cgardner
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Dedicated FM
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mh2000 wrote:
>>Functionally there is no difference between changing power manually or changing it via FEC adjustment in ETTL mode: IF THE SUBJECT IS STATIONARY.
No, when you change your framing at all your camera is metering off a different part of your subject, and unless it is a gray card you have to re-adjust your FEC etc. since the camera is adjusting to the changes it sees... so you have to anticipate the complex ETTL camera function. Dumb flash is easier to anticipate most of the time.
That's not the case with evaluative metering which is "smarter" than that.
For clarification, by "stationary" I meant nothing changes: subject/ framing. Perhaps I should have said "if camera, flash, and subject are kept in the same relationship". The underlying point was that FEC and manual power settings both do the same thing: change the flash duration which affects exposure, but ETTL mode will compensate exposure if the scene changes, M will not. ETTL doesn't always compensate perfectly as scenes change where something is in the frame affects the exposure decisions with evaluative, and because the metering zones are still rather large and the metering can only deduce by comparing reflections what tone and how far away the nearest objects are. It makes a logical guess, but will only guess perfectly about half the time. The rest of the time the result is close, but requires tweeking to get the results you had in mind. Bottom line? The camera metering can't read your mind it can only react to the reflected light you put in the viewfinder in a programmed, logical way.
At the end of the day Canon flash / metering is what it is, and you need to learn to deal with it or move on to something else. I'd used something else, manual flash and various TTL metering systems for years, but I quickly came to the conclusion I just could not predict in advance how the camera would meter the scene. So rather than try to out guess with spot metering or dialing in adjustments beforehand, I started pointing, shooting, evaluating, then adjusting from the camera's "I think this is correct" baseline of EC=0 and FEC=0. I found I got to the correct exposure faster than way.
The best aid for determining ideal exposure turned out to be the "idiot light", the warning that blacks out the playback when the highlights clip. But to use it to record a full range of tones I needed to see when the highlights are clipping. I've always used control targets like gray scales in my day job in reproduction and printing, but a solid white looks the same, 255.255.255 if overexposed by 1/3 stop or 33 stops, so I looked around for something white but with texture and grabbed the nearest thing: a white towel. With some experimentation and comparison of camera warning to results in the RAW file I noticed the texture in the towel and skin started to clip at the same time. So the towel texture is an ideal indicator for correct skin exposure when setting portrait exposure.
In most other situations there will usually be something white in the scene to use for evaluation.
The question the metering tries to answer is: how far away from me does the exposure need to be correct? When adjusting flash power its better to think in terms of where in the scene exposure is perfect rather than lifting and lowering the scene because the nature of flash is that its only correct at one distance. So if the point of correct exposure is in front of what you want correctly exposed use + FEC to move it backwards in the scene, and if the point of correct exposure is behind where you want it use -FEC to move it foreward in the scene. Logically you want the stuff needing correct exposure closest to the flash because otherwise anything closer would be overexposed (i.e., composing for flash)
Also in evaluative mode the camera is looking at 35 or 63 zones and knows which is the lightest, the darkest, the overall range and if it fits the sensor, whether the middle is brighter than the edges (i.e. person in the foreground), top brighter than the bottom (outdoor shot with sky in it) and make logical deductions as to flash output needed. So it will react and adjust to changes is framing much more responsively than simple averaging metering would. It creates a virtual map of the scene in the same way sonar or radar does, then tries to guess how far away and how reflective the closest objects are. It assumes the photographer isn't going to compose the shot with unimportant reflected junk in the foreground, or if they most understands how it will skew the metering.
The reason cameras have an FEC adjustment is because scenes can't always be composed in the viewfinder for optimal metering (i.e. the most important stuff closest to the flash). Sometime its necessary to blow out the foreground to get stuff further back correctly exposed with the knowledge the blown foreground can be dealt with via cropping or cloning when the file is edited.
With averaging if you shot a dark wall with a big white spot on it, moving around the spot in the frame wouldn't affect the exposure because the simple sensor mode would average it the same. But evaluative would change the exposure depending on whether the spot was in the center if the frame, the edges, top, bottom, etc. because of its logical assumptions about type if scene.
The more contrasty the scene, such as outdoors in backlight, the better ETTL evaluative seems to perform. One of the first tests I did after getting my flash is aim the camera out an open window to see how the camera would handle that tricky situation. Much to my surprise the inside was nicely exposed with the flash and the scene in the window nicely exposed also at FEC. Not perfect mind you, because there wasn't enough contrast on the wall to allow the metering it know what tone it was.
When bounce or modifiers like the FongDong are used the resulting scene and seen by the camera is a flat and uniform as an overcast day. With less contrast detected the metering isn't able to deduce what is in it based on the reflected tone. Also whenever the flash head it tilted the metering no longer uses the focus distance information because the focus distance and distance of the light path are no longer the same.
Chuck
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I'm wireless right now with my 430ex. You don't need a 550 or 580 to be wireless. 
