Metering - How to account for the light from the flash

Question

I pretty much use Manual mode (in my Nikon D40) and learning quite a lot from it. Though I understand "Metering" for correct exposure, I've trouble understanding it while using Flash (both in-built as well as external flash, SB-600)

Say in a medium size room having few friends of mine with single source of light, the meter says, exposure is underexposed. I bring it back to 'correct' position my tweaking ISO, Aperture and Shutter Speed.

But that makes sense only with "existing" light in the room. But I'm going to take the photo "with flash". How metering works on such scenario.

Answer 1

Basically, you expose for the ambient ("existing") light however you want, and the flash will automagically provide the rest of the light needed for a proper exposure (for values of "proper" as decided by the vagaries of your camera's metering system).

Your flash photo in effect consists of two separate exposures; one that sucks in the ambient light in the room and one that captures the light blasted out from your flash. The sum total of the two is your photo; how you balance them decides the mood.

To take this to one extreme: If you do not want to use the ambient light at all, use a low ISO, a fast shutter speed, and a small aperture. The ambient part of the exposure will come out pretty much black, and you will have an almost totally flash-lit picture. (It will probably be rather sucky.)

To take it to the other extreme: If you want to rely almost entirely on the ambient light, use a high ISO, a slow shutter speed and a large aperture. The flash will now only give a very little 'lift' to the total exposure of the photo, and you will get spades of atmosphere from your candle-lit dinner or whatever it is.

Properly balancing the ambient part and the flash part of your total lighting is much of the trick of getting good flash photos!

Answer 2

It depends on a couple of things, but basically there are a few scenarios. I'll explain first what they are, and then why you might choose one over the other.

Using the Flash as a primary light source

If flash is your primary light source, and you have ETTL (Or whatever Nikon's equivalent is), then what will happen is your flash will become your dominant light source, and the correct amount of flash will happen via the ETTL protocols.

Using the flash as a fill flash

If your image is already exposed properly, then your flash will simple give a taste of a flash so as to overcome the darker areas in the image. It's perfectly acceptable to use the flash in such a scenario, it will lead to a less contrasty image, and will probably look better with your friends, as there will be fewer shadows under the eyes and similar phenomena.

How to choose which mode

So, assuming you have ETTL, your image should be correctly exposed, so long as you aren't over exposed. The question then remains, what do you set your ISO, aperture, and shutter speed to? There are a few factors, so let me try and explain.

1. If you have distant, weak background light that you want to keep exposed, and if shutter speed isn't much of an object (IE, the subject won't move, or there is enough light to minimize any movement that might exist), then you probably want to use your flash primarily as a fill flash. If your subject is much darker than the background, then just expose for the background and set your flash in ETTL mode, and it'll expose it correctly.
2. Aperture is an artistic decision, as always. Flash is just another knob, but it won't really change anything.
3. If you want to light your subject, but don't care much about the background, or your subject is close to the background, then I recommend that you set your Aperture and Shutter Speed so as to minimize camera shake and get the desired affect, and let the flash light the scene. Look at other posts to see how to use the flash to light the scene, but that's the general idea.
4. If the scene is very underexposed, then the flash will cause the foreground object to be lit, but the background to appear black (Unless the background was brightly lit, or close to the subject and in the direction of the flash). The more underexposed, the more black the background will appear.

I usually set my camera to manual mode and intentionally underexpose when I'm indoors or outdoors at night, trying to underexpose by about 1-2 stops at most, unless it's a high light situation and I'm cranking up the ISO/Aperture to make sure I have enough light. I usually try to expose correctly when I'm outdoors, and just use the flash to fill in some shadows.

Hope this helps!

Answer 3

Flash exposure is a more complex subject than I can answer but I believe the basics are that you mostly ignore ambient light when using flash as your main source of lighting.

Either you use a dedicated flash that your camera communicates with, in which case your camera controls the flash exposure. For example, you'd choose an aperture and the camera will choose the flash interval.

For fully manual flash exposure you have to use the guide number for the flashgun. Older flashguns had a scale on the back which would tell you what aperture to use for any given subject distance at any selected ISO.

There's an article at Sublime Light which illustrates flashgun usage for beginners.