Trouble with ambient light and flash

Ok I have two questions. Say I want to use my flash as a 30% fill to the ambient. The ambient is 1/200, F2.8 ISO 200
For me to get a 30% fill on shadows I would go a 1/3 stop higher at F3.2 on flash? 60% would be f3.5 right?

Other question is when outside at night trying to get my background ambient light brightened up I should set my aperture where I want it, shutter at fastest? And use ISO to brighten background?

keano12 wrote in post #17807120
Ok I have two questions. Say I want to use my flash as a 30% fill to the ambient. The ambient is 1/200, F2.8 ISO 200. For me to get a 30% fill on shadows I would go a 1/3 stop higher at F3.2 on flash? 60% would be f3.5 right?

I have no clue about calculating the exposure because I'd simply either use my meter to get a stop or two difference or make a guess and a trial shot or two.

However, you've possibly overlooked something that is very important. Unless your "ambient" light is made up of mostly sunlight, you will very likely have a color issue. Mixing different colored light sources (incandescent ambient and your flash, for example) causes all sorts of nearly impossible-to-correct issues. You will probably want to use a gel on your flash to have the color qualities of the flash emulate the color qualities of the ambient light.

keano12 wrote in post #17807120
Other question is when outside at night trying to get my background ambient light brightened up I should set my aperture where I want it, shutter at fastest? And use ISO to brighten background?

This is not a good approach.

First off, changing the ISO setting would affect both the elements of the scene lit by the flash and the elements of the scene lit by ambient light equally. In other words, the ratio of foreground (flash) and background (ambient) would stay the same regardless of the setting.

Use the shutter speed to affect the background level while keeping the same aperture and ISO (set for the flash exposure).

Let's step away from specific ratios a moment. Unless you are using a flashmeter that calulates ratios getting a specific percentage is not as simple as you think.

All things being equal you control ambient via the shutter speed. It's the only control that doesnt affect the flash contribution. If you change aperture or ISO it changes the flash exposure, but because the flash is delivered in a brief burst changing the Shutter speed doesn't affect it. So the longer the shutter speed the more ambient light you can get in, the shorter the less ambient you get. Of course there are 2 limitations on this method, the first being the limits of max sync speed of your camera. The second is if you slow your shutter speed enough eventually you will overexpose the image (but for night shots that's not really an issue).

Now back to the original question. I'm not sure what you mean by setting the flash to 3.2.

If the background is static then think in time to keep noise down.

Lots of good online stuff out ther on the "balancing act" of adjusting flash and ambient lighting. One good explanation is at http://www.cambridgein​colour.com/tutorials/c​amera-flash-2.htm

+1

and maybe:

http://strobist.blogsp​ot.co.uk …-balancing-flash-and.html
( . . there's a part two, as well.)

Given that I am working in full Manual mode here. When I balance ambient and flash, I start at 1/125 and enough aperture to cover whatever DOF I want. I dial in ISO until I get the balance I want. If it goes over 6400, I'll drop the shutter speed. In average home indoor lighting, I usually end up between 800 and 3200, depending on lighting and subject to BG distance. It usually takes 3-5 shots to get it right. But sometimes it doesn't work at all.

gonzogolf wrote in post #17807134
Now back to the original question. I'm not sure what you mean by setting the flash to 3.2.

I'm assuming he means adjusting the flash power output to the point where his incident meter (in flash mode) meters its contribution at f/3.2, at whatever ISO he's using. That's the only interpretation that would make sense to me.

Fair Warning, pedantic TL;DR answer for those who actually want to know how to add ambient and flash meter values if your light meter doesn't do it for you.

keano12 wrote in post #17807120
Ok I have two questions. Say I want to use my flash as a 30% fill to the ambient. The ambient is 1/200, F2.8 ISO 200
For me to get a 30% fill on shadows I would go a 1/3 stop higher at F3.2 on flash? 60% would be f3.5 right?

If the flash is also going to illuminate the part of the subject you're trying to expose correctly (e.g. a person's face), rather than just add light to a shadowy area elsewhere in the scene, you can't just add flash without overexposure. It depends on what you mean by "fill". I'll assume the more difficult case, which is that you have a shadow on your subject from directional ambient light, and wish to even it out by replacing 30% of the ambient light on your subject with flash from another direction to more evenly illuminate the subject.

Lets start simple and say you want ambient to fill flash in a 30/70 ratio. The ambient light meters 1/125 f/2.8 ISO200. I've changed the shutter speed from your example to deliberately be below sync speed, I'll go into more detail on this below. To move to 30% flash, you need to decrease the ambient light contribution by 30% (about half a stop) and replace it with flash. To reduce the contribution from ambient light by 1/2 stop, you have any of the following options (there are other options, but these are the most common with a DSLR):

1. Increase shutter speed by 1/2 stop from 1/125 to 1/180.
2. Lower ISO 1/2 stop (e.g. from ISO 200 to ~ISO 140) if your camera can adjust ISO in half stops.
3. Put a 1/2-stop ND filter on your lens
4. Stop down 1/2 stop, from f/2.8 to f/3.5
5. Some combination of 1,2,3,4 for a net reduction of 1/2 stop. See notes below about which of these affect flash exposure.
6. High-Speed sync solutions that allow higher shutter speed, to complicated to calculate in this example and not readily meter-able (chimping/histogram time)

As long as your shutter speed stays below your x-sync speed, option 1 is the easiest to deal with because it won't affect flash exposure. Options 2 through 4 affect flash exposure and ambient equally, and option 5 will affect flash exposure as much as the sum of non-shutter-speed components (easiest to just re-meter if you adjust anything other than shutter speed).

Assuming you start with a shutter speed that is at least a 1/2 stop slower than x-sync, such that you can use shutter speed alone to drop ambient contribution 1/2 stop, then what remains is to set your flash to provide the missing 1/2-stop of light. What should the flash output meter for these to add up correct? Well, if I were doing the math in my head, on location, I'd fudge 70/30 at this point to a 2:1 ratio (66.6/33.3) and just set the flash to meter 1-stop below ambient. We reduced ambient 1/2 stop, so that meter reading we got that said f/2.8 tells us the contribution is now equivalent to ~f/2.4. A stop below that is ~f/1.7, so that's what you'd want the flash to meter at. Most light meters would display this as 1.45

When you have independent meter readings for multiple light sources at the same point in space, in the form of the f-stop denominators, i.e. "2.8", the way the lights add up is the square root of the sum of the squares of these numbers. So, two sources each metering f/2 firing together would give f/2.8. sqrt(2^2+2^2) = 2 * sqrt(2) = 2.828... The ratio of the strength of the lights in the ratio of the squares of these numbers, so 2.8^2/2^2 = 2, the ratio in stops is the base-2 logarithm of this, so 1 stop in this example, log2(2) = 1.

If you actually solve these equation simultaneously for the precise values that give a 70/30 ratio and total exposure that would meter at 1/125 f/2.8 ISO200, which is solving a^2/b^2 = 70/30 and sqrt(a^2+b^2) = 2.8 using the quadratic formula, you get the values f/2.345 and f/1.535, so in practice, you'd target those with your light meter. Given the latitude of a digital RAW file, you don't actually need to get out the calculator or spreadsheet in practice, you can just change the values on your light meter and get in the right ballpark.

Of course, a light meter that calculates ambient to flash for you as a percentage does all this math internally and makes this all trivial.

Now, what about the original example, where the metered shutter speed was 1/200, possibly with no room to make it any faster due to it being the x-sync speed (fastest shutter speed at which the whole sensor or film frame is exposed all at once). If this is the case, you simply need to work with a smaller aperture (say, f/4), lower ISO (if available) or add an ND filter if you really need to stay at f/2.8 and your flash doesn't support any form of High Speed Sync, hypersync, etc.

keano12 wrote in post #17807120
Other question is when outside at night trying to get my background ambient light brightened up I should set my aperture where I want it, shutter at fastest? And use ISO to brighten background?

Depends on whether or not you have a moving subject, tripod etc. If you are trying to capture a moving subject without blur and with only available light, higher ISO is your only option when you can't open the aperture any wider. If nothing is moving, or some blur of things is acceptable (like things swaying in breeze) then a longer shutter speed (using IS or tripod if needed) is generally going to get you a lower-noise shot.

Just remember that if you concern about the background exposure, adjusting the ambient exposure of the subject by changing the shutter speed, automatically change the exposure or mood of the background too.

I don't disagree with anything anyone has said here but isn't there a simpler solution available by letting the computers in the camera and flash (assuming they are capable) assist us? Leave your camera's manual settings and set FEC to -1/3 using ETTL with the flash. Wouldn't that get us really close?

GeeMack wrote in post #17808067
I don't disagree with anything anyone has said here but isn't there a simpler solution available by letting the computers in the camera and flash (assuming they are capable) assist us? Leave your camera's manual settings and set FEC to -1/3 using ETTL with the flash. Wouldn't that get us really close?

Exposure consistency is the main downfall of this method.

GeeMack wrote in post #17808067
I don't disagree with anything anyone has said here but isn't there a simpler solution available by letting the computers in the camera and flash (assuming they are capable) assist us? Leave your camera's manual settings and set FEC to -1/3 using ETTL with the flash. Wouldn't that get us really close?

To the best of my knowledge, there is nothing in our cameras or our flash units that would allow for dialing in a flash-to-ambient ratio and automatically setting things up for it in the camera. This would take some VERY sophisticated sensors and software to do. For one thing, the flash exposure calculations (using ETTL) are only made after firing a test "pre-flash" burst so the camera can analyze the light that's needed from the flash. In addition, there are separate bias values for Exposure Compensation (ambient light) and Flash Exposure Compensation (light from ETTL flash unit).

What you want would require a complex modification of the camera's software as well a complex operation procedure and I doubt that the manufacturer would feel that there are enough customers who would really want the feature to warrant the development expense and the associated higher price of the equipment.

Have a look at this Keano12.

https://www.learn.usa.​canon.com …ash_use_EOS_art​icle.shtml

GeeMack wrote in post #17808561
Have a look at this Keano12.

https://www.learn.usa.​canon.com …ash_use_EOS_art​icle.shtml

You seem to be presuming that he's using canon speedlites. Nothing to indicate that in his post.