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A-TTL and E-TTL. What is the difference?

Phil -- , Jan 04, 1998; 07:06 p.m.

For a novice like myself, alot of technical jargon can seem a little overwhelming when trying to decide which camera to upgrade to, especially when you just want to get out and get some shots. If someone could spare a minute explaining the difference between A-TTL and E-TTL, I would be very grateful. I have also heard that the EOS systems regularly overexpose the subject with fill in flash when it is not centralised or the frame is recomposed. Is this simply overcome (if it suits the composition) by using a focusing point to the side when composing your subject and link the exposure to this focusing point?

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Tom Shea , Jan 04, 1998; 07:37 p.m.

Phil, E-TTL utilizes a pre-flash to measure the light reflected back from the subject. When the shutter is pressed, a very short burst is fired from the flash unit. The metering system reads the results and adjusts the "real" flash duration as it flashes a moment later. All of this takes a very short period of time and I cannot actually see the pre-flash or tell that it has happened because all of this happens so quickly.

The metering pattern is rather broad - center, left weighted or right weighted, depending on which of the focus points is used.

My experience with E-TTL has been very positive. However, like any system, one must understand what is going on and make the necessary adjustments. E-TTL does tend to overexpose portraits in my experinece, so I use the flash exposure compensation when I shoot portraits with it. Since the Elan IIe has flash exposure compensation separate from the overally exposure compensation control, you can set them separately. This provides great flexibility and control. But there is no magic - you have to pay attention and understand what is happening.

Actually, my main use for E-TTL is for macro photography. I have been extrememely pleased with the results for macro work.

If your want a similar system that uses a pre-flash and a spot meter, the Contax RTS III is the only camera that does this.

Philip Greenspun , Jan 04, 1998; 11:33 p.m.

I'm not convinced of the virtues of E-TTL and certainly A-TTL isn't worth anything. Nikon did this right with their D system. With Canon, you might as well just use manual ambient exposure and either regular TTL or manual flash for tricky situations.

Peter Legisa , Jan 05, 1998; 07:24 a.m.

I use A-TTL sometimes on my 540 EZ. It fires a visible (and audible) red preflash and a sensor on the flash measures the response and selects a small enough aperture hopefully preventing overexposure. (At close range and with a lens wide open the TTL mechanism alone sometimes cannot quench the flash fast enough.) A-TTL works in many situations when you do not have the time to set things manually. It cannot be used in macro work. Over greater distances it is sometimes showing false settings in the viewfinder - but curiously enough the pictures are more or less OK.

Glen Johnson , Jan 05, 1998; 10:50 a.m.

The difference between A-TTL and E-TTL is covered by one of the other posters. I suppose that Philip is trolling, so here's a bite.

If you focus with the central cell and then reframe, the EOS flash sensor will be fooled in to thinking that you really want to make whatever is central as the main subject. If your real subject is off center, it will probably not be properly exposed by the flash. If you use the camera as recommended by the manufacturer, then you will either select the correct focus cell manually, or you will let the camera select the focus cell by itself (similar to the way that all of the Nikons through the N90s work). Once the correct focus cell has been selected for your subject, then that segment of the flash sensor is weighted most heavily when the unit figures out when to quench. If you are shooting with transparancy film, you may still want to add compensation for especially light, or especially dark subjects.

E-TTL is not essential, and I would not make a decision about which camera to buy based on the presence or absence of this feature.

I have NEVER had a problem with the EOS TTL or A-TTL flash. Not for light subjects. Not for dark subjects. Not for off center subjects. Not with one 540 EZ. Not with two 540 EZs. Not with the on camera flash from the A2E. Not with the off camera shoe cord. Not with the StoFen Omni Bounce. Not with the LumiQuest bouncers. Not with the EOS wired TTL gear. Not even with an umbrella or soft box.

If you read the directions and use flash compensation (just like you need to do with the Nikon stuff if you're shooting transparancy film), you get correct exposures. If you can't get decent results, then your gear may be defective and need repair.

The equipment fixation is fostered on us to some extent by the manufacturers, by usenet, and even by photo.net. After all, equipment is what people want to know about. Equipment is what sells magazines. Equipment angst is what makes people buy new camera gear. We are, at the least, willing victims.

Does anyone REALLY believe that there is a magjc flash algorithm out there that will solve all of the flash problems of the world? Does anyone really believe that one camera company is a lot smarter than the others and makes perfect products, while the others are dullards, and in 1998 aren't able to master the technology?

If you know what you are doing, you will get decent flash pictures with transparancy film with products from every manufacturer, including the aftermarket manufacturers. If you do not know what you are doing, you will still get decent results when you use print film - at least if you have a good printer. I use both the EOS and the Nikon systems, including flash, and they are both very nice.

When you get to the level where subtle differences in flash are critical, you get serious studio gear, a couple of soft boxes, umbrellas, light stands, reflectors, gel filters, a flash meter, etc. And you use polaroids to check out the result before you commit to transparancy film.

Chris Nicholson , Jan 05, 1998; 11:20 a.m.

Another nifty feature of E-TTL is "FE Lock" (Flash Exposure Lock). You point the center meter cell at the subject, hit the FE button and the 380EX emits the pre-flash and calculates output power; recompose and shoot and flash output is unaffected by what's undet the metering cells at exposure time. This feature is just another way of giving the flash system some explicit guidance in difficult situations where the imperfect "auto" system might be fooled. I don't think A-TTL offers such a feature.

On the other hand, I don't think there are any E-TTL flahses that also allow full manual control; some would argue that this is a fatal flaw.

Alder Wong , Jan 05, 1998; 11:46 a.m.

At 12:21 AM 1/6/98 +0800, you wrote: > >Another nifty feature of E-TTL is "FE Lock" (Flash Exposure Lock). You point the center meter cell at the subject, hit the FE button and the 380EX emits the pre-flash and calculates output power; recompose and shoot and flash output is unaffected by what's undet the metering cells at exposure time. This feature is just another way of giving the flash system some explicit guidance in difficult situations where the imperfect "auto" system might be fooled. I don't think A-TTL offers such a feature.

Yes, T90 and 300TL offer those FEL functions. It has been more than 10 years technology.

:-) Alder

Alder Wong , Jan 05, 1998; 11:50 a.m.

>Phil, E-TTL utilizes a pre-flash to measure the light reflected back from the subject. When the shutter is pressed, a very short burst is fired from the flash unit. The metering system reads the results and adjusts the "real" flash duration as it flashes a moment later. All of this takes a very short period of time and I cannot actually see the pre-flash or tell that it has happened because all of this happens so quickly.

According to Canon Manual both A-TTL and E-TTL do so. The difference is that E-TTL allow Faster Flash Syn up to 1/2000s. And E-TTL is only available with Eos-50/Eos55 (Japan/Asia Model Number) and Flash EX380 Together.

FEL is first introduced by Canon with T90 and 300TL more than 10 years ago.

:-) Alder

SHUBROTO BHATTACHARJEE , Jan 07, 1998; 08:11 a.m.

A-TTL (Advanced TTL) uses an infra-red preflash for ranging the subject. It then sets the aperture to balance between depth of field requirements at that distance and the background exposure required; then fires the mai flash burst and controls the flash exposure (duration) by measuring through the lens OF THE FILM PLANE.

E-TTL (Evaluative TTL) uses TTL metering for ambient and flash exposure, but NOT off the film plane. A preflash is fired for ranging, subject location and brightness, and its results are compared with the evaluative metering for the background. The main flash burst is then fired at a predetermined (calculated) level/duration, based on the preflash and evaluative ambient measurements.

The E-TTL system also makes flash exposure lock possible. The locked value is held for 16 seconds : long enough for a flash shot to be taken with the self-timer.

An E-TTL flash also offers high-speed sync with th EOS-50/50E (A2/A2E) and EOS 500N (Rebel G).

An E-TTL flash rverts to A-TTL operation if a camera other than these two is used.

Trust this helps!

Shubroto Bhattacharjee Melbourne, Australia

Mickey Mouse , Jan 07, 1998; 09:13 a.m.

>A-TTL (Advanced TTL) uses an infra-red preflash for ranging the >subject. It then sets the aperture to balance between depth of field >requirements at that distance and the background exposure required; >then fires the mai flash burst and controls the flash exposure >(duration) by measuring through the lens OF THE FILM PLANE. >E-TTL (Evaluative TTL) uses TTL metering for ambient and flash >exposure, but NOT off the film plane. A preflash is fired for >ranging, subject location and brightness, and its >results are compared with the evaluative metering for the background. >The main flash burst is then fired at a predetermined (calculated) >level/duration, based on the preflash and evaluative ambient >measurements.

Just want to add that preflash is also used in A-TTL if you use bounce flash or just tilt a little bit with the flash head.

And to support Alder, FEL is first introduced by Canon T90 and Flash 300TL in 1985.


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