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 p.1 #11 · Canon flash with slaves vs Nikon | |
The "brains" of the system controlling the flash is the camera metering system and that's where Nikon appears to do a better job than Canon, especially on the bodies with only 35 metering zones on the viewfinder. The zones are just too large to detect small objects and light entering the eye piece of the viewfinder can affect exposure readings because metering is done off the viewfinder. I look forward to the day when I can buy a Canon body with an EVF because EVF cameras meter by dumping and reading the sensor 40 or so times a second and can detect clipping down to the pixel level. My wife's Panasonic P&S does a better job at automatic exposure than my 20D or 50D for that reason. There also is no x-sync on a EVF camera either 
The ST-E2 is the weak link of the Canon system. Its obsolete and long overdue for replacement, but probably will not be updated because of the recent move on the 7D to make the camera flash a "Master" commander. The range of ST-E2 sucks and it only does ETTL ratios and only for two groups A:B.
If you use a 550ex /580ex/580exII or 7D as Master it is possible to control three groups A:B and C in ETTL ratio or manual mode. It sends commands via visible pre-flashes from the main flash head and has a much greater range than the ST-E2 or the build-in flash on the 7D.
In ETTL A:B sets the foreground lighting and C is intended only as background lighting as in this shot because C is metered separately and set relative to the A:B output:
A:B is set using a ratio scale, then C is balanced to A&B with a separate Group C FEC adjustment.
In ETTL mode lighting is quite simple. First dial in the ratio based on what tone you want the shadows. Then when shooting monitor the highlights with the overexposure warning - which shows when and where highlights are clipping - and adjust FEC as you shoot and the scene content changes to keep the highlights just below clipping. The oft maligned white towel is simply an aid when shooting portraits where there is no white clothing to make it easy to see when highlight clipping is occurring. The red channel in skin and the texture in the towel clip at the same time. Keep the towel below clipping and you get perfect skin exposure, simple as that. Also note how the range of the scene fits the sensor perfectly per the histogram. Get detail in both ends and the middle falls into place automatically because the photographic process is engineered to produce a linear response. Its just a matter of fitting the scene contrast to the range of the sensor.
(That was shot in M mode, which I use for location portraits)
Canon ETTL ratios are not the same as studio Highlight:Shadow ratios on a face. Discovering what the various ratios look like is a matter of testing. Here's the test I did with my 580ex flashes when I first got them...
The Master (A) is on a flash bracket illuminating both sides of the binder, with the Slave (B) just hitting the right side. What impressed me initially in the test and still does is how after first dialing in the FEC with the ratio set at 1:1 the exposure of the highlights stayed consistent as I just flipped through the other ratios. There isn't much difference between 8:1 and 2:1 because A is the fill and with those settings it overpowers the key light which overlaps it. When shooting I typically start at 1:4 which makes it easy to see the modeling, then adjust ratio up or down as needed after setting FEC to get the level of softness I want in the lighting ratio.
Like any TTL system changes in scene reflectance affect the flash exposure. After testing all the metering modes I found evaluative to yield the best results when combined with my use of the highlight clipping warning. Evaluative takes 35 or 63 "spot" readings (depending on body) and compares ambient to pre-flash. Basically it looks for "holes" in the ambient pattern to fill. It would see a back lit person like this...
Pre-flash after the ambient is subtracted...
The metering can deduce there is person in the foreground creating a "hole" in the ambient pattern. Here's an actual example... Backlit scene exposed to keep the highlights below clipping in the notorious white towel on the test target (standing in for the naked woman who would be otherwise wearing the towel on her head in the shot)...
I then reach up and turn on the 580ex flash set at FEC=0
The evaluative metering figures out how to "fill the hole" and make the foreground exposure correct. It looks a bit over exposed, but the target in the foreground is actually matching the sensor range perfectly...
I used manual flash with Vivitars for many years and Canon works the same in M mode, but with much more convenience. There's no metering involved so A, B and C can be used in any role with power set remotely from the 580ex Master.
Another part of the problem using ETTL and flash in general is that many people just don't understand how flash works. Regardless of how the power is regulated it will only create optimally exposed highlights at a single distance from the flash at any given power level, and will fall off 2 stops each time that distance is doubled. That means flash shots much be composed with what is most important closest to the flash with everything needing correct exposure the same distance from the flash. That's true of manual flash, ETTL, or studio lights.
ETTL mode reacts more to the closer objects because they reflect more pre-flash. So if shooting the family around the dinner table, if the white table cloth is closer to the camera than the faces the faces will be under-exposed because the metering reacts to the closer tablecloth. When one understand how flash works they just don't compose flash shots that way. FEL and spot metering can be used to correctly expose the faces, but that will not suspend the laws of physics and prevent the table cloth in foreground from being overexposed.
ETTL provides no feedback on power level actually used. If using bounce or modifiers its easy to reach 100% capacity and not realize it. If shooting a 50% capacity adding +1 FEC will max out the flash. +2 or +3 will do nothing because there is no more power to dial in. Canon flash has a "quick flash" mode that will fire the flash when its only 1/6th to 1/2 charged. If you are in the habit of shooting rapidly and get progressively underexposed shots the problem isn't the metering its the fact you are shooting faster than the flash can recharge.
Outdoors overlapping flash and ambient highlights does reduce contrast (the primary reason for adding flash) because it will hit the highlights and shadows at the same time. To be effective flash needs to be able to lift the shadow side of the ambient light independently: the sun needs to be used as back-rim light with the flash "filling the hole" the stuff in the foreground makes in it. Once that is understood it is easier to use flash effectively to create natural looking lighting outdoors.
The Canon flash system is what it is. I just learned to deal with it after using manual flash for 30 years.
I was a Nikon film shooter for many years with several cameras. I didn't switch to Canon for digital because of the flash system, but because I thought the image quality of its CMOS sensors and lenses were better at the time. The flash system just came as part of the package. I used my 20D with my Vivitar 285HVs for a year before buying the 580ex, and bought them primarily for the high-speed sync capability outdoors. Its silly to buy $4,000 worth of fast lenses then be forced by x-sync to shoot at f/11.
I had just as much trouble as everyone else until I figured out how evaluative metering worked and started to set FEC per the clipping warning. Do it that way and it really doesn't matter what metering mode is used. Regardless of metering mode the first shot of any new scene is seldom perfectly exposed, but its just a matter of being able to interpret the feedback in the clipping warning and histogram to be able to dial in the amount of EC and FEC to make the second one perfect. I'm an old dog, but for me the new tools are the best trick around and I use them to best advantage to make the job easier.
The fundamental dilemma of exposure is that many scene contrast exceeds the range of the sensor requiring the metering system to make a decision what part of the scene to expose correctly. Put a white cat and a black cat together in a sunny window. The camera can't expose everything correctly so it guesses where to peg the exposure. Then its up to the photographer to decide whether he wants the white cat, the black cat or the scene outside the window correctly exposed by overriding the camera's guess.
Manual, ETTL? The problem is the same. A big part of the solution to the dilemma of the scene fitting the sensor is learning to using two flashes, indoors and out, to change the scene contrast. Indoors its done by using even fill and overlapping the key light on top. Outdoors its done by shooting into the shadows of the ambient, then using the two flashes on the shady side the same as they are used indoors with the even flash fill assisting the even fill from the sky and the second off axis "key" flash creating the highlights on top of both. Want natural looking flash lighting? Just pay attention to where the natural light comes from and put the flashes in the same place.
ETTL ratios make lighting nearly idiot-proof: Set the ratio based on the desired shadow tone, then set FEC based on the clipping warning to get the highlights exposed correctly and the parts of the scene lit by the flash will fit the sensor, the definition of correct exposure in the technical sense. If you are using other methods and not getting good results you are probably over-thinking the problem or introducing too many variables.
Will every shot be perfectly exposed the first time? No. But if you think it should be you have unrealistic expectations. The technology just isn't that good yet; it can't read your mind
Chuck
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) the just use M mode and power settings.