How to correctly expose when using a flash?

Question

I have been using the Manual mode ever since I picked up my Nikon D3200 about 6 months ago. I am pretty well versed with the exposure triangle & how to use it.

However, recently - I am starting to discover flash as a non-evil thing. And reading the theory about it, has left me confused.

Everywhere I read, people always say...

Situation : subject is facing camera with sun in the background (bright background, dim foreground)

1. First expose properly for the ambient/background
2. Then expose for the subject/foreground

What does this really means? Should I select a focus point on background, adjust my exposure & then focus on my subject again & then adjust the flash compensation to adjust for proper exposure? (this might mean multiple photographs till I find perfect exposure).

It would be great if people who are well versed with flash photography could answer this for me.

Thanks,

Answer 1

When a scene combines ambient light and light from a flash this is a way to control the balance between the two.

The difference between the ambient light and the flash is this: The flash is a very short burst of light much shorter than the time the shutter is open (assuming you are shooting at or below your camera's flash sync speed). The ambient light, on the other hand, is constant over the entire time the shutter is open.

The changes you make to aperture and sensitivity (ISO) will affect both the objects illuminated by the ambient light and the objects illuminated by the flash. But any changes you make to the shutter speed will only affect the objects illuminated by the ambient light. To adjust the exposure of the object or person illuminated by the flash, you adjust the power level of the flash instead of adjusting the shutter speed.

To understand how the shutter speed is irrelevant to exposure in terms of the flash, imagine that you are in a totally dark room. If the flash is illuminated for 1/1000th of a second, it doesn't matter if the shutter was open for 1/200 second or for 200 seconds - the same amount of light will be captured in both cases: the light that was present for only 1/1000 second. (With digital, there might be additional noise generated by the heat produced by the sensor staying energized for that length of time. With film there would be absolutely no difference.)

Now, back outside to our portrait subject. If you want to shoot manually, first meter to determine the proper exposure of the background and set your camera's ISO, Aperture, and Shutter speed to match that exposure value. Then meter to determine the proper exposure of the subject, compute the number of stops difference between the two and add enough flash to make up the difference. What power level you will need to use for the flash will depend upon several variables: The flash's Guide Number that indicates how powerful it is, the distance from the flash to the subject, and the aperture and ISO you selected when metering the background.

Answer 2

I am going to throw in one more answer with a very pragmatic explanation as well.

1. Set your exposure in manual, with your sunset exposed the way you like. I frequently will run this ~1/2 stop underexposed, as it often looks nicer. At this point it generally best if you keep your shutter speed below your sync speed, which is 1/200 on your body.

2. Leave your flash in TTL mode and take a test shot, depending on your metering it may overexpose (or under) and just use flash exposure compensation to tweak.

I know this is not "all manual", but it easy and produces great results. If your subjects look photoshopped into the background, odds are the exposure ratio is too great and you need to bring up the background exposure and/or reduce flash power.

There is also a white balance issue to consider, as the setting/rising sun is often much redder (as it has to travel through a lot more atmosphere), so putting a 1/2 CTO (orange) gel on your flash will sometimes give a more natural look.

If you do want go all manual I would not worry about numbers, but again go with the pragmatic approach.

1. Set background exposure as above

2. Pick a flash power say 1/4 and make like a chimp.

3. Is your subject overexposed? bring down the flash power, under? bring it up. Side note, if you are min flash power and still overexposed, adjust your background exposure to use a smaller aperture and conversely (and more likely) if they are underexposed at full power, adjust your background exposure to use a wider aperture.

After some practice you will get a feel for a good first guess -- don't forget as you get closer you need less power and further away you need more.

Answer 3

If you are working manually, you want to adjust your ISO, shutter speed and aperture to get a good background, you then want to adjust your fill flash (which is going to be the primary use of the flash in this context) until it sufficiently lights the foreground. This way you get properly lit foreground and background.

You can either spot meter the background or depending on camera body evaluative metering may handle it. Also, if the foreground is a fairly small portion of the overall scene, average may also work ok. The trick is that you are trying to keep everything within the dynamic range of the sensor.