Flash exposure compensation vs manual flash adjustment

Hi there,
Could someone please explain the difference between setting the flash manally or using FEC? I can set one or the other in my G5 but I don't really understand the difference.
Thanks in advance.

When you set the camera to fire the flash automatically, the camera determines how much power is required from the flash. When it thinks that it's gotten just enough light from the flash, the camera shuts off the flash. However, the camera doesn't always get the flash exposure right. When the subject is dark, the camera will use more flash output and cause the picture to become overexposed. When the subject is too light, the camera will underexpose the picture.

You can rectify this by using FEC. You can use, for example, an FEC of -2/3 stop when you are shooting very dark subjects so that they don't come out overexposed. Conversely, you can use +2/3 stop when the subject is too light so that it doesn't get underexposed.

With FEC, you are always adding or subtracting a set number of stops, or fraction thereof, from whatever the camera thinks it should use.

On the other hand, when using the flash in manual mode, the flash will always fire at full power or, if set via the camera or flash, at some fraction of the full power. The flash output is fixed and the camera will not attempt to control the output of the flash. To get a good flash exposure, you have to manually set the appropriate aperture value based on the guide number of the flash for the power selected and the distance from the flash to the subject. This is not something to be attempted by the faint of heart, anyone turned off by math or someone who can't judge distances to save his life.

PacAce wrote:
On the other hand, when using the flash in manual mode, the flash will always fire at full power or, if set via the camera or flash, at some fraction of the full power. The flash output is fixed and the camera will not attempt to control the output of the flash. To get a good flash exposure, you have to manually set the appropriate aperture value based on the guide number of the flash for the power selected and the distance from the flash to the subject. This is not something to be attempted by the faint of heart, anyone turned off by math or someone who can't judge distances to save his life.

Leo, your explanation is outstanding in its clarity. I would respectfully disagree with the complexity of using manual flash settings - its a practice I've adopted on a semi-permanent basis with my G5-420EX combination, and its get splendid results. You are right that hashing out the right ratios, especially for studio work, can get a bit mathy, and really requires a dedicated flash meter, but for the vast majority of us creative users, I think the empirical method of dialing in the right amount of flash over two or three shots, works great. Now of course this approach is only possibl with a digital camera, allowing instant feedback on the results of such experiments. But for everyone who's ready to experiment a bit (think slave flashes, diffusers, etc.) manual flash operation is likely to be highly rewarding. From there, operating entirely in M mode is just a click away

Regards,

K

PS - if you're ready to take the manual flash compensation plunge, read up on the basics of artificial lighting, because there are quite a few new variables in the mix, which can lead to a fair bit of frustration.

kreego wrote:
Leo, your explanation is outstanding in its clarity. I would respectfully disagree with the complexity of using manual flash settings - its a practice I've adopted on a semi-permanent basis with my G5-420EX combination, and its get splendid results. You are right that hashing out the right ratios, especially for studio work, can get a bit mathy, and really requires a dedicated flash meter, but for the vast majority of us creative users, I think the empirical method of dialing in the right amount of flash over two or three shots, works great. Now of course this approach is only possibl with a digital camera, allowing instant feedback on the results of such experiments. But for everyone who's ready to experiment a bit (think slave flashes, diffusers, etc.) manual flash operation is likely to be highly rewarding. From there, operating entirely in M mode is just a click away

Regards,

K

PS - if you're ready to take the manual flash compensation plunge, read up on the basics of artificial lighting, because there are quite a few new variables in the mix, which can lead to a fair bit of frustration.

Yes, I completely agree with you. My last sentence was made with tongue in cheek (as indicated by the emoticon.

Any suggestions? I've used my 420ex in manual with decent results, but I mostly set aperature based on other considerations (DOF, or 5.6 for sharpness, primarily), then adjust the flash output for the desired effect - over 2 or 3 shots, like you said. If there's a more inteligent way to go about it, I'd love to learn.

Scott, here's what I bear in mind when I mix flash and ambiant light. For a given F-stop, the flash power and shutter speed will influence how much light arrives on each part of your scene (flash part, and ambiant part). Now, what you want to work out is whether your scene is going to be dominated by the flash, by ambiant light, or by an even mix of both.

<>Flash power ONLY effects the part of the scene covered by the flash. When fully powered down, this can be at very short range, indeed.

<>Shutter speed ONLY effects the part of the scene covered by ambiant light. The faster you set the shutter speed, the less light is available for the the ambiant part of your scene. Just make sure you set the 420EX into the fast-syncro mode.

<>Shutter speed effects BOTH the flash and ambiant parts of the scene. As you would expect, the more you stop down, the darker everything gets.

So, a good way to start is first take a reading in Av or Tv mode, *with the flash turned off*. Switch to M mode, and the settings are retained (luckily!!). Now turn the flash on, and power it way down (it will also be in manual mode) and take a couple of shots, adjusting shutter speed to fine-tune the ambiant portions, and the flash power, for the flash covered parts. You may have to mess with the aperature if things seem wildly unbalanced ie blown out flash part, or greatly underexposed ambiant part.

Here's an example:
http://www.betterphoto​.com …owNumber=6&memb​erID=62047

Hope this helps!

K


PS - Leo, I feel I'm one of those folks you're describing, with math aversion and distance ineptia I just wing it and experiment - the fun nevers ends!
PPS - Asking questions on this forum and others dedicated to photography also has done wonders for my understanding

Thanks for the informative reply, but now I am confused! When the subject is dark wouldn't you use a FEC of +2/3 stop, not -2/3 stop? Maybe this is where I am going wrong?
My dilemma is that whenever I am at a wedding I try to take photos inside the church or reception, and they are always too dark. Even when I set the aperture as open as possible, and I don't like to increase ISO too much. I am trying to figure out which is best to use, FEC set at +2/3 (or -2/3 as you said above?) or manual flash set at maximum, keeping in mind that the aperture stays fixed. Do I just need to go out and buy a 420ex, because the on board flash is too weak?

Hi shniks,

Now that's less a problem of exposure compensation than of sheer amount of flash-generated light. The on-board flash has a very short range, maybe up to 3 meters, which means that it might work ok for portraits, but not at all well for group shots.

I see two solutions : Use the program (forget the name) that fires the flash AND leaves the shutter open longer (you'll probably need a tripod) or get an external flash, like the mythical 420EX that almost everyone, including yours truly, swears by.

Its been said many times - the quality of light from an exterior flash is greatly superior to the onboard flash, because its farther away from the line of view, and because its capable of generating many times more light. Not to mention the virtues of indirect (bounced) flash illumination. Do a forum search for more information...

Regards,

K