D lens data screws up TTL BL direct flash

Started Dec 4, 2012 | Discussions thread
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D lens data screws up TTL BL direct flash
Dec 4, 2012

Don't know where to post this, it is a flash situation, affected by D lens.

I avoid TTL BL flash mode in favor of TTL (when I can), but I have just become aware of a tragic situation about the Nikon TTL BL flash system. I knew TTL BL was flaky, but the D lens distances that TTL BL direct flash sometimes uses are often garbage. The D lens is not really needed anyway, always works fine without it. The problem is the stupid system design uses it anyway, knowing that is garbage. Seems unthinkable.

TTL BL is of course a TTL metering system, analyzing preflash reflected from the scene. However direct flash in an open background typically sees a dark background (inverse square law falloff), affecting the meter, and tends to overexpose trying to fix it. So for direct flash, the default TTL BL mode looks at the D-lens distance, and may say "What is that meter doing here? That is too much flash for this distance.". So as a safety guard preventing overexposure, it cuts back on flash power level, to suit its own distance. Sounds OK, but the problem is, much of the D lens data is garbage. Lots of it just says infinity, which is no problem, it just disables this feature (which is a good thing). But some cases may say 1.4 to 2.8 meters when the focused distance is 6 meters. That is a significant problem.

Here is a for example: Nikon 16-85mm lens, at 16mm zoom, direct flash, focused on white card at 20 feet. No near stuff to confuse anything. D300, SB-800, no compensation.

Flash GN mode, distance set for the measured 20 foot subject distance. No problem. White card is 248.

Flash TTL mode. No problem. White card is 244.

Flash TTL BL mode. Exif shows subject distance to be 1.4 meters (4.6 feet), but it is actually 6.1 meters (20 feet).

This is trivial to duplicate with some lenses. This is just walkup point & shoot stuff. I can see why many users just don't like flash. Many are using only direct flash, often the internal flash. The default is TTL BL, and they must see no way around this. I'd hate to have to live like that. Seems very serious, puzzling why it has not been mentioned before.

I really wasn't aware it was this horrible. I mean, this is EVERY TIME with the 12-24 or 16-85, if around 10 to 20 feet. I myself use bounce when possible, and do avoid TTL BL at every chance, so I wasn't seeing this, I was unaware. I had no clue it was as bad as this. Seems unusable, at least for lots of users who only use direct flash.

The only way to turn the problem off is to use TTL flash mode, instead of the TTL BL default. But Nikon is not selling many TTL flash these days, SB-910 is only current model with the menu. If no TTL menu, Spot metering will select TTL, but which is another rough spot for beginners. TTL mode is generally better anyway, certainly indoors, and it does not suffer this fault.
Really hard to understand how Nikon can ship stuff like this. They have to know the D-lens distance is just crap. Marketing probably wants this because Canon does it, etc, etc. But it is not just some menu we can ignore, it screws up the flash pictures. They could make me happy by offering a camera menu so I could turn this stupid D lens feature off. Obviously works without it, better without it.

Not all lenses are this bad, but the 12-24 and 16-85 (great lenses) are pitiful in this regard. Some lenses give it up early (at short range) and start reporting infinity, which is same as disabling it, which is a good thing.

But IMO, the real fault is not the lens. D-lens is nothing needed anyway, everything obviously works fine without it (better without it). The problem is the irresponsible design uses it, presumes to actually use these garbage numbers, which screws up flash unrealistically. Unimaginable to me, I am really outraged/disillusioned to think Nikon does it this so poorly. Inexcusable.

I see what my lenses and flash do, but wonder if I am causing this somehow? I really don't think that is possible, but comments are welcome.

Here is my chart testing various Nikon D lenses for distance they report. Infinity is a very good thing, it turns this problem off. The problem for TTL BL direct flash is when D lens says distance is way less than it actually measures. Stupid design believes it.

Numbers generated by stretching a measuring tape from a closed door (at least 20 feet). Move a tripod along the tape, every five feet, focusing on the door frame, and click the picture. Make notes about file name and distance to avoid confusion later. Subject Distance is in the Exif, in meters. 3.28 feet in one meter. Some numbers vary a bit each time, focus seeking I suppose.

Nikon D300
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