How do I predict exposure when using flash?

Question

So I've got the basics down for the exposure triangle, but sometimes you just need to add more light to the room and the flash can be the most convenient way of doing that. The problem is that the exposure on the camera is not taking into account the extra light from the flash. I'm wondering if there is a way to tell the camera to predict an exposure based on the flash I'm using or if there are any rules of thumb I can apply.

For the record, I only have a built-in flash on my Nikon D7000. (Although I want this question to apply to all dslrs in general.)

UPDATE:

As an extreme case, is it possible to do this in a very dark room? I've found in that case the major problem is that the autofocus doesn't work.

Answer 1

With continuous light (ambient light), your exposure is based on the amount of light, shutter speed, ISO, and aperture size. (See What is the exposure triangle?)

However, light from a flash is very brief; generally 1/10000 to 1/1000 second, depending on power level (more power, longer flash). This means that shutter speed, up to a certain point, shutter speed doesn't matter.

There are 5 factors that affect how much exposure you get from a flash:

1. Flash power. Scientifically, this is measured in lumen-seconds, but it's more practical to use Guide Numbers, which are given as a distance. More on guide numbers in a bit.
2. Distance from flash to subject. The intensity of the flash decreases proportional to the square of the distance to the subject. This is known as the inverse-square law
3. Your camera's aperture size. The aperture has the same effect on flash as it does any other light. Open up by one stop, say, f/5.6 to f/4, and you've doubled the exposure.
4. The ISO sensitivity of your film/sensor. Again, ISO has the same effect on flash as any other light. Double ISO, double the exposure.
5. The zoom setting, or beam spread, of your flash. This determines how concentrated the beam is. Zoom to 24mm, and you'll get a lot of spread, but the flash will be less bright. Zoom to 85mm, and the beam will be tight, and brighter.

Your flash will give you a guide number, or GN, measured in meters and/or feet. On the D7000, your pop-up flash has a guide number of 12/39 (fairly typical for an SLR pop-up flash), which means 12 meters or 39 feet. This is measured at ISO 100.

A guide number is the distance at which you'd get a proper exposure if you used a full-power flash, ISO 100, and an aperture of f/1.0.

Remember the inverse-square law, which tells us that flash intensity falls off proportionally to the square of the distance? Conveniently, our aperture settings also follow a square law, since aperture size measures diameter, but the area, proportional to diameter squared, is what determines the exposure)

To use a Guide Number to determine your aperture, first set your flash power to full, and set ISO to 100. Divide the Guide Number by the distance between the flash and the subject. This gives you the f-stop that you should use to get a proper exposure.

For example, if you're shooting a subject 8 feet away with a GN of 39, you need an aperture of f/4.9. If you cut the distance in half, to 4 feet, you need an aperture of f/9.8.

Cutting flash power by a factor of four cuts your guide number in half. Similarly, increasing ISO by a factor of four doubles your guide number. Changing the zoom setting on a speedlight will affect guide number as well.

All this math is inconvenient for the on-the-go shooter, and a lot of it goes out the window when you start using modifiers, bounce flash, etc, so the camera manufacturers came up with Through-the-Lens flash metering, also known as TTL. In TTL flash operation, your camera fires the flash at a reduced power before opening the shutter, known as a pre-flash. The camera uses its metering system to determine how much flash power is needed while the shutter is open.

As you mentioned, the camera doesn't try to "correct" the meter to take flash into account. This is actually a good thing, as it let's you know how much darker the background and any shadows will be, compared to your subject. With TTL flash on, assuming your subject isn't too far away, you always know that it will be properly exposed (at least, according to the camera's meter).

Now, since the shutter speed doesn't affect the brightness of the flash, you can keep your shutter open longer or shorter, depending on how much ambient light you want. Of course, you can get motion blur, etc. An excellent resource on balancing flash with ambient, and all things flash, is the Strobist blog.

Answer 2

Flash pulse has a very short duration, and, as long as you keep the exposure long enough not to interefere with your max. sync speed, so you can safely ignore shutter settings when it comes to flash exposure.

What you can do when shooting with flash is one of the two things:

ignore ambient light and just use the flash

This way, all you want to do is shoot with flash, which usually sets its power automatically via E-TTL or something. So you set you "usual" exposure just so it does not get it the way. This means flipping to manual mode and setting:

• shutter: your max sync speed (usually around 1/200 sec)
• aperture: according to how much DoF you need. smaller aperture eats up more of your flash power
• ISO: whatever you want, lower ISOs means you have to use more flash power, and you have lower noise

Obviously, if the resulting shutter/aperture/ISO combination is sensitive enough to catch any of ambient light that is already present in the scene, you'll have picture that has both. When you encounter that, you can either adjust aperture or ISO to kill it, or moving on to the next option

combine flash with ambient light

This is a bit more tricky, because you want both of these different lighting aspects to work together. Basically, you have four ways to alter exposure:

• flash power: this obviously changes just exposure of the flash, usually set automatically by camera. A lot of times you can alter it by flash exposure compensation and similar controls, or just flip flash to manual mode
• shutter: as long as you don't run into sync speed issues, this does not influence flash at all, works the way you're used to and affects only ambient exposure
• aperture: affects DoF and ambient exposure, also affects exposure of the flash, but if you have that on auto it just compensates it automatically
• ISO: well known effect on noise, similar to aperture in regard of flash exposure

Knowing all this, you can switch to manual, and fiddle with it until you get what you want (above it's 400 ISO and f/3.5, 1/40 sec to get background exposed decently and flash exposure lock on the dog to get it flashed properly).

Another option is to switch to aperture priority and ISO and aperture, which will make the camera auto-adjust shutter to get proper ambient exposure, and flash power to get proper flash exposure. You can then fiddle with AE compensation to alter ambient and with FE compensation to alter flash.

Answer 3

Do It By Eye

(?not for purists?)

I use off-camera flash quite a lot (I'm a fully brainwashed signed-up follower of http://strobist.com), so this advice might work differently for your pop-up...

If you are in a situation where you can take several shots, go by eye. Pick a setting which might work and give it a go. Check the results (pic and histogram) on the back of the camera and adjust the exposure (aperture) accordingly.

Typically, for doing walk-around candid portraits I might set:

shutter: 1/200 apperture f/2.8 flash power 1/16 (can't remember the guide number on my strobes - they're Vivitar 285s).

I'll take a couple of test shots with a willing volunteer, and usually I'll adjust the flash power to get the right exposure. From then on, I will usually control the amount of flash reaching the subject by moving closer to / further from them by a small step.

This works for me.
If you try it a few times and you will get a feel for what works.
(some of my shots taken like this are here: http://www.flickr.com/photos/ajstuff/sets/72157625381959500/)

Answer 4

All of the above answers are good. From my experience with Nikon DSLRs there is one setting that will influence exposure when using Flash in TTL Mode: "Metering Mode". When using Matrix and Centre Weighted Metering the system will try to balance the flash and ambient light. But when using Spot Metering Mode the system does not try to balance the exposure thus the flash is not in Balanced Mode. When using a hot shoe flash like the SB910 you will see that even with the flash set to "Balance Mode", as soon as you go to Spot metering the "BL" for Balanced Flash goes out.