Nikon D100 and D-TTL - Explained

There is an excellent discussion going on over at DPreview between myself, a friend of mine, and Thom Hogan:

http://www.dpreview.com/forums/read.asp?fo...message=2870832

The thread explains WHAT D-TTL (The protocol that the D100, D1x and D1H use) is and how it is different than regular TTL.

Here's one quote:

> This is just something that I cannot understand....  

>  

> D-TTL is quite an evolved process. Why would a camera able to do  

> that, not being able to do a more basic process like TTL?  

 

No, it's no more complicated than Nikon's usual TTL on film bodies. In fact, the digital version (D-TTL) essentially is a simplification of the film body TTL, forcing the flash to always use pre-flash, whereas on the film bodies it is only used in the balanced fill flash TTL modes (not in Standard TTL).

 

> Is that a decision driven by marketing instead of technical issues  

> in order to sell more DX speedlights?  

 

No. It's driven by the fact that the Nikon CCD doesn't reflect light the same way film does. The film bodies measure flash output from the reflection off the film. Curiously, Fuji has retained during exposure measurement, so their CCD/filter must reflect about the same as film. (It actually may be more trivial than that--Fuji doesn't put a black frame around the CCD/filter the way Nikon does.)

And here's the kicker:

> - what if I put the D100 in an underwater housing with TTL (but not  

> D-TTL) compatible strobes? Do I have to go manual?  

 

That's the bummer. No underwater strobe supports D-TTL. If you're an underwater enthusiast you've got problems: buy a D100 and you're more likely to find a housing, but your strobes can only be used in Automatic and Manual flash modes; buy a Fuji S2 and you can't find a housing, but your strobes work as before

So, Conclusion: the Fuji S2 is looking more attractive for underwater photography, but the housing manufacturere (Sea and Sea and Aquatica) have already "jumped on the Nikon Bandwagon" even going so far as announcing housings for the Nikon SB80DX land flash...sigh.

I hope this clears up some of the confusion about D-TTL and TTL with DSLR's

James Wiseman

More on the differences between TTL and D-TTL. Written by Thom Hogan, who has both cameras:

> So with the NIkon, if it is all done pre-flash, where does the  

> "TTL" concept come in?  

 

TTL = thru the lens. The sensor sits at the floor of the mirror chamber looking back at the shutter curtain. The pre-flash occurs just prior to opening the shutter curtain (hint: turn you camera to Rear Sync and use a shutter speed of 1 second--you'll see the pre-flash, then the main flash; in normal operation, you can't differentiate the two).

 

The difference between D-TTL and the old-style TTL is subtle, but important. The pre-flash is very low power, and the camera is scaling up what it measures to the full flash output. Any small error in pre-flash sensing can result in a larger error in the actual flash output. In the old-style TTL (used by the Nikon film bodies and the Fuji S2 Pro), the camera does a second measurement on the reflections off the film (or CCD) during the exposure, and since the flash is firing at full power, the light hitting the TTL sensor is greater and the camera less likely to make an error.

 

Bottom line: TTL exposures are slightly more consistent than D-TTL. And things that rob power from the flash, bounce and diffusers mostly, don't tend to fool the system as often.

 

--  

Thom Hogan

J