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using flash...
When using flash, (manual on camera and ETTL on flash w/ compensation as needed), I pretty much understand the idea of dropping the background exposure and letting the flash fill in on the subject. What I'm having trouble with, I think, is knowing what qualifies as "background." I mean, it seems that there must be quite a separation between the background and your subject in order for this to be effective or else the flash will just overpower your manual exposure settings for the background. I wanted a lit Christmas tree in the background, slightly dark and blurred, with a nicely lit subject in front (about 6 feet in front - probably way too little space between, right?). I metered off the tree, lowered exposure -2/3 or -1 stop, tried boucing flash off the ceiling, and the entire scene was very nicely lit, but there was little difference between the foreground and background. If I drop EC on the flash, the subject becomes too dark.
I'm thinking some more and remembering that bouncing flash is like putting in a big window, which would of course light the whole scene. Hmmm...
Anyway, I'm so amazed at what a bounced flash can accomplish in a very dark room...just trying to get some details nailed down in my head!
I'm thinking some more and remembering that bouncing flash is like putting in a big window, which would of course light the whole scene. Hmmm...
Anyway, I'm so amazed at what a bounced flash can accomplish in a very dark room...just trying to get some details nailed down in my head!
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First let me be real clear. There are lots of people on this forum WAAAYYY more qualified than I to weigh in on this, but I got here first
A fundemental principle that helps my understanding is that the strength of your light falls of geometrically as distance increases. I think you prolly understand this. What you might be forgetting is that when you bounce the light off the ceiling you are increasing the distance BOTH TO YOUR SUBJECT AND THE BACKGROUND. But by a lesser amount to the background. Unfortunately geometry is not your friend in this instance. That even sounds like gobbeldygook to me.
Let's say you're 8 feet from your subject, and the background is 8 feet behind the subject. The background is twice as far from your flash (assuming on-camera) as your subject. So there's a full stop less light. Some smart person please correct this if it's 2 stops. Anyway, just to keep the geometry simple, let's say your camera (and flash) are four feet off the floor and you have a twelve foot ceiling. I'm being imprecise about the trigonometry, but if you bounce your flash 45 degrees off the ceiling, the light travels about 20 feet before it illuminates the tree, and only 4 feet more before it illuminates the background. That's a whale of a lot less than twice as far isn't it. That's one reason it's hard to get an appreciable difference in the amount of light on a relatively close background when using bounce flash. Draw a diagram for yourself.
OK, now somebody who knows what (s)he's talking about can maybe clear this mud up.
Natural selection is responsible for every living thing that exists.
D3s, D500, D5300, and way more glass than the wife knows about.
I think Icebear has it right. For this technique to work, your subject has to be significantly closer to the flash source than the background. If you bounce off the ceiling, then the subject and background are nearly the same distance from the flash (per the bounce path) so you don't get much of a difference in illumination. The flash illuminates the whole room fairly well.
Now, if you were 6 feet from your subject and your subject was 6 feet from the tree and the flash was pointed straight at your subject, you would get 1/4 as much light from the flash onto the background vs. your subject (2x the distance squared = 4x). Now, keep in mind the meter is exposing for the background (1-stop lower) and the flash is filling in what ever else you need for proper subject exposure. With the setup the way you described, you will only need about 1-stop of light from the flash on your subject and you will only end up with about a 3/4 stop difference between your foreground and background (1-stop under exposed + 1/4 of 1-stop of light from the flash that gets to the background = 3/4 stops underexposed).
I would guess that you probably want more of an effect than 3/4 stops. If that's the case, then just lower the manual exposure some more. If the subject is willing, I usually bracket these shots and take a bunch of different shots at a bunch of different exposures, using my LCD display as a guideline for whether I've covered the range enough.
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When you bounce it, you diminish the ratios as described in the answers above.
(The other factor is that indoors, when you bounce off the ceiling, you also get secondary rebouncing off the walls, floors, etc.)
The easiest way to make the background darker, is to get your flash closer to your subject, thus increasing the ratio between foreground and background. You do not have to change the camera position, just the position of the light source.
If your light is only 4 feet from your foreground, and 8 feet from the background, with a nice umbrella for a softer but focal directed beam of light, the ratio of the light should be two fstops = going from 4 feet, past 5.6 feet, to 8 feet. For three stops the background must be 11 feet from the flash with the subject still at four feet. Or move the flash closer to the subject at 2.8 feet.
Again, you can shoot from where ever you wish, it is the ratio of the flash to the subject, versus the flash to the background that controls lighting ratios with a single direct flash.
But when you bounce off the walls, the ratios are not so easy to figure in my head, anyway.
Moderator of the Technique Forum and Finishing School on Dgrin
Either I'm really confused or you meant to say "background" instead of "foreground".?.?.
I like the way you state the distances in terms of ordinarily recognized f-stops. It all becomes clearer now.
Natural selection is responsible for every living thing that exists.
D3s, D500, D5300, and way more glass than the wife knows about.
At least I did not have it spelled incorrectly. Thanks for the heads up, I have edited my original post.
Incidentally, when I posted the table for the fstop/light intensisty of the Vivitar 285HV , I tested the ratios of lighting at 2.8 feet, 4 feet, 5.6 feet, 8 feet, and 11 feet, with a Sekonic flash-meter and my camera's histogram, and the ratios work as expected with 1 stop of light change at each step from 2.8 to 4 feet, 4 feet to 5.6 feet etc. If you are using diffusers, like an umbrella, they may be some slight variation, but knowing these ratios will put you in the ballpark.
Moderator of the Technique Forum and Finishing School on Dgrin
Like the current TV ad says, "Way to go Captain Obvious"
Natural selection is responsible for every living thing that exists.
D3s, D500, D5300, and way more glass than the wife knows about.
Jeff
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Distance between subject and light vs background and light is the easiest way to control lighting ratios. But if you can't get your subject away from the background, you might still be able to control the ratios by controlling the direction of the light. For example, side light your subject so the light doesn't continue past to fall on the background. For extra control, you can use something between the light source and the background to keep the light from spilling onto the background. Doesn't have to be fancy--anything that blocks light can do the job. I've used black craft foam for the job.
I like that also. Even knowing the whole halving/doubling part being related to f-stops, I was still in the 1, 2, 4, 8 ft. mode.
An easy way to think about it: to directly light something, point the flash directly at it. To scatter light over a greater area, bounce it. A simple way to see this? Take a flashlight and practice pointing it at your subject, walk closer, bounce it off the walls/ceiling, and so forth. Output isn't the same as a strobe, but the effect is the same. The ones with a rotating front that can focus light more directly are fun. You can also put on different modifiers/diffusers and see how they work.
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elaine, about bounce flash:
Another way to get your desired result may be to bounce the flash from the side instead of from the ceiling. If you bounce the flash off a side reflector and have another reflector on the other side of your subject, then the subject should be well-lit. As long as you angle the two reflectors properly, they should prevent too much light from hitting the background tree if your subject is a few feet in front of it. Takes some trial and error to get the relfectors right but hey that's what chimping is for, right? Hope this makes sense.