This is Part 2 of the preceding TTL BL page.

TTL BL exposure is influenced by four factors, other than any normal metering of the scene:

1. Main idea of TTL BL is flash reduction to balance with (to not overexpose) existing ambient exposure. (Seems not always the expected zero effect in dim places. TTL BL Bounce sometimes seems to need +EV compensation.)
2. Similar to #1, reduction to prevent the addition (overlap) of multiple flashes (Commander Main/Fill setup typically needs about +1 EV manual flash compensation.)
3. D-lens distance reduces flash level when distance is reported shorter than metered exposure is planning (safe guard, direct flash only - the subject of this page)
4. Normal metered variations of reflected intensity from subject/background colors (dark/black is overexposed, light/white is underexposed, as is always normal for reflective metering)

In contrast, TTL mode is only affected by #4, i.e., TTL mode does NOT watch the others automatically (and someone has to watch 1-3, but it helps to be there, and see it). The others (1-3) are considered helpful automation for TTL BL mode (point&shoot), but sometimes cause their own problems.

This page looks at #3, D-lens distance. Sometimes D-lens data reports shorter than the actual distance.

The TTL BL D-lens algorithm is that when the metered flash attempts to use more power than Guide Number says is appropriate for the distance (maybe TTL BL metered a distant dark background), TTL BL pulls the flash power back to match the D-lens reported distance (a safety factor preventing overexposure, computed with Guide Number). Problem is, D-lens distance is crude, and some lenses are simply not correct, and none are precise. TTL BL exposure depends on metered value, but if the Reported lens distance is less than actual distance, then flash level will be pulled back (TTL BL direct flash only). If Reported distance is Infinity, or any value greater than actual distance, then the undisturbed metered value is used. I am making this next part up, but FWIW, my notion is thinking there may be a limit to the maximum correction allowed due to D-lens. Some of the results are bad, but not quite as bad as the numbers would suggest.

The SB-400, SB-700, the internal flash, and Commander remote flashes do only TTL BL by system default (when their menu selects the generic TTL (Through The Lens metering), it is TTL BL. The Exif reports TTL BL mode (in the Exif Flash Data section, at the end — but Adobe Saves omit that. Check your original file with another Exif viewer.) Spot metering does override any TTL BL to be TTL (but it also does Spot metering too).

Flash GN mode is shown as a comparison, because it is usually about accurate exposure for direct flash, when the accurate actual measured distance is given to GN mode. My notion is that GN mode is what it probably ought to look like. GN mode and TTL mode are not affected by D-lens distance, but the D lens reported value is still shown here, to indicate that sometimes it is variable at each shutter press (focus seeking I think). The camera focuses by rotating the lens for the best appearance in the pixels.

Reports on two lenses are here. Both are prized as great lenses, but the Nikon 16-85 mm is no prize regarding D-lens distance. Nor is the Nikon 12-24, but the Nikon 14-24 mm is, because it generally just ignores it and reports infinity. One point is to show the obvious difference (below) that it makes to TTL BL direct flash (same camera, same flash, same subject, same situation). It was carefully done, however there are variances, doing any of it twice may see two distance results (focus seeking). Also there are few unexplained variations, so not every sample fits the plan, but most do.

Situation for all the pictures: Nikon D300 (DX), f/5.6 ISO 200, 1/200 second shutter, matrix metering. SB-800, maximum range is 25 feet at ISO 200 f/5.6. Hot shoe direct flash, focus on white card edge (on the edge, half of the metered area is not white card). 18% gray card also included. The actual distances were carefully measured. The only changes made are as reported, to distance, zoom, flash mode. White Balance is the only correction/compensation done. WCV just means White Card Value from eye dropper.

The D lens distance is NOT used for GN or TTL flash mode, but can override TTL info for TTL BL direct flash mode.

Nikon 16-85 mm f/3.5-5.6G ED VR lens