Exposure compensation vs. flash exposure compensation

[paul_serafini1]

<p>I thought exposure compensation is used to increase or decrease the overall exposure of an image, and flash exposure compensation is used to maintain the <em><strong>same exposure</strong></em> but to rely less on flash and more on ambient light, thus giving a less harsh blast of light. However, when I dial down my FEC, my image becomes darker...if I dial it up, it becomes brighter.</p>

<p>So obvioulsy, I was mistaken and the FEC changes the exposure of the overall image.</p>

<p>So now I have three questions...</p>

<p>1. What's the difference between exposure compensation and flash exposure compensation? </p>

<p>2. Other than adjusting my exposure and flash output in manual, is there another way to accomplish this? Maybe I should shoot in Aperture priority, which will automatically set a longer shutter speed and rely more on ambient light.</p>

<p>3. I have heard of the rule of thumb about setting my flash for -1 for sunny days, -2 for cloudy, and -3 for overcast. I was under the impression that on sunny days, shooting at nearly full power will fill in eye shadows, etc, while maintaining the same exposure, and shooting at -3 on overcast days will add "pop". I'm now wondering why one would do this?</p>

 

[craigd]

<p>The way you express your previous understanding of FEC, it sounds like you thought using negative FEC would cause the camera to compensate for the reduction of flash by some other means (increased shutter time, wider aperture, higher ISO, whatever). That's not how it works. When you apply FEC, you are basically telling the camera, "You think I need X amount of flash, but I'm telling you I need this much more (or this much less)." Nothing else changes. The picture will in fact be brighter or darker because you have increased or decreased the amount of light provided by the flash.</p>

[robin_sibson1]

<p>What Craig said. The other half of the story is that non-flash exposure compensation affects only how the camera handles what is illuminated by available light (which is why using Av and Tv modes with flash in dark conditions can give results that are not what you want). In reality it is more complicated than that, for at least two reasons. One is that in many circumstances parts of the scene are illuminated by both flash and available light. Another is that the camera tries to be smart about working out whether you are expecting a bit of fill-in when there is plenty of available light, or alternatively whether you are expecting flash to be the principal illumination, and sometime this doesn't achieve what you are wanting/expecting.</p>

<p>Canon's documentation of the subtleties of its flash system is not as enlightening as it might be, and you may find that a web search turns up some helpful resources. But in any case, do your own experiments when the results are not critical so that you are on top of the system when it matters.</p>

[picturesque]

<p>1. Exposure compensation control applies to the ambient exposure. Flash exposure compensation applies only to the flash output. Both are separate systems that don't intersect (except in your brain through your own decisions about how you want to light a subject).</p>

<p>2. Research and read about the two kinds of ETTL--evaluative and averaging. With evaluative, regardless of camera mode, if there is a certain light level present (I think it is EV10 or so), the flash automatically drops the flash output in increments down to -1 1/2 stops. Below that light level, it does not drop the output. This is Canon's automated way of handling fill flash and the fact that light is additive. It drops flash output to 'make room' for ambient. This happens no matter what kind of flash comp you have set.</p>

<p>With averaging flash mode, there is no dropping of flash output automatically. You have to drop the output yourself by setting minus compensation.</p>

<p>Aperture priority will always go for what the camera meter thinks is correct ambient exposure. So you don't actually have much control there unless you use both exposure comp and flash comp together (they won't do it automatically). This mode also sometimes has what is called NEVEC (you can look it up), another automated way of handling the 'light is additive' thing. I have used the exposure and flash comp together before. Maybe -2/3 ambient/exposure comp and 0 flash comp. Remembering light is additive, the -2/3 comp makes room for the flash, which is not set for my usual plus comp (for flash as main light). Actually this is what NEVEC wants to do for you.</p>

<p>3. The obvious answer for sunny days is to fill in harsh shadows, particularly eye socket shadows. Cloudy days and even overcast days still produce the 'racoon eyes' eye socket shadows, just to a lesser degree. This is because the light comes from above (the sky). Remember that with evaluative, a little bit of minus flash is employed for you on top of whatever flash comp you set.</p>

<p>Beyond filling shadows, very low level flash can help with white balance (color cast from reflected grass or bluish shadows in the shade), and can even help stop motion, although you can't rely on that too much.</p>

<p>Now realize that with any automated system, when one says '-1 stop', the basis for measurement is already a 'guess' on the camera's metering system, so -1 flash comp means the flash has been comped down from what the camera thinks is right, not what is right in theory or, perhaps, in reality.</p>

<p>You may want to go to NK Guy's site, photonotes.org/eosflash or purchase his new book, which explains the ins and outs of Canon flash.</p>

[matt_needham]

<p>I think of them as doing pretty much the same thing: adjust the meter to expose for something other than middle gray. Exposure comp affects normal metering. FEC affects the way the camera meters the ETTL pre-flash.</p>