Nikon flash system – TTL vs TTL BL
Nikon’s TTL flash system is generally accepted as being quite exceptional. The camera and speedlight working together according to various algorithms to control the flash exposure. The TTL flash exposure will depend on various factors – the tonality of the subject and scene; the brightness of the scene; and how the camera interprets the sections of the metering pattern. The other factors quite possibly also includes data from the lens.
How these factors inter-relate, we can only guess at. We can make educated guesses; and many photographers have taken time and effort to do test shoots to see how the cameras and speedlights work. To make things even more interesting, Nikon offers two modes of TTL flash metering: TTL and TTL BL.
The way I understand this to work, is that with TTL BL flash metering, the camera takes the ambient light into account when calculating the flash exposure. With TTL flash metering, the flash metering would appear to be de-coupled from the ambient metering.
Similarly, with Canon, you have Average / Evaluative flash metering for TTL flash. (This is set on the camera body via the custom functions.) Again, the way I understand this to work (and I’ll gladly be corrected or fine-tuned on this), is that with Evaluative TTL flash metering, the camera takes the ambient light into account. And with Average TTL flash metering, the camera is less biased by the available light..
So which TTL flash exposure mode to use? TTL or TLL BL?
In the end, I work in a fairly simplistic way …
With Nikon I keep it to TTL BL, and with Canon I mostly keep it to Evaluative TTL flash metering.
I then adjust my flash exposure compensation to taste. I do this by pre-judging the tonality of my subject and scene, and making an educated guess as to how much FEC I would need. Then I fine-tune this by looking at the image on the camera’s preview.
This does strongly imply that you have to ride the FEC as you check your results while you shoot. Shoot, check and adjust.
Ultimately you HAVE to get used to how your camera and flash responds, by getting familiar with your equipment and shooting a lot. This to me is the key point here .. being able to roughly predict how my camera will react, and what the flash exposure would be like as a result. It comes with experience and shooting a lot.
My take on it is that I am better of using one of the modes the majority of the time, and get a feel for how my camera and flash would react. Rather that, than jumping between the modes, and hoping the camera and flash will sort it out, without much input from me as the photographer.
The general approach:
Keep to TTL mode if the flash is a dominant light source.
Change to TTL BL when the flash needs to act as fill-flash or when the flash is in relation to the ambient light .
This is the approach as generally advised on various websites and forums, and is how I used to do it with the D100 and D2H and D2x.
As it is, I mostly keep to TTL-BL on my Nikon D3 bodies. There isn’t a huge difference between TTL and TTL BL modes on the Nikon D3, like there was with the Nikon D100 for example.
With the D100, I would be change between the two modes, picking TTL when the flash was a dominant source of light … and TTL BL when I wanted fill-flash, or needed the flash exposure in relation to the ambient exposure. WIth the Nikon D3, I would just use TTL BL mode and adjust from there.
In my opinion, with the latest range of Nikon D-SLRs, it doesn’t make such a difference whether you use TTL or TTL BL. Since TTL flash metering is dependent on the tonality of your subject / scene (and all the other factors), you will have to ride your FEC anyway.
Once again, this means that there has to be some familiarity in how a specific camera and speedlight works together. This is how I noticed that I needed to adjust the FEC on my D3 bodies differently than I was used to on the D2x and D2H.
So whichever TTL flash exposure mode you use, for the same situation, you’d just set your FEC to different values … and still get correct exposure by looking at your camera’s preview, and making a visual judgement of how much more or less FEC you need.
We need to accept a certain flexibility in our technique … as opposed to expecting that by meticulously analyzing how our camera and speedlight work in test situations, that we could get every image perfectly exposed from the very first frame. I believe this one of those areas where there is a danger of over-thinking it and expecting real world situations to fall withing specifically anticipated behavior … when it is much simpler and better, to simply adjust your FEC as you need, on an on-going basis while you shoot.