Why does flash create a black background?

I know it happens, but I don't think I understand why...Why when you shoot with full flash does the background go black?

Short answer: It doesn't have to.

Longer answer: Each flash exposure is two exposures. One for the ambient light, and one for the flash. If you set your ambient light exposure such that it's significantly underexposed, you'll get the black background (black hole) effect. If you set your ambient light exposure (in the case of a black hole, longer shutter or wider aperture) closer to correct, you'll eliminate the black hole.

If your flash can't illuminate the entire scene, which is more than likely, you're going to need to use different camera settings. Try a flash shot in AV mode. Give you closer to what you're looking for?

There's a ton more information, and by people who are a lot smarter about this than I am, in the lighting forum. Check it out.

https://photography-on-the.net/forum/showthre​ad.php?t=138907

EWek11,
A simple answer is:
The flash wasn't set strong enough to reach and reflect back from the background to make it show up in photo.
Here is an example
Weed Seed

IMAGE LINK: http://johnbdigital.co​m/macro/weed_seed.htm

Another key is to understand that flash power decreases at the square of the distance. So in all cases where flash is the main/only source of illumination subjects that are half the distance from the camera as the background will be four times brighter. Subjects that are 1/3rd the distance from the background will be nine times brighter.

This is why it is a good idea to look up the term "dragging the shutter" to improve your flash photography.

ibdb wrote in post #4329380
Short answer: It doesn't have to.

Longer answer: Each flash exposure is two exposures. One for the ambient light, and one for the flash. If you set your ambient light exposure such that it's significantly underexposed, you'll get the black background (black hole) effect. If you set your ambient light exposure (in the case of a black hole, longer shutter or wider aperture) closer to correct, you'll eliminate the black hole.

If your flash can't illuminate the entire scene, which is more than likely, you're going to need to use different camera settings. Try a flash shot in AV mode. Give you closer to what you're looking for?

There's a ton more information, and by people who are a lot smarter about this than I am, in the lighting forum. Check it out.

https://photography-on-the.net/forum/showthre​ad.php?t=138907

I agree with David on this. It is all about the ambient exposure and the flash exposure when you want to get a flashed pic that doesn't look like a P&S flashed pic or a blacked out background. Check out the link he posted, great info there.

You could increase the flash exposure to try and capture some of the background too (check the guide number for the 580EX for the ISO you are using). But then, I find it looks like a "flashed" pic and you may still have some dark or black parts, the further back in the scene you go. The foreground subject will get too overexposed by the flash exposure though if you try this!

What is important, I think, is that you should set the camera to expose for the background ambient light and then let the flash expose the foreground subject. Use FEC to dial down or up the flash for the foreground subject. I usually dial down the flash so that the subject isn't overly flashed. E-TTL with the camera set to manual mode, should do a good job for you. Don't worry if the subjects are moving and the shutter speed is way too low. The flash will "capture" them. If the objects in the background is moving (people or camera shake) then use a tripod or secure yourself of the camera because the settings you set in the camera for the ambient light will probably not be quick enough to freeze them.

John_B, if you would have reduced your shutter speed down to something like 1/4 or even 0.3 sec or slower then the background would have exposed with the flash illuminating the foreground subject.

JeffreyG, yep agree, dragging the shutter (to expose for the ambient light) is the way to go.

If the background is completely dark anyway then dragging the shutter won't help and increasing the flash power will mean an over exposed subject. In that case, a second flash unit or a lamp ot some moveable light source to illuminate the background would help.

Do you have an example with settings used?

Here is my example to try and show what I mean. The "principle" is STILL the same whether shotting a scene like this, a large room with fall off, outside etc.

Note: These are technical pics, they have no artistic value

Full EXIF is intact.

Setup: The lights in this room are OFF. The lights in the kitchen or ON and dimmed down.

See how the first pic does NOT show the ambient light through the doors into the kitchen? That is because of the relatively fast shutter speed of 1/60.

ISO: 400, Aperture F/4, Shutter: 1/60, Onboard flash no FEC.

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Everyone-
Thanks for the examples and the info. I definitely understand this concept much better now. I also shot almost a hundred shots today with the on-board flash and many different settings and I think I have a clear understanding of these principles now. So thanks so much for that!

Now, as far as FEC goes, how does one know how much is needed? Once the shot is metered for the background, and then the subject is added in front of this background, is it trail and error (and experience obviously) or are there some rules of thumb to apply?

EWek11 wrote in post #4333611
Now, as far as FEC goes, how does one know how much is needed? Once the shot is metered for the background, and then the subject is added in front of this background, is it trail and error (and experience obviously) or are there some rules of thumb to apply?

Always do a test shot or two with your settings with a test subject. Find someone (if it is people) who will stand there for a couple of test shots. Adjust the FEC if the subject is too bright (blown highlights) or too dark, for the ambient light.

Then you can happily snap away once you have done your test shots and as the light changes, compared to your test shots, you can adjust accordingly.

Agreed. Experience will help a lot -- but your camera has a histogram. Take advantage of it.