iTTL vs. dTTL, DLSR flash explained...

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RonFrank

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I posted this in response to another thread, but certainly worth a standalone thread.

Here is the difference between iTTL, and dTTL (you asked)

D-TTL uses a five-element flash sensor and looks at the shutter curtain for reflections. One problem is that it sees outside the 1.5x area on the DSLRs. D-TTL pre-flash is a sequenced series of 18 pulses from the flash. Only one flash can be used if you're in a D-TTL mode.

I-TTL uses the 1005-element color matrix meter in the viewfinder and has much better ability to distinguish what's happening in specific area of the frame. I-TTL pre-flash has been extended to allow communication between flashes and camera so that multiple flash (up to 10), groups of flash (up to 3 groups of 3 flashes), and wireless exposure control of flashes is all possible.

What does this all mean? iTTL has a LOT of extended capabilities, and also is fooled less often due to a much more comprehensive pre-flash sequence, and much better and well defined metering area. It allows TTL BETWEEN flashes, and better yet, one can control flash ratio's AND it's wireless.... Really cool STUFF!!

Does it work better UW. I'm not certain anyone can answer unless they are using a flash housing as Ikelite is about to release the first UW iTTL flash. Will those flashes support the full iTTL implementation with wireless multiple flash capabilities, and the master to slave control that is available on the Nikon SB800?? God only knows, or maybe Ikelite.

I can tell you iTTL with even one flash works better on land vs. dTTL in SOME situations. One thing to keep in mind is that as advanced as these things get, if you use manual mode, and have the flash exposure dialed in based on distance, it is repeatable, and right on. This will apply UW as well as on the surface assuming constant lighting, constant subject distance, and constant water clarity.

Where dedicated TTL with a flash really helps is when everything is changing, subject distance, lighting, and when it works, it works damn well, and it DOES work for the majority of subjects.

However the BEST photographer is one who uses their skill and mind, and is NOT totally dependent upon technololgy, and pushing a button to get an exposure. Let's face it, if you are in a situation where iTTL is NOT working, or even the Camera exposure meter (which as these TTL meters are based on the same technology, they are fooled by similar subjects), you either loose the shot, or rely on your knowledge and understanding on how to shoot manual.

IMO anyone who is shooting a multiflash setup UW BETTER take the time to become a photographer. If you are spending 5K and do not have time to even learn how to use the equipment, well IMO THAT is VERY pathetic!!!!
 
Old school as it may be, I still shoot manual strobe.

<end confession>

Not that I'd pass up the chance to try i-TTL. It would be foolish to ignore something that might be great!

All the best, James
 
Ron, James:

I am curious. When I shoot my Nikonos V with a wide angle lense, I ignore TTL as it is pretty much useless. Manual all the way. With macro, I have figured out the adjustment to the iso knob to give me pretty good ttl.

I am seriously considering a housed dSLR. Does TTL with a strobe like Ike's DS-125 work on wide angle shots or are you limited (like the Nikonos) to macro in TTL?

Not that I am afraid of working my camera in manual (after all my Nikonos IIIs are still my workhorses), but UW TTL is atractive to me.

---Bob
 
Bob, like you, I just pretend that the TTL on the Nik V simply doesn't exist.

My only real experience with TTL in a SLR (D or film) came from a F4. This just didn't cut it for me, either WA or macro. At least not something I'd count on to produce quality. I quickly added a couple pieces of tape to the macro port that have the close-field exposure corrections on it, and have used manual for macro ever since. (I did find that the TTL gave good results as long as the item I wanted exposed correctly was dead-center in the middle of the frame. Which is to say, not often.)

The D100 doesn't do the TTL thing. I'm not into chimping much anyway, so mostly all I do is a quick look at the histogram and rely on instincts from film.

Much as I hold out hope for underwater TTL, it seem as if the gap between underwater stobes and manufacturer protocols is widening, not closing. This is too bad, because the terrestrial strobes are truly amazing, and something I cheerfully count on to produce better results than I can on the fly.

Sheesh! All I want is a quench signal. How tough is that? Obviously, harder than it looks....

All the best, James
 
ScubaBOBuba:
Ron, James:

I am curious. When I shoot my Nikonos V with a wide angle lense, I ignore TTL as it is pretty much useless. Manual all the way. With macro, I have figured out the adjustment to the iso knob to give me pretty good ttl.

I am seriously considering a housed dSLR. Does TTL with a strobe like Ike's DS-125 work on wide angle shots or are you limited (like the Nikonos) to macro in TTL?

Not that I am afraid of working my camera in manual (after all my Nikonos IIIs are still my workhorses), but UW TTL is atractive to me.

---Bob

Because the Nikonos is a range finder, and because it is rather old technology, the TTL behind this camera is rather outdated, and limited.

With iTTL Nikon has really broken down the barriers and limitations, and it's a VERY effective tool. It will work with any lens at any setting (assuming the flash has enough coverage) and with 3D matrix metering and the way it judges subject distance and the use of the 1005 element meter makes it work flawlessly in the majority of shooting situations.

To date UW photographers have NOT had this option, but bet on it becoming a standard if UW flash manufactures can match what Nikon has done with the SB800.

Let's put this in perspective. If one is busy shooting a sea spider, and looks over and spots a Manta Ray moving at speed in Manual mode there is little chance that one can adjust the flash quickly enough to get proper flash exposure. With iTTL, there is a very good chance that if one can frame that ray, and adjust the focal length quickly that the flash expososure will be dead on without any thought on the part of the photographer.

iTTL is seriously worth strong consideration once available, and if or when I get a DSLR underwater, it will at the top of my list of required features in an UW flash setup.
 
RonFrank:
To date UW photographers have NOT had this option, but bet on it becoming a standard if UW flash manufactures can match what Nikon has done with the SB800.

Let's put this in perspective. If one is busy shooting a sea spider, and looks over and spots a Manta Ray moving at speed in Manual mode there is little chance that one can adjust the flash quickly enough to get proper flash exposure. With iTTL, there is a very good chance that if one can frame that ray, and adjust the focal length quickly that the flash expososure will be dead on without any thought on the part of the photographer.

Housed SB800 works very well with D70 but with one catch, one strobe only. My buddy uses one and for macro it is very good. However, I don't think anyone has quite figure out how to connect 2 SB800 together for underwater use yet. Certainly connecting 2 SB800 via sync cord does not work at all, probably in part due to wireless network that SB800s use in multiple flashes. I don't even think that you can hardwire 2 SB800s on land and use iTTL, if I read the manual correctly. So at least for SB800, it is going to be for macro only for now.

Ikelite stated that with their iTTL converter, you can use dual Ikelite strobes in iTTL.

How big is a sea spider? I would imagine that if you have the right lens for sea spider, it is probably wrong for a manta anyhow :wink: but yes, sometimes it would be nice to be able to shoot quickly without too many adjustments and get the right exposure right away.
 
https://www.shearwater.com/products/perdix-ai/

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