Автор: victory13
Дата: 09-06-13 20:29
E-TTL (evaluative TTL, for film and digital cameras).
With the Canon Elan II/50 camera in 1995, Canon introduced another form of flash technology - E-TTL, for “evaluative through the lens” flash metering. While still metering through the lens, E-TTL is completely incompatible with its predecessors and works on a very different technical basis. E-TTL fires a low-power preflash of known brightness from the main bulb to determine correct flash exposure. It measures the reflectance of the scene with the preflash, then calculates proper flash output to achieve a midtoned subject, based on that data. It uses a preflash, but doesn’t suffer from A-TTL’s drawbacks for two reasons.
First, the E-TTL preflash occurs immediately before the shutter opens and not when the shutter release is pressed halfway. Unlike the A-TTL preflash, therefore, the E-TTL preflash is actually used to determine flash exposure and isn’t fired during the ambient (existing) metering stage. Some users may be surprised to learn that E-TTL actually fires a prefire flash before the main flash at all. Using regular settings the process happens so quickly that the preflash is difficult to notice, though you might catch glimpse of it before the mirror blackout - an exception being second-curtain sync.
Second, the preflash light is analyzed by the same evaluative metering system that the camera uses to meter ambient light. This means it meters through the lens and is harder to fool than external sensors, isn’t confused by bounced light and does not read anything off the surface of the film. For what it’s worth, unlike the TTL flash meter, the E-TTL metering sensor cannot be seen by the curious - it’s hidden away up in the pentaprism (or roof mirror in low-end EOS cameras) housing.
E-TTL is also generally superior to TTL and A-TTL when it comes to fill flash. The E-TTL algorithms are usually better at applying subtle and natural fill flash light to daylight photographs. E-TTL exposure is also linked to the current AF focus point, which in theory results in finer-grained exposure biasing than most multiple-zone TTL flash sensor systems. E-TTL support is built into all recent EOS film cameras and all EOS digital cameras since the D30.
The usual E-TTL sequence of operations, not counting the optional flash exposure lock (FEL) feature or wireless operation, is as follows:
• When the shutter release is pressed halfway the current ambient light levels are metered by the camera as usual. Shutter speed and aperture are set by the camera or user depending on the current mode - PIC (icon) modes or P, Av, Tv or M.
• When the shutter release is pressed all the way the flash unit immediately fires a low-power preflash from the main flash tube. (ie: white light)
• The reflected light from this preflash is analyzed by the same evaluative metering system that the camera uses for metering ambient light levels. The appropriate power output (ie: flash duration) of the flash is determined and stored in memory. The entire sensor area is evaluated and compared to the ambient metering, and the area around the active focus point is emphasized. If you are in manual focus mode then either the central focus point or averaged metering is used.
• If the photo is being taken under bright lighting conditions (10 EV or brighter), auto fill reduction is applied (unless it has been disabled by a custom function, as is possible on some bodies) and the flash output is decreased by anywhere from 0.5 to 2 stops. However, the E-TTL auto fill reduction algorithm has never been published to my knowledge, so nobody outside Canon knows exactly how it works.
• The mirror flips up and the shutter opens, exposing the film - or sensor chip if it’s a digital camera.
• The flash tube is then fired at the previously determined power level to illuminate the scene. Start time of the flash burst depends on whether first or second curtain sync has been set. The OTF sensor in the camera, if present, is not used in E-TTL mode.
• The shutter stays open for the full duration of the shutter speed time.
• The shutter closes and the mirror flips back down. If the flash unit has a flash exposure confirmation light and if the flash metering was deemed adequate then the light glows.
Camera units which support E-TTL:
All type A EOS cameras (see below) including all EOS D-series digital cameras.
Flash units which support E-TTL:
All EX series Speedlites: 220EX, 270EX, 380EX, 420EX, 430EX, 430EX II, 550EX, 580EX, 580EX II, MR-14EX, MT-24EX.
Limitations of E-TTL.
By far the biggest drawback of E-TTL is that the preflash can cause people who blink quickly to be photographed mid-blink. The preflash normally occurs an extremely brief period of time before the main flash, but there’s still enough time for rapid blinkers to react. The problem is heightened when using second-curtain sync with slow shutter. It’s not uncommon for group photographs to have a number of people with droopy or closed eyelids. A similar problem can affect nature photographers who photograph skittish birds. The only reliable way around the issue is to fire the preflash manually by pressing the FEL button, waiting a moment, then taking the actual photo. If you do this it’s wise to warn your subjects that there’ll be two flashes because otherwise they might look away after the preflash, thinking the photo has already been taken.
Another problem is that the use of preflash can trigger studio slave flash units which work by detecting the light from the triggering camera - analogue optical slaves. This results in flash exposure going very wrong, since the optical slave is triggered too soon. The preflash can also confuse handheld flash meters, making manual flash metering very difficult.
More abstractly, E-TTL is a very automated system and isn’t well documented for the user. For instance, as noted above, Canon have never published details on the E-TTL auto fill reduction algorithm. It takes a bit of experimenting to figure out how the system is likely to respond. And there’s relatively little user selection or choice in operation modes. Most flash units don’t, for instance, let you manually choose TTL, A-TTL or E-TTL flash metering at will.
E-TTL has also been a problem for a lot of users of early EOS digital cameras (see TTL and E-TTL and digital EOS cameras below) because of the way E-TTL flash metering is performed. Some of these issues are addressed by E-TTL II, which is described in the next section.
Finally, not every E-TTL feature is supported by every type A body and E-TTL flash unit. Some wireless E-TTL features and other functions such as the modelling light, for example, require both newer type A EOS bodies like the EOS 3 or EOS 30 and flash units like the 580EX or 420EX. Part III of this article describes which features are available for which combinations of camera body and flash unit.
E-TTL II.
Introduced in 2004 with the EOS 1D mark II digital camera and the EOS Elan 7N/EOS 30V/7S film camera, E-TTL II is an improved version of regular E-TTL which includes two key innovations.
Improved flash metering algorithms.
First, E-TTL II examines all evaluative metering zones both before and after the E-TTL preflash goes off. Those areas with relatively small changes in brightness are then weighted for flash metering. This is done to avoid the common E-TTL problem of highly reflective materials causing specular highlights in a flash-illuminated image and throwing off the flash metering. Normally E-TTL II uses evaluative algorithms for its flash metering, but the EOS 1D mark II has a new custom function (CF 14-1) that lets you use centre-weighted averaging rather than evaluative metering for flash metering if you prefer.
Distance data incorporated into some calculations.
Second, E-TTL II can use distance data when it’s available. Many EF lenses (see list in next section) contain rotary encoders that can detect the current focus distance. For example, if your camera is focussed on an object 4 metres away then the lens will send this approximate focus distance data to the camera body.
Under certain conditions the distance data is factored into the calculations for determining proper flash output. This is particularly useful if you use the focus and recompose method without setting FEL - the new system can help minimize flash metering errors under these conditions. Canon describe the new system as essentially metering flash data across a flat plane rather than a point.
Up until now distance data hasn’t really been used much by EOS cameras. Some PIC (icon modes) apparently incorporate distance data into their exposure calculations, but that’s really been about it. E-TTL II is the first really useful application of this information that Canon have implemented, and is obviously very similar to the fashion in which Nikon have long relied on distance data for their flash metering system.
Cases in which distance data is not used.
Distance data is not always used by E-TTL II. There are three very significant cases in which distance data is not used, aside from the obvious case when it isn’t available because the lens doesn’t provide it. These three conditions are bounce flash, macro flash and wireless E-TTL flash.
When you’re using bounce flash (ie: when the flash unit’s head is in basically any position other than full-on straight or, with those flash heads which support it, with 7 degree downward tilt) then there is no way for the camera to know the distance the light took to reach the subject from the flash. Light will be scattered off walls or ceilings or reflectors and won’t travel directly to the subject. Since bounce flash is a common technique to improve the quality of a flash-illuminated scene it means that the primary advantage of E-TTL II in this situation is just better evaluative flash metering.
The other two conditions are similar. With macro flash you’re too close to the subject for the lens to determine useful information, and with wireless E-TTL flash the camera will have no idea where the flash units are positioned in relation to the subject. Note that E-TTL II can still use distance data if the flash unit is connected to a camera via an Off-Camera Shoe Cord. (there was some confusion about this early on, but Canon USA’s Chuck Westfall has confirmed it) This means that users of flash brackets won’t be left out, though it does mean that if you position the flash unit closer to or further from the subject than the camera, or if you point the flash unit away from the lens axis while keeping the flash head locked in a straight ahead position, then you might throw off the flash metering slightly. You can’t directly disable the use of distance data if the lens has it, though in this case you could take the simple precaution of setting the flash head to a very slight off-centre bounce position that would disable distance data while not significantly altering the flash coverage.
To summarize, there are two important points to keep in mind. First, E-TTL II does not require any changes to either the flash units or lenses used with an E-TTL II camera - the changes are all basically internal to the camera body. And second, while E-TTL II does use distance data when it’s available and when it’s appropriate (eg: when using direct non-bounce flash), it doesn’t prevent you from using older lenses.
Camera units which support E-TTL II:
EOS 1D mark II, EOS 30V/33V/7S/Elan 7N/Elan 7EN, EOS 20D/20Da, EOS 350D/Rebel X Digital/Kiss N Digital, EOS 400D/Rebel XTi Digital/Kiss X Digital, EOS 450D/Digital Rebel XSi/Kiss X2, EOS 500D/Digital Rebel T1i/Kiss X3, EOS 550D/Digital Rebel T2i/Kiss X4, EOS 1000D/Digital Rebel XS/Kiss F, 1D mark IIN, 1Ds mark II, 1D mark III, 1Ds mark III, 1D mark IV, 5D, 5D mark II, 30D, 40D, 50D, 7D.
Flash units which support E-TTL II:
All EX series Speedlites: 220EX, 270EX, 380EX, 420EX, 430EX, 430EX II, 550EX, 580EX, 580EX II, MR-14EX, MT-24EX.
Canon EF lenses with distance data for E-TTL II.
The following lenses are capable of returning distance data for use with those cameras which can use them. This list was published by Canon USA’s Chuck Westfall in March 2004 and is reasonably comprehensive, though does have a few omissions.
Note that most of the lenses with distance data capabilities contain ring USM focus motors. In fact, the first three lenses with distance encoders were introduced in 1990 along with the EOS 10/10S - the 35-135mm 4-5.6 USM, 70-210mm 3.5-4.5 USM, and 100-300mm 4.5-5.6 USM. It’s also not clear what the resolution is of a typical lens distance decoder. Photos I’ve seen of the decoder rings (not quite like children’s toys in a cereal packet) in one lens suggest that the distance data is fairly approximate, with each combination of distance contacts returning a certain distance range.
I have no information as to whether any third-party lenses compatible with the EF lens mount are capable of returning distance data.
.....
If your lens doesn’t appear on the list above then it may or may not have distance data capabilities. However, here are a few current and recently discontinued lenses in the EF lineup which definitely do not have distance data. Note the 50mm 1.4 USM and the 85mm 1.2L USM mark I (not mark II) are in this list.
EF 15mm 2.8 fisheye
EF 24mm 2.8
EF 28mm 2.8
EF 35mm 2.0
EF 50mm 1.4 USM
EF 50mm 1.8 II
EF 85mm 1.2L USM
EF 135mm 2.8 SF
EF 28-80mm 3.5-5.6 II
EF 28-90mm 4-5.6 II USM
EF 28-90mm 4-5.6 II
EF 35-80mm 4-5.6 III
EF 55-200mm 4.5-5.6 II USM
EF 75-300mm 4-5.6 IS USM
EF 75-300mm 4-5.6 III USM
EF 75-300mm 4-5.6 II
EF 80-200mm 4.5-5.6 II
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