It's a long article, but don't get bored and don't skip the heart of the problem below. It can seriously affect the flash exposure. The first half of this page is about how it ought to work. The last half page is about how it in fact works instead.
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, and about accuracy, which necessarily brings up strong basics of flash metering, even more important in their own right - the good stuff we need to know about.
Spot, Center, or Matrix metering choices only affect metering the ambient. The flash has its own system, which does not use Spot metering (the flash exposure does not use Spot Metering, but the ambient metering does). However, TTL BL "balances" the flash exposure with the ambient (TTL BL reduces flash level to be fill flash, to not overexpose the sum with ambient, but TTL mode does not, it comes ahead on). But if Spot metering, only the small spot area matters (to ambient), so there is no concept of background ambient to balance the TTL BL flash with - so the flash system switches to TTL mode if ambient is doing Spot metering. Spot metering is a way to switch the flash to TTL mode - however it may also greatly affect your ambient if ambient is normally bright (and meterable).
Incorrect D-lens distance data (in wide zoom lenses) is one common cause of underexposure in TTL BL direct flash. D-lens data is only used by TTL BL direct flash. Bounce flash (tilting or rotating the flash head) does not use (ignores) the D-lens data. TTL mode does not use D-lens data. Commander remotes do not use the D-lens data. Or, if your camera model (with Commander) has the FV Lock function, FV Lock will simply ignore the D-lens data, and will use the direct TTL BL metered value.
Caution: A TTL BL non-tilted flash head (in an umbrella for example) on a hot shoe extension cord (Nikon SC-17, SC-28, SC-29) will use D-lens distance data, even if the TTL BL flash is at a different distance than the lens. Nikon advises to switch hot shoe extension cord flashes to TTL mode (but they no longer provide the obvious way to do that). Spot metering or FV Lock or tilting or rotating the flash head are work-arounds.
Regarding TTL BL direct flash, first how D-lens distance works correctly. Then further below, how it often works incorrectly.
Next is a 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. This test is NOT about metering, it is about how an incorrect D-lens distance override the TTL BL metering. All of the Nikon lens made in the last 20 years are a D-lens (with the embedded CPU chip, i.e., a CPU lens). The D-lens can report distance at the focus point (it is not very 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. And sometimes the D-lens distance is simply wrong.
The test uses the fact that we know reflective meters will overexpose black scenes, and will underexpose white scenes. Large areas of either black or white 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 also mention that we ought to learn Flash Compensation.)
The D-lens distance can sometimes correct overexposure in direct TTL BL flash. And sometimes its error can cause underexposure.
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. 70-200 mm D-lens (86 mm) at 5 feet (this is a very good lens case, the lens reported 4.92 feet, 98.4% of actual, which is close enough - but other lenses can't do that, more below.) 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.
The results below are to show how it is SUPPOSED to work, when things go right (the rest of the page complains often things don't go right). The results seem dramatic enough, and obvious enough, we can see it at a glance. These exposure results are basic fundamentals about how reflective metering works.
The TTL results did overexpose the black scene (as normal and expected), but the TTL BL results came out like the GN results instead. This is due to the effect of the system checking TTL BL direct flash with the D-lens distance info, computing the guide number, and saying "Whoa man, that's too much flash power for this subject distance", and limiting overexposure via the guide number distance check. Which can be a good thing (as here), and Guide Numbers are independent of the scenes colors.
The TTL results did underexpose the white scene (as normal and expected), and the TTL BL results came out like the TTL results. This is the same metered exposure, but just with no effect from the D-lens check on underexposure (D lens check only guards against overexposure of TTTL BL direct flash).
But the D-lens distance info obviously DID NOT affect the TTL BL expected underexposure of the white scene (D-lens does not increase flash exposure). The TTL BL result is slightly more underexposed than TTL mode, as is routinely typical of TTL BL.
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 dark area in the center area. But this scene is white overall, which is a strong influence the other way, towards underexposure. The point here is, the meter meters what it can see. White scenes typically underexpose. Or 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, because the metering of the TTL preflash sees that too. 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 ensure getting his desired result. But the photographer did not get involved above, here he relied only on the automation.
This test used an older 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.
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, are 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 to be at the focus sensor, so they already know where that is, and distance does not matter anyway, there is nothing sunshine can do about it. Subject distance only matters to direct flash, when it is very important. The D-lens system is flawed, too poor to count on, especially flawed by zoom lenses that report focus distance incorrectly at different zooms.
Pretending that it somehow works though, my notion of the concept is this: Direct flash into an open background indoors often sees a lot of dark background at distances greater than the subject (inverse square law, it's dark back there). 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, not used in TTL mode). The D-lens distance reported by zoom lenses is not always correct however.
D-lens: Nikon lenses (since 1992) have a CPU chip in them, which can report focused distance to the camera. Those with it were called a D-lens (distance). They all have it now, but they don't bother to say D if they have later features to tout instead. Later lenses are called other names (G, AF-S, etc), but they are still D-lenses too. The D-lens distance info can be a watch guard for hot shoe TTL BL direct flash, protecting against overexposure for direct flash (TTL BL mode), 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. So TTL simply meters a preflash to measure the reflected light directly (instead of the distance). But here, we are speaking here only about direct flash.
GN mode: (NOT part of TTL BL) The purpose of including GN mode here is to explain 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 (the D-lens distance is too inaccurate for GN mode to use). 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. It works.
The D-lens Logic: In this way, the TTL BL direct flash power level is compared to the level suggested by the flash distance (guide number is the inverse square law requirements). If the metered direct TTL BL flash value seems excessive in comparison to the known and reasonable logic of Guide Numbers, then the TTL BL exposure is reduced, to avoid overexposure (could be caused by metering dark direct backgrounds). Rhetorical, but how else could flash use distance?
For flash, the D lens distance is considered only for TTL BL mode direct flash. NOT considered for TTL mode, and/or is NOT considered if the flash head is tilted or rotated (bounce flash, which is a very different flash path distance).
The issue causing attention to the lens distance 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 that I perceive", 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.
Hot Shoe Extension Cord: If you put the flash off-camera on a hot shoe extension cord (like the Nikon SC-28 or SC-17 cord), the camera still thinks the flash is connected to the hot shoe (CLS communication still exists). Then TTL BL direct flash can be adversely affected by the camera lens distance, instead of just using the metered flash from the actual flash distance. The flash and camera distances are of course the same when the flash is on the hot shoe (and if the reported distance is correct). But if using the hot shoe extension cord, the flash and camera are probably not at the same distance. If the flash is closer than the camera, probably no big problem, the system should use the TTL BL metering then. But the flash being farther is a real problem (for TTL BL direct flash). Actually, it is not so much about the flash being farther than the lens, it is about the flash being farther than the distance the lens may inaccurately report. It can work well if the reported distance is correct. But if the flash head is not tilted or rotated (still at regular 90 degrees to the body, for example even if in an umbrella with flash head aimed away from subject), then TTL BL exposure can be affected badly by an incorrect D-lens distance (camera distance). The D-lens report says "Whoa, that is too much flash power for this reported lens distance, and then it cuts back on the flash power (to prevent possible overexposure). It doesn't understand the situation of course. It affects TTL BL direct flash mode. The fixes include using Manual flash mode, or a tilted or rotated flash head (bounce), or actual TTL flash mode, or Spot Metering to convert TTL BL to be TTL mode. Or using the wireless Commander mode knows not to allow the off-camera remote flash to respond to D-lens distance. Or using FV Lock will ignore the lens distance info too.
NOTE: Non-Nikon brand flashes are the exception.
The Nikon system still defaults to TTL BL with them, but they do not suffer with the D-lens problem shown next below. Third party flash exposures use the metered value, and appear Not affected by bad D-lens distance. They don't have a head tilt switch, which seems able to ignore D-lens effects. That and the price are big pluses for them.
See a similar report about the same problem with hot shoe extension cords. Non-Nikon brand flashes are exempt from D-lens problems.
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. Bounce flash never uses the D-lens distance. However, in some cases of direct flash, yes, hot shoe TTL BL automation can watch distance and sometimes does reduce the flash power level, trying to prevent the overexposure expected from double checks of high guide number calculations (caused by common dark backgrounds affecting the direct TTL BL 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 errors.
The Focused Distance Reported
|Measured||24-70 mm FX D-lens reported|
|Measured||70-200mm FX D-lens reported|
|Measured||24-120mm FX lens reported|
|Measured||14-24mm FX D-lens reported|
|Measured||60mm D macro||105mm VR macro|
|Measured||16-85mm DX lens reported|
|Measured||12-24mm DX lens reported|
|Measured||18-70mm DX lens reported|
The Downside: The D-lens distance check is great in theory, when it actually works, as a TTL BL direct flash point&shoot automation. But the D-lens distance data is often not correct, which causes problems for direct flash (approximating distance from lens rotation is very difficult, not at all the same thing as subject distance). Prime lenses are better at it, but zooms are a crapshoot (a dice game of chance with a risky unpredictable random result). Zoom lens distance accuracy is very suspect, often worse at wide angle zooms (incorrect is the best way to bet) - or it is not always even available (reporting it as infinity cannot help the flash, but at least it does no harm). This feature is sometimes good (the first results above), and is sometimes bad (the next results below).
The table at right below is my check of nine better Nikon lens. Carefully measured distances were set up, and then compared the D-lens focus distance actually reported in the Exif (at minimum and maximum zoom). (See a better Exif Tool that can see focus distance in the Exif). None of the numbers are very accurate, but the numbers shaded in yellow are the problems (reported less than the true value). For example, the first 24-70 mm lens at 24 mm focused at 4.5 meters reports 2.5 meters, which causes nearly a one stop underexposure error for TTL BL direct flash. The D-lens data is a little crude, (usually better up closer). The chart shows some surprises. Even on a tripod, the values were not always repeatable each time in every case, sometimes could vary each time the shutter button was pressed (focus seek, shifting focus), especially on lenses like the 16-85 mm lens, which refocuses between minimum and maximum zooms. The far right hand columns show the percent difference.
It certainly can cause TTL BL direct flash problems when incorrect. If the D-lens distance number reported IS LESS THAN the actual measurement (cases marked in yellow in the table), it will limit direct flash power in TTL BL mode.The accuracy of the rest is not any better, but they report a distance greater than actual, which does not shut down the flash. For example, TTL BL direct flash with the 12-24 lens at 12 mm at 20 feet (6.1 meters). Chart shows it reports 2 meters at 6.1 meters. This is variable, sometimes it says 2 meters, next minute same may show infinity. Infinity is OK, it stays out of the way and does not interfere. If the D-lens reported distance is greater than the actual measurement, there is no underexposure protection involved, and TTL BL direct flash exposure is not affected (so no problem). But if you get the 2 meter 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. This 1/3 distance should be about three stops underexposed then, but in practice, it seems slightly less bad, but still very bad.
For example, below are some quickie flash pictures at night with the Nikon 16-85 mm lens, to show the concept discussed. All are point&shoot, all the same except for flash mode, which was as indicated. Focus is always on the hanging white card measured at 19.0 feet. Nikon D300 and hot shoe SB-800 direct flash. Matrix metering, ISO 400, f/8, 1/160 second. Cropped slightly, but no exposure compensation in any.
Pictures below are the 16-85 mm lens at 16 mm zoom. The 18-70 mm lens would not have done this, but the 12-24 mm would (see table). Buying a more expensive lens is no help, it is a bigger general issue, involving many zoom lenses, affecting direct flash. You'd think the designers would know this is a problem.
Now that would seem to be very dramatic picture of a real problem (again, only TTL BL direct flash is affected, but which is default). Only Nikon brand flashes. Other flashes without a head tilt switch are immune to this problem.
This is NOT a metering issue. It is not a flash problem, but it is a problem for the flash. The camera does the metering, and does it well, but here, an incorrect D-lens distance got in the way meddling with the metered result (which Only affects TTL BL direct flash). It would seem to be a serious design error, the hardware obviously cannot support the plan. Apparently no one else gets concerned about this, but my camera does these things which I sure wish it would not do.
What happens is this: The white card subject was measured to actually be at 5.79 meters (19 feet). However, the Exif reported this "Focus distance" to be 1.68 meters (5.5 feet). That's a problem, so while the preflash was metered to indicate the actual 19 foot exposure level (needing significant flash power, nearly maximum), the D-Lens data said "Whoa man - that's obviously too much flash for a subject distance of only 5.5 feet". So unfortunately, the flash power was reduced (about 3.5 stops) to prevent the supposed overexposure at 5.5 feet/1.68 meters (the way it figured it, described above). Not a plus unless it is correct. Zoom lenses are often not correct.
This 16-85 mm lens is not the only lens affected this way, and it is extremely repeatable, with this lens at 16 mm. But we simply do not know when this will happen, which lenses at which zoom and distance values. We just know the flash system is squirrelly sometimes.
Again: This topic is only about direct flash with TTL BL flash. Not about bounce, not about TTL mode. But the internal flash, the SB-700 and SB-400, only provide TTL BL mode.
We could just apply Flash Compensation. Camera Flash Compensation only has a +1 EV range, but on Nikons, Exposure Compensation value adds to the Flash Compensation value. Saying - When ambient is dim and negligible, Nikon EC amounts to the same thing as FC.
This seems BIG. This very easily repeatable. Why there is no uproar about incorrect Nikon D-lens distance screwing up TTL BL direct flash mode (the default, for example the internal flash) is a mystery to me. We try to work around it, but at Minimum, the camera ought to have a menu to NOT DO THAT, to be able to ignore D lens data in zooms.
Next are two ways to work around this D-lens problem with TTL BL direct flash.
4. TTL BL again the same, but now simply using FV Lock, which also suppresses D-lens distance data. White card tone is 230.
So FV Lock can be a work around for the D-lens zoom problem. The camera models with Commander have the FV Lock feature.
Nikon manuals say of FV Lock: "This feature is used to lock flash output, allowing photographs to be recomposed without changing the flash level and ensuring that flash output is appropriate to the subject even when the subject is not positioned in the center of the frame."
So the logic seems to be, if we might re-aim the camera elsewhere, which D-lens distance is appropriate? We can get a different result with FV Lock. So FV Lock ignores D-lens distance, and lets the TTL BL metering stand. The evidence is clear, and again, it is extremely repeatable. The Exif says FV Lock is still TTL BL mode, but there was no ambient here (certainly not at f/8 1/160 second), and the TTL BL flash level is within 1/3 stop of TTL now. FV Lock can vary that much with TTL, and FV Lock seems a very big help here (speaking of TTL BL direct flash with a zoom lens).
This D-lens distance issue is only a problem for TTL BL direct flash, but never for any bounce nor for any actual TTL mode. However, TTL BL is the only choice for the Nikon internal flash, the SB-700, the SB-300-400-500, and almost all third party flashes. Only the Nikon SB-600, SB-800, SB-900/910, and a few Metz models have a menu to choose TTL or TTL BL. But Nikon removed that menu choice from the newer models. They should put this necessary option in a new camera menu. Because choosing actual TTL mode (vs TTL BL) is a solution, except for most flashes now, Spot Metering mode is the only remaining way to get there.
5. TTL BL was still selected, but now using Spot Metering to switch metering to TTL mode, which also suppresses D-lens distance data. White card tone is 239.
So Spot metering mode can be a work around, indoors, or in dim areas. But which is more problematic in bright sunshine. Spot metering can drastically affect the exposure of bright ambient, so we definitely need to know what we are doing. But indoors, the ambient level is normally insignificant anyway (the reason we use flash).
Note that the flash system does NOT actually do Spot metering. The flash has its own metering system, which does not use Spot metering (only ambient does). Spot metering can affect bright ambient in its way, which in turn can affect TTL BL fill level. But the ambient was Black here with no flash, too dim to affect the metering, so here, or normally with flash indoors, it does not matter which spot we aim at, ambient is not in play. And the camera was in Manual mode (so metering could not change the camera settings anyway). This case (dark ambient) is NOT actually doing any Spot metering, it just switches the flash metering from TTL BL to be TTL (because Balanced flash makes no sense with Spot metering - when only ONE spot is considered). But the ambient being dark and not in play does allow Spot metering to be an acceptable choice for flash indoors, more so than in bright sunshine, because spot metering in sunshine is going to seriously affect the ambient settings.
6. Non-Nikon brand flashes (Yongnuo, Neewer, etc) don't have the head tilt switch, and seem not to suffer with this problem. See those results.
The camera seriously needs two new menus:
Saying, not in this case, but we do have to deal with the sunshine. Use of Spot metering causing TTL flash mode means there is no automatic balanced (reduced) fill level in sunshine - so TTL mode requires you to manually apply about -2 EV flash compensation to prevent subject overexposure by both TTL and sunlight. And Spot metering of course can greatly affect the way the ambient is metered. Saying, neither factor is likely much of a concern in normal indoor flash pictures where ambient is insignificant level. Just don't forget to reset the metering mode when you go back outdoors.
If the reported D-lens distance error is less than the actual distance, it limits the TTL BL direct flash power that was properly metered. If the reported D-lens distance error is greater than the actual distance, then it is simply ignored. The system only tries to protect against overexposure, not underexposure. My experience is that we often see significant problem at the wide end of zooms. The D-lens is intended to help, but it is implemented too poorly in many zoom lenses, instead we get random wrong results.
The D-lens distance data simply is wrong sometimes (especially in wide zoom lenses), and of course it is one cause of TTL BL direct flash underexposure problems. You can always compensate the result, or use TTL mode (TTL mode does not use D-lens data).
To disable the D-lens distance check (because it often interferes with TTL BL direct flash):
This D-lens distance can even vary a little between two shutter presses (focus-seeking at each shutter focus), especially on these worst cases. A 20 foot case (6.1 meters) at 16 mm zoom might indicate 1.4 to 2.8 meters on this 16-85 mm lens (but which is not the 6.1 meters). It seems some people are not bothered by this, but I think it is a real big deal, adversely affecting TTL BL direct flash exposure. Depending on which lens (and which distance and which zoom), it can cause severe underexposure, shown here.
The D-lens focus distance data only affects direct flash in TTL BL mode. It does not affect bounce or TTL 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 of the ambient, which if significantly bright, introduces new issues for novices). Or, FV Lock can improve TTL BL (but FV Lock is only on cameras with Commander). FV Lock also ignores TTL BL direct flash D-lens checks, and uses the metered value. 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 (it is good in sunshine though). 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 distance data wrong, but it varies, any two shutter presses may report a different distance, at least with wide zooms. The other lenses in the chart might not see so much issue (only a problem when the D-lens data is LESS THAN the actual distance). Reporting infinity distance is great, that means this feature turns off. These are all fantastic lenses, and the only real issue is that the TTL BL flash system design wants to look at inaccurate distance numbers. The D-lens distance data seems a real detriment for zoom lenses that cannot do it. For example, the GN mode is all about distance, but it does not use the D-lens data (how could it?) The system works just fine if not using garbage data. Nikon marketing could tout a new menu disabling D-lens data was to allow TTL BL mode off camera on a SC-28 hot shoe cord, instead of having to admit the zoom lens distance was poor enough to screw up flash exposure.
To prevent these lens errors from causing flash underexposure, I think we obviously need a new menu "Do Not use D-lens distance for TTL BL direct flash". Otherwise, we can use bounce, or FV Lock, or we can get out of TTL BL mode. Most flashes no longer have a TTL override. We could switch to TTL mode via Spot Metering, but realize then for the ambient exposure, you have to learn to evaluate the spot. This is all very non-intuitive, we need something more straight forward. I think this TTL BL vs TTL mode ought to be a new camera menu too, since it is the camera that does this metering. 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.
Here are some pictures with same setup as on next page. D300, 16-85 mm lens, 16 mm zoom, 7.5 feet (2.3m) actual measured distance, and direct flash. All that changes is flash mode. D-lens reports different distances at each focus try. White Balance is the only adjustment made. WCV = White Card Value from eye dropper.
Note that the third picture (TTL BL without FV Lock or Spot metering) is the only case here that D-lens distance info will have any effect. And that effect seems a poor result, at least with this lens choice. The Exif for FV Lock does still say TTL BL. The camera manual shows the FV Lock metering area which is much larger than Spot metering, but not quite Center Weighted. The D-lens distance info must be ignored by FV Lock, and TTL BL often improves significantly (see more). Try it, and decide for yourself.
The bottom line is that 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).