 100 ISO, 105mm.