Nikon TTL History
What is TTL? D-TTL? iTTL?

There have been three flavors of Nikon automatic TTL flash systems, all incompatible with each other:

  1. Film TTL, called TTL
  2. Early digital TTL, called D-TTL
  3. Current digital TTL, called iTTL

But the nomenclature is worse - Today, the term TTL has three different meanings, at least for Nikon:

The context of the TTL term usage has to be mentally analyzed to decipher meanings.

  1. TTL is used to generically refer to automatic Through The Lens metering, not necessarily even flash. The menus use it this way, to just mean automatically metered flash. It can be of any type, generally, without specificity. For example, a flash menu may say TTL (general automatic flash), but it means it does what the metering system is going to do, which may actually be iTTL (a specific type of general TTL).
  2. The term TTL previously specifically meant the TTL auto-metered method used on Film cameras, A) above. Film TTL was always called TTL, which made sense back in those times, but digital is a new situation, different now, and we had to find new terms today. Today, with current digital DSLR, Nikon's mode is called iTTL now. And there was a short previous first Nikon digital try (until 2003) called D-TTL. But in reference to older film gear, TTL specifically specifies film TTL.
  3. In 2003, the SB-800 was the first iTTL flash for the Nikon D2H DSLR camera that could use it. But even the term iTTL has two meanings to consider. It has both TTL or TTL BL modes. TTL BL is balanced flash, which is the default flash metering mode of iTTL cameras. In bright ambient, balanced means it reduces flash level to be fill level to merge it with bright ambient.
    Specifically, TTL BL means balanced, in contrast to regular TTL mode which is regular TTL flash automation, when flash power is adjusted to whatever was actually metered, Not balanced flash, Not reduced by ambient level. The camera menu call it all TTL, meaning whatever the flash is going to do. A few flash models (SB-600, 800, 900) have a menu to select either TTL or TTL BL, but iTTL will be TTL BL unless there was a way to override it. Otherwise, normal bright light meters TTL BL balanced, and dimmer ambient meters TTL.

    Think of iTTL this way. The ambient light is metered by the camera meter, which sets the camera exposure (shutter speed, f/stop and Auto ISO) to a correct exposure. The TTL flash metering sets the flash power level to give a correct exposure with flash. However, those two correct exposures combined is 2x necessary, which is 1 EV overexposed (at least the near part the flash lights is). That was always a problem with film TTL, when the early computer chips used in cameras back then did not have sufficient ability to do more. It worked OK indoors (in dim ambient compared to a sunlight exposure) but bright sun had to be compensated by the user. So then -1 2/3 EV TTL was the automatic standard sunlight flash exposure (which is 24% flash, and probably 1/3 EV overexposure of the near subject). But today, iTTL mode TTL BL balance flash automatically similarly backs off on flash power for the flash to just be fill level (unless a flash menu to set TTL instead of TTL BL specified otherwise). In any case, the sum of any two lights is always brighter than the one brightest, so even though flash power is backed off, there could still be mild overexposure, perhaps 1/3 EV or so, often acceptable.

So what we casually call TTL flash (Through The Lens metering) might correctly refer to film TTL, but is probably instead iTTL today. And in iTTL, it is possibly TTL regular flash mode, but most often is TTL BL balanced mode. We sort of have to know what we are talking about. 😊

This can be confusing, but in short, 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 for those cables, it definitely means Film TTL.

The current iTTL system is wireless, meaning it does not use any cable (except it can use a hot shoe extension cable, Nikon SC-17, SC-28, SC-29). These hot shoe cables work with either film TTL or digital iTTL. Note that Wireless does NOT necessarily mean radio, it just means wireless, including optical communication by light signals (flash pulses in Commander mode). TTL BL requires normally bright ambient, and the iTTL flash can become TTL BL. Dark and underexposed ambient means the iTTL flash is TTL, meaning full exposure of the metered flash, Not TTL BL balanced.

And for flashes and cameras that can provide a PC connector, a standard PC Sync cable can be used for manual flash.

How Nikon Flash Actually Works

A) TTL - We casually say TTL for any automated flash, but TTL today technically might mean Film TTL, because TTL was also used as the name of the specific automatic flash system used in film cameras since 1980 (except the last Nikon film F6 model, which can do both TTL and 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 later classics, popular in the late 80's and 90's, before digital.

The film camera TTL had a photo sensor that metered the full working flash power in 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 flash control, the multiple flashes were all metered simultaneously as one, but the quench to off provided by the three wire cables was simultaneous control for multiple wired flashes (providing overall exposure, but no lighting ratio control). However, wireless optical slaves did not offer quench and were in full Manual control. However, the effect of any close or powerful flash was much stronger than any more distant or less powerful flash, unless there was manual control.

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.

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 (the sensor anti-aliasing filter is one factor), so the digital flash in the D-TTL camera metered weak preflash reflection from the front of the closed shutter curtain surface (which a spot 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 or individual flash control, and no FV Lock. It was a first try for digital flash. This reflection system was abandoned in the next iTTL version.

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. The low level signaling flashes occur before the shutter opens, which then opens, and the full prescribed flashing occurs. It's pretty fast, and humans perceive it as one flash. 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 a Commander, which can control groups of remote flashes (in iTTL or manual mode). The Commander is a flash with Commander capability (either the camera internal flash or a hot shoe flash, or the SU-800 commander-only device) which has the special commander mode to flash specially encoded low-power bursts to signal the remote flashes about mode and power level. The camera individually meters each iTTL flash (assuming only one flash in each of three groups), and then the Commander signals the remotes to set the individual power levels accordingly, and then the final signal to trigger the flashes. This adds an unnoticeable 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 the earlier command flashes).

The older SC-17 hot shoe extension cord 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 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.

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 (Creative Lighting System) is a communication system between camera and flash, including many additional features. The Commander system named AWL (Advanced Wireless Lighting) is simply one of the several features supported by the CLS communication system. AWL is the Commander system providing multiple wireless remote iTTL flashes (individually metered and adjusted by the commander and camera).

Flash Naming Nomenclature and Compatibility:

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.

Multiple flash units in the Nikon System

Any system (digital or film) using PC sync cords, or plain optical slave triggers, or any regular manual radio trigger is fully usable in Manual flash mode with any old TTL film or iTTL flash unit, including any manual studio-type flash, with any film or digital cameras, with flashes in Manual mode (but this idea is incompatible with iTTL mode, and is incompatible with iTTL Commander). Manual flash simply only triggers the flash (using only the one large center pin on the hot shoe), but cannot otherwise control or meter it individually, other than just to flash it. 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 iTTL Commander system, with radio links.

Best advice IMO: For any formal indoor portrait sessions, you want manual flash mode anyway, for full control. With even a modern digital camera (and using a flash meter to set the individual flash power levels for lighting ratio, see a good simple light setup), the plain optical triggers work great with professional results for flashes in Manual mode. These optical slave triggers are built into studio flashes, and in many modern hot shoe flashes. I use a PC sync cord to trigger the Fill light (which is very near the camera), and it easily triggers all the other flash units used.

For multiple iTTL flash, Nikon offers the AWL system (Advanced Wireless Lighting) using an optical Commander in either a camera flash or a hot shoe flash.

Nikon provides this DSLR and Flash Compatibility Chart. It mentions older iTTL flashes, but does not mention film TTL cameras.

Nikon Creative Lighting System (CLS)

CLS (in iTTL models) offers these major features (description from a Nikon SB-800 flash manual (from page 5):

i-TTL mode
This is a TTL auto flash mode in the Nikon Creative Lighting System. Monitor Preflashes are fired at all times. The subject is correctly exposed by the light from the flash lighting and the exposure is less affected by the ambient light (SB-800 page 37).

Advanced Wireless Lighting (AWL)
AWL is the Commander system providing multiple wireless remote iTTL flashes (individually metered and adjusted by the commander and camera). With the AWL Commander, individual wireless multiple flash operation in the TTL (i-TTL) mode can now be accomplished with digital SLRs. In this mode, you can divide the remote flash units into three groups and control the flash output independently for each group, expanding your range of creative multiple-flash shooting techniques (SB-800 page 76). And see more AWL description.

Most Nikon iTTL flashes offer the CLS option AWL (the SB-600, SB700, SB-800, SB-900, SB-500, SB-5000, but NOT the SB-300 or SB-400) and Nikon iTTL camera bodies offer the Commander (except NOT the D300x class).

CLS and AWL features are available only when both the Nikon camera and the flash are compatible.

Flash Value Lock
Flash Value (“FV”) is the amount of flash exposure for the subject. Using FV Lock with compatible cameras, you can first lock in the appropriate flash exposure for the main subject. This flash exposure is locked in, even if you change the aperture or composition, or reposition the camera aiming, or zoom the lens in and out (SB-800 page 61).

Flash Color Information Communication
When the hot-shoe SB-800 is used with compatible digital SLRs, color temperature information is automatically transmitted to the camera. Then if the camera mode is Auto WB, the camera’s white balance is automatically adjusted to give you the correct color temperature when taking photographs with the SB-800.

Auto FP High-Speed Sync (HSS)
Allows High-Speed flash synchronization at your camera’s faster shutter speeds. This is useful when you want to use a wider aperture to achieve shallow depth of field to blur the background (SB-800 page 60). And see more HSS description.

Wide-Area AF-Assist Illuminator
In autofocus operation, the SB-800 emits AF-Assist illumination over a wider area. This enables you to perform autofocus photography in dim light even when you change the camera’s focus area with cameras supporting this function (SB-800 page 62).

See your equivalent camera’s instruction manual for details on the Nikon See your equivalent camera’s instruction manual for details on the Nikon Creative Lighting System.

The first Nikon digital models D1 and D2 used the D-TTL system for automatic metered flash, but since then (the last 20 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.

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