How to test if eTTL is working?
Posted 17 June 2004 - 07:46 AM
Mathias answered my emails, I quote (and translate) with permission:
"ETTL uses a special preflash: a low-power, modulated, quite long (500us) flash with constant power, not the typical exponential shaped flashes. This kind of preflash cannot be reproduced by ANY subtrobe I know, because it was not needed to modulate the tube until ETTL showed up. The reason of the un-typical preflash is unknown to me. I guess the camera can better know which light is ambient light and which is from the preflash. And this preflash gives the camera more time to evaluate the proper exposure. My converter fires a normal preflash with very exact timing, I got good results with the 300D but cannot say this will work with other ETTL-SLRs...
... I cannot say if the DS125 will work with my converter. Only strobes and cameras I tested by myself are officially supported."
Posted 17 June 2004 - 08:11 AM
Dual Ikelite Strobes
Photo site - www.reefpix.org
Posted 26 January 2006 - 11:09 AM
Posted 23 March 2008 - 06:14 AM
I'll give you my qick explanation of TTL theory and then try to give so things to look out for when try to make TTL work with your system.
The basic idea of TTL flash is that the camera reads the light hitting the subject from the flash and then cuts off the flash when it sees that the subject has received the right amount of light. That's the basic idea and when it works if frees you from having to change your aperture or flash output or flash position each time your FLASH to SUBJECT distances changes as when you move closer and farther from a subject.
Basic TTL was the first, simplest, and even in theory the most accurate TTL of all. Why? Because it read the flash intensity off the film itself at the very instant that the exposure was being made.
With the advent of digital cameras the major manufacturers decided that they wanted to sell you a new flash in addition to the camera. So they said that the old way of reading the flash at the film (sensor) plane would not work with the very dark digital sensor. Fuji proved that this was a lie with their S2 digital camera which would work with all the regular TTL strobes. BUT all the Nikon and Canon cameras required that you use a "digital" camera flash that first emitted a less intense "preflash" (which in the case of Nikon could be anywhere from 2 to more than a dozen very fast pulses) which is read by a sensor that reads off of the focusing screen BEFORE the mirror raises and the sensor is exposed. This reading is then used to calculate the intensity needed for the main flash which is only "predetermined" not read or modified during the actual exposure.
This "preflash" system has potential for introducing several kinds of "new" exposure problems or errors because it does not allow for any difference in subject distance between the time of the preflash and the main flash. If the subject distance changes significantly between the two flashes, as it might with a very fast moving subject, the picture can receive more or less light that it ideally needs. If the subject has a very fast reaction time as some animals do, it may "jump" and be in a slightly different position within the pictures. Or just do something like blink. The time between the two flashes is quite short, perhaps something like 1/60th or 1/30th of a second. This usually presents no problem. But with some animals I believe it sometimes does.
Beyond the basic system is how the different manufactures implement it.
Nikon calculates the TTL flash or the i-TTL flash on the basis of how much light is needed from the flash to provide ALL (100%) of the light needed for the picture. On their shoe mount strobes there is usually an option to select something like "fill flash" or "matrix balanced fill flash". This usually simply reduces the output of the strobe by about two f-stops (to 25%) with the idea that ambient light is going to provide most of the light necessary for the picture and the flash if just for "filling" some shadows.
No underwater strobe I know of (except a housed shoe-mount strobe) has ever offered this kind of option. And as a result, people trying to do "fill flash" pictures often over expose their shots because they do a correct ambient light reading (which privides 50-75%) of the light the picture needs. And then just fire a full TTL shot which provides another 100%. Total 150-175% and gross over-exposure. Recent film and digital SLRs have compensation controls which allow you to reduce the flash (in Nikon) from .3 to 5.0 f-stops. And thus if you are providing 60-75% of the light needed with ambient light, a starting point of -2.0 compensation is about right. To do this easily and effectively, just take your ambient light reading in the M (manual) mode WITH the compensation set at 0.0. Then with both the shutter speed and aperture set on the camera reset the compensation to the new - amount for the flash to reduce its output appropriately.
If you are in one of the auto-exposure modes, changing the compensation will also reduce the ambient light part of the exposure and lead to errors. If you want to make several different fill flash exposures you can use auto-bracketting functions to help you. BUT you must do several things. Check the custom functions and make sure that in the M exposure mode that bracketting is applied to ONLY the flash exposure and not the ambient light. Then pick the bracket increments you want. For macro shooting with no ambient light and using flash alone brackets of .3 stop are generally OK. For fill flash I usually use increments of .7 or 1.0 stops. Remember, first read the ambient light with no compensation set on the camera. Then apply any compensation you need for the shot with the least compensation you want and use the bracketing for shots with slightly more or less.
When I'm shooting flash-only macro shots with my F5's I first check the ambient light reading. If the ambient light is off the analogue metering scale to the minus side (2 full f-stops) plus one more stop, I know that the ambient light can be disregarded. In many cases I want to shoot just two shots, one at -.3 and the second at -.7. For this I would set the bracketing choices to be 0 and -.3. And then turn the compensation to -.3, which is then added to the two bracketted values. The advantage to using auto-bracketting for this is that you don't have to change the compensation value (which is often awkward on many housing and can require using both hands and losing visual contact with the subject). With the auto-bracketting you just push the trigger twice to get the two different shots.
When shooting fill flash pictures I first read and set the ambient component of the exposure in the M mode. And then add flash - typically at -2.0 and -3.0. Or a bit more or less depending on how much light hitting that part of the subject that will receive the fill flash.
I am not familiar with the "custom functions" of today's Canon cameras. In the film days Canon cameras often had a custom function that allowed you to set whether or not you were doing "fill or outdoors" flash. And in those cases the camera reduced the output of the TTL flash, whether it was a Canon flash or any other flash including underwater flash like an Ikelite, by about two f-stops.
If you want to check whether your TTL system of any kind is working. First shoot a shot with the lenscap on. Are you getting any "underexposure" warning? In Nikon this is given by the flash ready light (looks like a lightning bolt) blinking several times after the shot. If no warning signal something is not right.
Then shoot a shot. Does it look about right on your LCD (assuming no warning signal). If it looks OK, then vary the aperture in either direction by about one f-stop. IF it looks the same as the previous picture your TTL is probably working OK. During the "exposure" shots can you see that the flash output is noticably LESSS that the shot with the lens cap on? This is also a very good indication that the TTL is working and this is what I use as an indicator as my F5's shoot film and do not give me an LCD image to check.