The obvious major function of TTL BL flash mode is to provide automatic balanced flash in sunlight (automatic reduced flash level for proper fill level in brighter light). See Part 4 about that. The story here is about a subtlety, but one which necessarily brings up strong basics of flash metering, even more important in their own right - the good stuff we need to know about.
This page shows a very quick test about how Nikon TTL BL mode uses (or does not use), the D lens distance info for hot shoe TTL BL direct flash. All of the Nikon lens made in the last 20 years are a D lens. The D-lens can report distance at the focus point (it is far from accurate, but sometimes it is close). This test will show that the D lens can prevent TTL BL direct flash overexposure, which can frequently occur in direct flash due to dark backgrounds (from inverse square law falloff). But underexposure of TTL BL is not affected by the D lens.
The test uses the fact that we know reflective meters will overexpose black scenes, and will underexpose white scenes. Large areas of either typically come out gray, because that is simply what the camera reflected meters do. So expecting that, a pair of black or white scenes were used here, and without being too fussy or precise (no attention was given to compensate exposure), we can consider "proper" exposure to be white that is white, and to be black that is black, instead of both being gray. (Genuinely useful info here would be to mention that we ought to learn Flash Compensation.)
About all that flash exposure can do with distance is to compute Guide Number. The test method here is to compare TTL BL results to the SB-800 GN mode (Guide Number method). If they both use guide numbers, the exposures ought to match. We have to know distance for the Guide Number method, but if the numbers are known, GN is usually a pretty good result, better than some - because, GN is independent of the reflected subject colors, similar to how incident metering is also independent of subject colors. These two methods (GN and incident meters) give an exposure based on the actual light level instead of being about how well the subject's specific colors reflect light (downside of reflected meters).
The pictures below: Nikon D300, f/8 1/125 second, ISO 200. 70-200mm D lens. SB-800 hot shoe direct flash. No compensation, all point and shoot, except the distances were measured for the GN mode. The only adjustment is white balance. The backgrounds are black and white paper. I always use Center Weighted metering myself, but this is instead Matrix metering for those that think it might matter (it doesn't for flash indoors, because flash uses its own metering area in the center of the frame). Again, this is not about theoretical perfect exposure, as there was no attempt here to compensate anything. There is a two stop difference in exposure between 10 feet and 5 feet, but all the systems accounted for that (each set of rules came out the same for the two distances, which is a pretty big deal).
The results below seem dramatic enough, and obvious enough, we can see it at a glance.
The TTL results did overexpose the black scene (as expected), but the TTL BL results came out like the GN results instead. This is due to effect of the D lens distance info computing the same guide number, and limiting overexposure via the guide number distance check.
The TTL results did underexpose the white scene (as expected), and the TTL BL results came out like the TTL results. This is the same metered exposure, with no effect from the D lens on underexposure.
GN mode (guide number) TTL BL mode TTL mode
I focused on the horse's nose. Note this area includes the dark direct shadow. We don't see that dark shadow when focusing, but metering of TTL preflash obviously sees it, which is a significant area at the focus point here. Then if seen, metering computes a darker average of its overall view, which lightens the exposure to compensate - just meaning, maybe not as dark as the middle gray the all white scene would have been. But this scene is white overall, which is strong influence the other way, towards underexposure. Maybe if focus had been on the ears or the feet, maybe it wouldn't see a shadow. The point here is, the meter meters what it can see. Which for TTL direct flash metered on camera, obviously frequently includes preflash shadows and dark open backgrounds, which tends to overexpose. For bounce, direct shadows disappear and backgrounds are lighted better, which may tend to underexpose. But photographers can learn to see the scene, to expect this before the shutter. Call it platitude if you want, but experience is literally about expecting the same result as you saw last time in a similar situation (but for experience to help, you do have to think about it).
Which picture is the correct result? Possibly none of them are exactly what we would want. These are extreme subject situations, so we know better, know that some attention will be required. The camera automation is a starting point, but the photographer will need to pay attention, and then do what he sees he needs to do (flash compensation), to insure getting his desired result. But the photographer did not get involved above, here he relied only on the automation.
This test used a five year old D300. Nikon sometimes tweaks things, so just to check, the new D800 produced very similar, but slightly different results (viewed here). It does not affect the discussion.
TTL BL mode is the "only" automatic flash menu choice available for the Nikon internal flash, or the SB-400 and SB-700 external flashes, or the Commander menu for Remote flashes. Plain TTL mode is not available. Their menu may be generically abbreviated TTL, but is a TTL BL system, and it is TTL BL by default - Unless Spot Metering is selected, which always switches to TTL in every case (no concept of background or balancing with Spot metering, but Spot metering introduces its own issues). Only the current SB-910, and discontinued SB-600/SB-800/SB-900 flash models have a menu to select TTL to override TTL BL.
If you put the camera off-camera on a hot shoe extension cord (like the Nikon SC-28 cord), TTL BL can be affected by the camera distance, not the flash distance. So, this case needs to use TTL mode, or to keep the flash and camera at the same distance from subject. Or, you can use Spot Metering, or you can angle the flash head away from 90 degrees to disable the lens distance effect. Or, using the wireless Commander mode knows not to allow the off-camera remote flash to respond to lens distance. And remember, Flash Compensation is the way to control an automatic flash.
Theory: Nikon only says a few vague gee whiz marketing words, and does not tell us how the system works. Automation mysteries: matrix, databases, balanced flash, etc, just labels, but no details. They say 3D Matrix metering takes into account the subject's distance, but never say how and why, or when and how often. Cameras don't know distance. The D-lens is a crude tool attempting to approximate it from focus rotation. The subject is assumed at the focus sensor, so they already know where that is, and distance does not matter anyway, there is nothing ambient can do about it. Distance can only matter to direct flash, when it is very important. My notion is this: (and simple experiments clearly support it, but the D-lens system is flawed). Direct flash into an open background indoors often sees a lot of dark background (inverse square law). Metering sees it too, and its goal is to make things brighter, which frequently causes overexposure of direct flash. So the D lens distance can compute guide numbers, and if it thinks the metered flash power exceeds what is reasonable for the distance, it cuts back on the flash power. We can see this happen, shown here, above and below (it is only a factor for TTL BL direct flash). The D-lens distance reported is not always correct however.
GN mode: (NOT part of TTL BL) The purpose of including GN mode here is to simulate the D lens distance info effect, for the obvious comparison above (SB-800 menu shown here). GN mode is a manual flash mode, but it computes the manual power level from Guide Number. The hot shoe flash knows aperture and ISO and zoom from the camera, and it knows the guide number chart too, so mode GN looks up the right flash power level for this distance we enter. GN mode is basically just a calculator for the Guide Number system, and it is pretty good stuff for direct flash, if you know the distance. Again, it only works for direct flash, and you enter the distance manually. Nikon calls the mode: Distance Priority Manual Flash. Unlike the TTL modes, a really wonderful advantage of GN mode is that it is independent of the subject colors (independent like incident metering in that way), so that generally, if the light itself is made right for the distance, black things will come out black, and white things will come out white. But GN mode is direct flash only, and you have to enter the subject distance.
D lens: Nikon lenses (since 1992) have a chip in them, which can report focused distance to the camera. Those with it were called a D lens (distance), and they all have it now. The D lens distance info can be a watch guard for hot shoe TTL BL direct flash, protecting against overexposure for direct flash, with the flash head is level straight ahead. There is an electrical switch to indicate when the flash head is tilted or rotated, because the GN distance applies only if direct flash. Guide Number is not practical for bounce. We can't measure the bounce flash path distance very easily (up and down), and there are losses at the ceiling reflection surface too. But TTL simply meters a preflash to measure the light directly (instead of the distance). Here, we are speaking here only about direct flash.
The issue is that direct flash often sees an open background as being dark, due to the inverse square law falloff. TTL BL is of course a metered system, and it is affected by the dark it sees (intends to do its best to lighten it). But with D-lens data, the system can also still do the same Guide Number check, and for direct flash, TTL BL watches distance. And when this says "What in the world is that meter doing now? This flash level is too much for this distance", then direct flash can reduce the flash level to the GN value, to prevent probable overexposure (with results seen in the black pictures above). Using this D lens distance does make the assumption that the subject (where we probably want the flash to be correct) is same as the focused distance. In contrast, the actual GN mode does not know about the subject, and we enter its distance manually - which is less convenient, but more versatile, and has advantages.
Some claim TTL BL mode "always uses distance" to compute its exposure. It doesn't of course. That is exactly what GN flash mode is, but TTL BL mode is in every sense a TTL metered system. Obviously it computes flash exposure based on metering the preflash reflected from the scene. However, in some cases of direct flash, yes, hot shoe TTL BL automation can watch distance and sometimes does pull the power down, trying to prevent overexposure (caused by expected dark backgrounds affecting the TTL metering). This feature is intended to be a safety check on an abnormal situation, but seeing that happen a lot could cause us to imagine it always uses distance. TTL BL obviously is adversely affected much of the time on some lenses due to their D lens distance errors (direct flash only). But lenses vary, all do not report the same distance.
Downside: The D-lens distance check is great in theory, as a point&shoot automation. But the D-lens data is not always correct, which causes problems for direct flash (approximating distance from lens rotation is very difficult, not at all the same thing as subject distance). Or it is not always available (reporting it as infinity at least does no harm). This feature is sometimes good (the first results above), and is sometimes bad (the next results below).
The chart at right below is my check of eight Nikon lens. It carefully measured the focused distance, and compared it the D-lens distance info reported in the Exif. The D-lens data is a little crude, (usually better up closer). The chart shows some surprises. The values were not exactly repeatable each time, in every case (focus seek, shifting focus), esp on the 16-85 mm lens, which refocuses dramatically between 16 and 85mm zooms.
It certainly can cause problems when incorrect. If the D lens distance number IS LESS THAN than the actual measurement, it will limit direct flash power in TTL BL mode. For example, TTL BL direct flash with the 12-24 lens at 12mm at 20 feet (6.1 meters). Chart shows it reports 2 meters at 6.1 meters. This is variable, sometimes it says 2m, next minute it may show infinity. Infinity is OK. But if you get the 2m value when focused at 20 feet (6.1 meters), this D lens will erroneously limit the TTL BL flash exposure to instead be correct for 2 meters (6.6 feet), even when the actual distance is 6.1 meters (20 feet). This 1/3 distance should be about three stops underexposed at 20 feet. In practice, it seems slightly less bad, but still very bad.
For example, here are some quick point&shoot flash pictures with the Nikon 16-85mm lens, to show the concept. All are the same everything except flash mode, as marked. All are direct flash at 16mm zoom. Focus is always on the white card at precisely 20 feet. This night scene was selected to have no bright ambient, no bright or dark background, and no foreground objects, all of which can affect metering. This is solely about "the 20 feet". D300, matrix metering, SB-800. No compensation, only adjustment is white balance. Three flash modes are compared (only change is flash mode):
GN flash mode. Actual distance is used in GN mode, when the GN distance was set for measured subject distance to the white card. White card tone is 248.
TTL mode. D-lens data is not used in TTL mode. White card tone is 244.
TTL BL mode. ONLY change was switching to TTL BL flash mode, which of course sees the same preflash, metered in the same way. White card tone is 174 here. Exif says D lens reported this subject distance to be 1.41 meters (4.6 feet). But the focused subject is measured to be 6.1 meters (20 feet). So unfortunately, the flash power was reduced to prevent the expected overexposure at 1.41 meters (the way it figured it, described above). The D lens data simply is wrong sometimes, one big source of problems. You can always compensate any result, but to avoid surprises, one conventional wisdom is to only use TTL BL when you actually want point&shoot to automatically balance ambient and flash, but use TTL mode indoors, or when not balancing.
This D-lens distance can vary a little (focus-seeking at each shutter focus), esp on these worst cases. This 20 foot case (6.1 meters) at 16mm zoom might indicate 1.4 to 2.8 meters on this lens, but this case is not near 6.1 meters. It seems many people are not bothered by this, but I think it is a real big deal, adversely affecting TTL BL flash exposure. Depending on which lens, it can cause severe underexposure, shown here.
The D lens data only affects direct flash in TTL BL mode. But many users use only direct flash, and TTL BL is the default mode, so they are going to see a lot of this. No wonder lots of people dislike using flash, I would too, if I had to live like this. Switching out of TTL BL mode, instead to be TTL mode, easily solves this (ignores D lens data).
However, Nikon is not shipping many flashes today that allow that, the SB-910 is the only current model with the menu to override TTL BL mode. If no menu, switching to Spot Metering reverts back to TTL mode (but it also does Spot metering, which introduces new issues for novices). And while I cannot say using FV Lock causes TTL mode, it can improve indoor TTL BL, sometimes when it needs it (but FV Lock is only on cameras with Commander). For me, FV Lock can help TTL BL considerably (direct or bounce), but has less effect on real TTL mode. I use bounce whenever possible, and avoid TTL BL all I can, so I really was not aware it was this bad. Depending on which lens, it is very bad.
The Nikon 16-85 and 12-24 are great lenses, but their D lens distance data is abysmally poor. Not only is the D lens data wrong, but it varies, any two shutter presses may seek and report a different distance, at least with wide lenses. The other lenses in the chart should not see so much issue (only a problem when the D-lens data is LESS THAN the actual distance). Seeing infinity distance is great, that means this feature turns off. These are all totally fantastic lenses, and the only real issue is that the TTL BL flash system design wants to look at useless numbers - my notion being only so marketing can say they do too. The D lens is unnecessary, and what would seem needed is a camera menu option to turn off using D lens distance for the flash. For example, the GN mode is all about distance, but it is manual entry, NOT using the D lens data (how could it?) The system works just fine if not using garbage data.
The only solution is to use bounce, or to get out of TTL BL mode. Switch to TTL mode (perhaps via Spot Metering, but realize you have to learn to evaluate the spot). TTL mode is perhaps not always exactly right either (is affected by the reflection from the subject), but at least it is more predictable and consistent, less unknown automation mucking around in it. But many flash choices provide only TTL BL, with no other choice. The system default is TTL BL now, and this is how they implemented it - IMO, to use unusable data. Unbelievably poor plan. I knew I didn't care for TTL BL, but I was not aware it was this bad (speaking of direct flash).
My own notion is that FV Lock also effectively switches to TTL mode. Technically it doesn't, at least the Exif does still say TTL BL, but certainly FV Lock makes a very similar change, the rules change, and indoor TTL BL often improves. Try it, and decide for yourself. Here are some pictures with same setup as on next page. D300, 16-85mm lens, 16mm zoom, 7.5 feet (2.3m) actual measured distance. All that changes is flash mode. White Balance is only adjustment made. WCV = White Card Value from eye dropper.
GN flash mode, 2.3 meters
reported as 1.4m, WCV 245
TTL flash mode
reported as 2.8m, WCV 247
TTL BL flash mode
reported as 1.4m, WCV 210
Flash level reduced for 1.4m
instead of 2.3m (>1 stop)
TTL BL, FV Lock
reported as 1.4m, WCV 238
Spot Metering on gray card
TTL BL changes to TTL,
reported as 1.2m, WCV 247
The bottom line is that the there are two exposure skills to develop for using TTL flash. The necessary one is to learn to use Flash Compensation, which is the only control you have over the automated metering. If you don't like it, simply fix it. If it is off a little or a lot, still simply fix it. The other skill is a helpful shortcut - it is to develop the ability of simply looking at the scene, and "seeing it" (and recognizing its dark or light subject, and its dark or white background, etc) to learn to recognize in advance how its dark or light areas are going to influence the metering, affecting the compensation you will need to do (point is, it can largely be done in advance now). Both are quite easy after slight experience (usually just about like last time in the similar situation, i.e., experience). You simply have to think about it. Then regardless if the automation dances around or not, you will know what to do to get the result you want.
Next is Comparing TTL BL with two lenses, more data in more detail about this D lens/TTL BL direct flash situation (image intense).