There have been three flavors of Nikon automatic TTL flash systems, all incompatible with each other:
But the nomenclature is worse - Today, the term TTL has three different meanings, at least for Nikon:
The context of the TTL term usage may be necessary to decipher meanings.
So what we casually call TTL flash (Through The Lens metering) might be film TTL, but is probably instead iTTL today. And it is possibly TTL regular flash mode, but often is TTL BL balanced mode. We sort of have to know what we are talking about. :)
This can be confusing, but the first film flash TTL system was named TTL, and today's digital flash TTL system is named iTTL.
For example, the Nikon cables SC-18, SC-19, SC-26, SC-27 are named "TTL Cables", but that use definitely means Film TTL.
The current iTTL system is wireless, and does not use any cable (except it can use a hot shoe extension cable, Nikon SC-17, SC-28, SC-29). Note that Wireless does NOT necessarily mean radio, it just means wireless, including optical communication by light.
A) TTL often specifically means Film TTL, because TTL was also used as the name of the specific automatic flash system used in film cameras since 1980 (except the film F6 model, which can do iTTL too). Nikon's first film TTL flash was the SB-12 for the F3 camera in 1980. The SB-24 and SB-26 were classics, popular in the late 80's and 90's.
The film camera metered the full working flash power via the flash reflection from the film surface after mirror was up and shutter open, in real time during the actual exposure. When exposure was sufficient, the flash was quenched (stopped). One of the four pins in the camera hot shoe is the quench pin. ( iTTL flash does not use the quench pin, but it still exists, and even for iTTL, it can be used to quench the flash to save power when the shutter closes)
There were two methods to use multiple TTL flash units (meaning film):
1. The AS-10 adapter and the sync cables with three-pins (SC-18, SC-19, SC-26, SC-27), could quench remote multiple flashes (controlled by the camera via a cable to the flash).
2. the SU-4 optical slave (foot-mounted on each remote flash) had an AUTO mode which followed the triggering flash, quenching when the triggering hot shoe flash stopped (controlled by the TTL flash on the camera — when it stopped, the remotes stopped).
This was not individual control, the individual flashes were not metered, but for TTL, the quench was simultaneous control.
B) D-TTL - early digital cameras, 1999, D1 series and D100 (and D2 family is backwards compatible). Flash models ending in DX were D-TTL flashes: SB-28DX, SB-50DX, SB-80DX. Also the SB-600 and SB-800 were backwards compatible with TTL, D-TTL, or iTTL.
Film surface reflection does not work for digital (anti-aliasing filter is one factor), so the digital flash in the D-TTL camera meters weak preflash reflection from the front of the closed shutter surface (which was painted light gray for this) after the mirror was raised, but before shutter opens. Hot shoe flash power level was preset from preflash calculations. There was no quenching, and no provision for off camera remote TTL, and no provision for multiple TTL flashes, and no FV Lock. It was a first try for digital flash.
C) iTTL - Beginning in 2003, with D2H and D70 with SB-800, and up through the newest recent current models (This even includes the one F6 film camera). The iTTL flashes are SB-300, SB-400, SB-500, SB-600, SB-700, SB-800, SB-900, SB-910, SB-R200, and SB-5000 (not in that order, the SB-800 was first, and of these, SB-500 was most recent). There are also several third party flashes compatible with the Nikon iTTL interface (and many of those also compatible with the wireless Commander Remote mode). The iTTL metering is done by the camera system, and the flash simply has to accept the proper power level command.
iTTL meters a low power preflash from the flash before mirror is raised, metered using the viewfinder RGB meter. Preflash is with lens wide open, and preflash calculation sets the power level in the flash to be appropriate for the aperture which will be used, with no additional attention or monitoring (no quenching concept as in film TTL). Multiple flash is done wirelessly via Commander, which can control groups of remote flashes. iTTL can individually meter each flash (assuming only one flash in each group), and it sets individual power level accordingly. This adds a delay, but adds wireless flash, multiple TTL flash, and it now also allows FV Lock, to lock a flash exposure for multiple frames, or simply to meter independently of shutter time (FV Lock prevents pictures of subject blinking from timing of the earlier command flashes). More Here.
Flash Naming Nomenclature and Compatibility:
Here is a good look at the history of previous Nikon flash models. Plus the site has an impressive overall coverage of pretty much all Nikon gear for collectors.
For any automation, both the camera and the flash must have the same capability. Otherwise, the flash only works in manual flash mode (or possibly an Auto non-TTL flash mode, where the flash itself meters and controls the flash).
Older non-iTTL flashes can still do Manual flash mode on the newer iTTL bodies. And also can still do any flash Auto mode (where the flash itself meters the exposure), however, if the old flash cannot do the newer CLS communication, then the camera cannot otherwise communicate with the flash (cannot send ISO or aperture info for example, so it would have to be entered in the flash manually.) But the iTTL body can still trigger the Manual flash mode. And any simple optical slave triggers should still trigger manual mode.
Nikon provides this DSLR and FLash Compatibility Chart. It mentions older flashes, but does not mention film TTL cameras.
A) TTL (film TTL)
There was multiple TTL flash, but there was no individual control of the flashes. All TTL flashes fired at full power, and when the combined light seen was metered as sufficient, all flashes were terminated (quenched) at the same instant (normally using sync cables to each flash). However, the effect of any close or powerful flash was much stronger than any more distant or less powerful flash.
The three-pin cables (Nikon SC-18, SC-19, SC-26, SC-27) were named "TTL cables", specifically meaning Film TTL (only). The three-pin cables provided communication of trigger and quench to all flashes. Current iTTL cameras do not use that system (iTTL does not even use any cable, other than a hot shoe extension cord). And the current flashes do not have that three-pin connector. So beware, this "TTL cable" does not mean iTTL, it means film cameras.
The Nikon SC-28 type of cables (hot shoe extension cords, including SC-28, SC-29, and older SC-17) also have a three-pin socket (for connecting older film TTL flash units). These can also do iTTL or D-TTL with the ONE single flash in their hot shoe, but not multiple flash, except film TTL.
The older SC-17 still works the same as the SC-28, except it just does not have the newer shoe pin locking mechanism. Also the SC-28 places the cord at one side instead of in front (in front on SC-17 can block the internal flash door from opening fully). The SC-19 adds an AF-Assist flash sensor at the hot shoe end, always pointing forward to the focus point (whereas the one in the flash might not be aimed at the focus point if the flash is off-camera).
The separate optical slave foot accessory (Nikon SU-4 unit, still available) had an Auto mode, where it quenched the flash when it saw the triggering flash stop (could work at short range). Therefore, this was another multi-flash system for film TTL — it simply followed the controlled hot shoe TTL flash which triggered it. Some current flash models also include this optical slave internally (for manual flash — NOT for use with Commander), still called SU-4 mode in those models menu.
B) D-TTL (early digital, D1 and most D2 models, until D2X)
There is no multiple flash D-TTL capability. Single hot shoe flash only.
C) iTTL (current CLS system, last 15 years, since SB-800 in 2003)
The iTTL wireless Commander offers the only Nikon method for multiple flash units, and offers true control of individual flash groups (limited to two or three of them). Its default mode is to meter individual flashes and set power of all flashes power level to be equal intensity at the subject. Then individual compensation can be done from there. More Here.
We tend to imagine that the term CLS means the Nikon Commander system, and people often say "CLS" when they actually mean AWL for "wireless Commander". But instead, CLS is a communication system between camera and flash, including many additional features. The Commander system named AWL is simply one of several features supported by the CLS communication system.
To make this point about names, the description of CLS from a Nikon SB-800 flash manual (page 5):
The SB-800 features the Nikon Creative Lighting System (CLS). This system offers additional flash shooting possibilities with digital cameras by taking advantage of camera’s digital communication capabilities. CLS is available only when the SB-800 is used with compatible Nikon cameras. The SB-800 offers these major features:
The first Nikon digital models D1 and D2 changed to the D-TTL system for automatic metered flash, but since then (the last 15 years), the iTTL system is used. HSS flash came out on the Nikon SB-25 flash in 1992, called FP High-Speed Sync then, but it did not have the Auto FP feature yet (automatically shifting modes with shutter speed).
There are no iTTL cables whatsoever. Multiple iTTL is only done wirelessly via the Commander (called Advanced Wireless Lighting, AWL). So (excepting the hot shoe extension cables like Nikon SC-28 for ONE iTTL flash), any cable when used with an iTTL camera can only be used with multiple flashes in Manual flash mode. The iTTL multiple flash method is the wireless Commander (not using any cable).
The word wireless just means no wires. It does not necessarily mean radio, the Nikon Commander system instead uses the infrared light component in the visible flash. Flash is visible light, but which also includes a strong infrared component. The remote flashes (and the SU-800 commander) do have infrared filters to filter out the visible light, which we may call infrared, but this filter is not required, because the visible flash naturally also contains a strong infrared component.
Any system using PC sync cords, optical slave triggers, or any regular radio trigger, is manual flash only (incompatible with iTTL, incompatible with Commander). However, there are a couple of expensive add-on radio trigger systems that do relay the multiple Commander infrared signals via radio, achieving the same TTL Commander system, with radio links.