In 2003, Nikon introduced the CLS flash control system – and there was light. CLS stands for the Creative Lightning System, and it is indeed a very good system. It brought Nikon in front of everyone and everything, until 2009 when a modification of i-TTL (a part of CLS) made the concept take a turn for the worse.
Rewinding a bit to film SLRs, Nikon had an – at the time – good TTL flash control system, that measured the light from the flash as it bounced off the film during exposure. TTL means through-the-lens, as opposed to measuring the general light level attained by firing a flash (which is what simpler flashes do). Nikon’s system was good, but for example Minolta had an as good system back then.
Then, starting in 1999 with the launch of Nikon’s first digital SLR (appropriately named D1), Nikon users were surprised at the modifications made to the flash system. The digital sensor and the anti-aliasing filter are more shiny and do not reflect light in the same way as 35 mm film. Thus, Nikon had to modify the TTL flash system as it was no longer meaningful to measure the exposure by the light reflected. Instead, Nikon had to rely on a set of pre-flashes to determine how much flash is needed. The new system was called D-TTL, as in D for digital. The pre-flashes are bounced off the focal plane shutter instead of off the film. To correct for the light in the scene, the shutter blades on D-TTL cameras are painted gray as a gray-scale card. This was an obviously kludgy solution, and Nikon knew early on that they had to design a new system for digital, not merely patching the old one. Another sign of this is the set of flashes that accompanied the D-TTL system. There were only three flashes ever produced that were compatible with D-TTL and they soon became obsolete as the new i-TTL system was quickly launched. To add insult to injury, Nikon decided that new cameras (except for the D2 series) would not be backward compatible with flashes only a couple of years old.
The new system called i-TTL (i for intelligent, implying that the old D-TTL system was indeed unintelligently conceived) was intended for the Nikon D100 in 2002 but was not ready in time for its launch. Thus, the D100 was left with the old dysfunctional D-TTL system. The first camera to incorporate the i-TTL system was instead the D2H in 2003 and the SB-800 Speedlight i-TTL flash was introduced with it. The D70 of 2004 was the first of the non-professional Nikon cameras to have the i-TTL functionality.
The i-TTL functionality was state-of-the-art and was essentially unmodified until 2009 when some engineers back at Nikon decided to “improve” the functionality without any notice to the user community. The “new i-TTL” system tends to choose extremely high ISO under the auto-ISO option when it is not called for to make a correct exposure. The reasoning behind the change is that as cameras became better at handling high ISOs, this should be used to make images have more exposed backgrounds. While this is not wrong as an observation, the mistake made was to force this into the functionality as a non-option. In the “original i-TTL” system, you could crank up baseline ISO for the auto-ISO option manually whenever you felt this was called for depending on the image effect you were after. You are in the driver’s seat, especially if you have a camera where the ISO is assigned to a button like the D70. In the “new i-TTL” system, this is done for you in a way you cannot anticipate or control. You have become a passenger in a flash unit driven by algorithms you cannot understand. A black box of lightning. A much longer argument against “new i-TTL” is made in this blog post. For example, owners of the Nikon D300s find themselves with a completely different i-TTL than those of the otherwise very similar D300. Those poor ones that have one of each are, to say the least, confused. What Nikon did was adapt the i-TTL behavior to novice users, trying to save some of their need to think. But they did this on all models, from toys like the D3100 to cameras targeted at advances users like the D300s. Big mistake. What they should have done? 1. either applied it only to toy cameras, or 2. better yet, made it configurable. “Original i-TTL” vs. “New unpredictable i-TTL”.
This is not a lecture on flash lightning, there are a number of good tutorials out there. Essentially, you must learn to master the balance between two kinds of light sources, flash and ambient. These can be of different luminance and color, even mixed. They have differing direction, duration, etc. But in all this, you want your camera to help you. The original i-TTL system does an excellent job of doing that provided you know how it works. Just like ordinary (non-flash) exposure, there are two modes of operation with flash. In non-flash exposure, you either use the PSAM modes where you, even though you could have some automation, it can be overridden since you are in the driver’s seat. Or you use the preconceived scene and green auto modes (scene modes are called vari-programs in the D70). The scene/green modes contain ready-made settings for exposure. These modes help inexperienced users to set the camera up and you are the passenger.
The same goes for flash exposure. Apart from the manual mode, there are two TTL modes. This is not well understood by most i-TTL users. Either you use Standard-TTL mode where you are in control and can apply corrections to the flash values. If you want -1EV for the flash compensation, it has the intended effect since you are the driver. Or you use TTL-BL (where BL stands for Balanced Light) where the camera logic does what it thinks is best for you regardless of what you asked for. Sometimes your corrections are applied, sometimes not. You are a passenger. This is unknown to many and explains some reports on erratic flash exposure by some Nikon DSLR users. It’s not an error but a design choice. One that has been poorly communicated, though. You need to know how you switch between the Standard-TTL and TTL-BL modes. The selection is implicitly done by your choice of metering and automation modes. If you select M = manual exposure or if you select Spot as your metering mode, Standard-TTL is selected for you. Otherwise, TTL-BL is selected for you. You cannot select the mode explicitly. This is of course a mistake by Nikon. It should have been selectable, just as exposure mode is.
To sum up: if you want control, use Standard-TTL mode. If you want to be a passenger (and sometimes you want), use TTL-BL mode. Make sure you know how to switch modes. If you want to be in control of ISO baseline values when using flash automation, stick with Nikon cameras designed prior to 2009 that have the original i-TTL system. If you don’t want to be in control, just being a passenger and accept what’s being handed to you, then you could be as happy with the new i-TTL system. This is much more important than you might realize at first. You should have the option to use flash as fill-flash in daylight scenes all the time, and that option is severely limited in the “new i-TTL” since you must go all manual to have control over the ISO value. In the “original i-TTL” you simple stay in automatic Standard-TTL and select baseline ISO for auto-ISO manually. I very much prefer the “original i-TTL” of the D70. Apart from TTL mode selection, the D70 is flash ergonomics at its best.