i-TTL

Can someone (Isaac?) please explain in simple terms how i-TTL differs from TTL flash. I'd like to know if my old TTL flash will work with a digital SLR and what I'll be losing to i-TTL.

In case, it's important, the old TTL flash is a Braun with a shoe for Nikon SLRs such as the old F801. The new digital SLR I'd like to buy (probably in the new year) is the D70s. Should I be saving for a flash gun as well? Conversely, does an i-TTL work with a film SLR?

Thanks in anticipation.

 

No one?

 

Hmmm, not sure how I missed this.

Ok.... Flash metering is not an easy thing to do. Compared to normal scene metering you need to know some more things to make a really good exposure:

1) Subject Distance
2) Subject Reflectivity

Most TTL flash metering systems rely on the second of these. They take what is called an "off the film" reading. This means that the shutter is opened and the flash is fired at the appropriate sync. The metering system measures how much light from the flash is reflected off the film and from this decides when enough is enough and kills the flash output.

This is the basis of Canon's TTL and A-TTL flash metering systems.

Nikon upped the game in the early-mid 90s by introducing distance information. Some AF lenses can tell how far away they are focused (not all though), and this information was fed to the metering system and used in conjunction with the reflectivity reading from the film to work out the correct exposure. This is the Nikon D flash system. This was seriously ahead of canon's existing systems at the time.

Next stage was the introduction of canon's E-TTL flash metering system. This system uses only reflectivity information gathered by firing a preflash just before the mirror is fired in order to use the main metering system to do the calculation. The advantage of this is that it can tie into the subject (eg off-centre spot metering or evaluative metering patterns) much better than an off-the-film reading. The problem that E-TTL has suffered since inception is that if the subject is very light or very dark the exposure goes horribly wrong.

Finally we come to the latest generation of flash systems. i-TTL in the case of Nikon and E-TTL II in the case of canon. Basically combine E-TTL and Nikon D and you get either of these systems. Canon added distance information reading into E-TTL and Nikon added pre-flash reflectivity metering into D.

Why? The answer is that you can't take an off-the-sensor reading from a digital sensor. It has completely the wrong characteristics unlike film, so to get the reflectivity reading you have to use a preflash method.

When this is combined with Nikon's RGB metering you get a formidable system - superior to any metering system canon have come up with (says a canon user of 13 years standing. IMO that's the area where nikon have always had and have kept a lead over canon).

Coming back to your Braun flash... if it is a TTL flash then I highly doubt it will work. I don't know for sure with nikon, but TTL flashes absolutely do NOT work with canon DSLR bodies. You may be able to use it as a manual flash.

As a slight aside, the Canon D30 and D60 DSLRs had a pretty dreadful AF and metering system. Combined with the inadequacies of E-TTL, exposure problems were rife, particularly amongst those shooting weddings. The solution that many turned to was the Thyristor flash, which has its own meter on it.

 

Thanks Isaac. Very informative. Looks like I'll need to budget for a speedlight (SB600 or SB800). As you say the alternative is to use it as a manual flash as I have been doing with my Nikon FM for years, but I want to make fewer mistakes now.

 

interesting stuff Isaac, have a virtual pint on the house :beer:

 

Well Dev, the nice thing about digital is that you can make as many mistakes as you like and bin them without it costing you anything. I used manual flash quite a lot with the D30's sub-par metering system. I use auto much more with the 20D because it actually works.