Examining Nikon TTL BL flash with D-lens Distance

Comparing TTL BL for two Nikon lenses, 16-85 and 14-24

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:

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 seems to need +EV compensation.)
Similar to #1, reduction to prevent the addition (overlap) of multiple flashes (Commander Main/Fill setup typically needs about +1 EV manual flash compensation.)
D-lens distance reduces flash level when distance is reported shorter than metered exposure is planning (safe guard, direct flash only - subject of this page)
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 the generic TTL (Through The Lens metering), it is TTL BL. The Exif reports TTL BL (in the Exif Flash Data section, at the end - but Adobe omits 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 16-85mm is no prize regarding D-lens distance. Nor is the Nikon 12-24, but the Nikon 14-24mm 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 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.

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

GN flash mode	TTL flash mode	TTL BL flash mode
16mm zoom at 5 feet (1.5m), reported as 0.8m, WCV 249	reported as 1.0m, WCV 246	reported as 0.9m, WCV 206 Flash reduced from 1.5m to 0.9m value
35mm zoom at 5 feet (1.5m), reported as 1.7m, WCV 242	reported as 1.7m, WCV 203	reported as 1.7m, WCV 211 Greater than 1.5m, not reduced
85mm zoom at 5 feet (1.5m), reported as 1.4m, WCV 238	reported as 1.7m, WCV 184	reported as 1.7m, WCV 197
16mm zoom at 7.5 feet (2.3m), reported as 1.4m, WCV 252	reported as 1.2m, WCV 250	reported as 0.9m, WCV 205
35mm zoom at 7.5 feet (2.3m), reported as 2m, WCV 244	reported as 2m, WCV 229	reported as 2m, WCV 214<
85mm zoom at 7.5 feet (2.3m), reported as 1.4m, WCV 246	reported as 1.7m, WCV 249	reported as 1.7m, WCV 199
16mm zoom at 10 feet (3m), reported as 2m, WCV 255	reported as 2.8m, WCV 255	reported as 2.8m, WCV 240
35mm zoom at 10 feet (3m), reported as 3.6m, WCV 250	reported as 3.6m, WCV 238	reported as 3.6m, WCV 223
85mm zoom at 10 feet (3m), reported as 3.6m, WCV 252	reported as 3.6m, WCV 205	reported as 3.6m, WCV 227
16mm zoom at 12.5 feet (3.8m), reported as 2m, WCV 255	reported as 2m, WCV 249	reported as 2m, WCV 231
35mm zoom at 12.5 feet (3.8m), reported as 5m, WCV 254	reported as 5m, WCV 242	reported as 3.6m, WCV 228
85mm zoom at 12.5 feet (3.8m), reported as 3.6m, WCV 253	reported as 3.6m, WCV 213	reported as 3.6m, WCV 209
16mm zoom at 15 feet (3.8m), reported as 2m, WCV 250	reported as 2m, WCV 240	reported as 2m, WCV 208
35mm zoom at 15 feet (3.8m), reported as 5m, WCV 244	reported as 5m, WCV 235	reported as 5m, WCV 213
85mm zoom at 15 feet (3.8m), reported as 5m, WCV 243	reported as 5m, WCV 212	reported as 5m, WCV 203
16mm zoom at 17.5 feet (5.3m), reported as 2.8m, WCV 246	reported as 2m, WCV 220	reported as 2m, WCV 176
35mm zoom at 17.5 feet (5.3m), reported as 5m, WCV 242	reported as 9.4m, WCV 229 No harm done, it's large, like infinity	reported as 5m, WCV 202
85mm zoom at 17.5 feet (5.3m), reported as 5m, WCV 242	reported as 5m, WCV 218	reported as 5m, WCV 194
16mm zoom at 20 feet (5.3m), reported as 3.6m, WCV 244	reported as 2.8m, WCV 194	reported as 2.8m, WCV 167
35mm zoom at 20 feet (5.3m), reported as 5m, WCV 242	reported as 9.4m, WCV 227	reported as 5m, WCV 209
85mm zoom at 20 feet (5.3m), reported as 9.4m, WCV 240	reported as 5m, WCV 224	reported as 5m, WCV 204

Nikon 14-24mm f/2.8G ED AF

This lens is the best case I have seen, of non-intrusive D lens info (less harm done to TTL BL direct flash).

GN flash mode	TTL flash mode	TTL BL flash mode
14mm zoom at 5 feet (1.5m), reported as 2m, WCV 249	reported as 2m, WCV 246	reported as 1.3m, WCV 206 This is limiting
18mm zoom at 5 feet (1.5m), reported as 2m, WCV 242	reported as 2m, WCV 203	reported as infinity, WCV 211 This is not limiting.
24mm zoom at 5 feet (1.5m), reported as 2m, WCV 238	reported as 2m, WCV 184	reported as 2m, WCV 197
14mm zoom at 7.5 feet (2.3m), reported as infinity, WCV 252	reported as infinity, WCV 250	reported as infinity, WCV 205
18mm zoom at 7.5 feet (2.3m), reported as infinity, WCV 244	reported as infinity, WCV 229	reported as 2m, WCV 214
24mm zoom at 7.5 feet (2.3m), reported as infinity, WCV 246	reported as infinity, WCV 249	reported as infinity, WCV 199
14mm zoom at 10 feet (3m), reported as infinity, WCV 255	reported as infinity, WCV 255	reported as infinity, WCV 240
18mm zoom at 10 feet (3m), reported as infinity, WCV 251	reported as infinity, WCV 238	reported as infinity, WCV 221
24mm zoom at 10 feet (3m), reported as infinity, WCV 251	reported as infinity, WCV 220	reported as infinity, WCV 239
14mm zoom at 15 feet (3.8m), reported as infinity, WCV 246	reported as infinity, WCV 218	reported as infinity, WCV 184
18mm zoom at 15 feet (3.8m), reported as infinity, WCV 244	reported as infinity, WCV 235	reported as infinity, WCV 213
24mm zoom at 15 feet (3.8m), reported as infinity, WCV 243	reported as infinity, WCV 212	reported as infinity, WCV 203
14mm zoom at 20 feet (5.3m), reported as infinity, WCV 244	reported as infinity, WCV 194	reported as infinity, WCV 167
18mm zoom at 20 feet (5.3m), reported as infinity, WCV 242	reported as infinity, WCV 227	reported as infinity, WCV 209
24mm zoom at 20 feet (5.3m), reported as infinity, WCV 240	reported as infinity, WCV 224	reported as infinity, WCV 204

Clearly better D-lens data (#3 at top of page) gives TTL BL better direct flash results. There is still some built-in bias against TTL BL (#1 & 2 at top of page), but crude D-lens data greatly exaggerates the problems.

Bottom line though, it really doesn't matter much. TTL has other issues too (light reflected from the subject's colors, #4 at top of page), so in all cases, we just watch results and use Flash Compensation as necessary. Just do what you see is necessary. Knowing why helps predict results, but does not change the need.

Then two old non-D lenses. Frankly, my theory was expecting excellent results, and I was greatly surprised to see maybe the worst case yet. These old lenses provide no D-lens data. PhotoME says the 50mm lens "reports" every distance as 0.01 meters, and the 55mm lens reports 0.06 meters (less than 1/4 inch). These are impossible numbers of course, and you'd think this would be known and planned, and expect the system to ignore them, but there is some strong effect. Sorry, no explanation is known, the obvious is unthinkable. Again, same camera, same metering system, same flash, same subject, etc. Only the lens changed.

Nikon 50mm f/1.8 AF, from late 1980's, no D-lens info

This lens is from 3 to 5 years before the D-lens era, it does not report it. TTL BL has trouble with this lens.

GN flash mode	TTL flash mode	TTL BL flash mode
5 feet, WCV 221	WCV 228	WCV 113
10 feet, WCV 245	WCV 244	WCV 191
15 feet, WCV 242	WCV 236	WCV 171
20 feet, WCV 239	WCV 238	WCV 185

Nikon 55mm f/3.5 macro, from late 1960's, no D-lens info.

This manual focus lens is from 25 years before the D-lens era, it does not report it. It is a non-CPU lens, setup in D300 menu. It has been modified to AI mount, to report aperture. TTL BL has trouble with this lens, and TTL is down some too.

GN flash mode	TTL flash mode	TTL BL flash mode
5 feet, WCV 223	WCV 197	WCV 97
10 feet, WCV 247	WCV 202	WCV 113
15 feet, WCV 242	WCV 202	WCV 150
20 feet, WCV 240	WCV 219	WCV 162

My own notion is that we seriously need a menu option to disable effect of D-lens data on TTL BL direct flash.
TTL mode was such an option, but it is disappearing now.

Workarounds:

Sometimes just changing to a lens with more usable D-lens data can make a big difference (see the chart on Previous page, of a few lenses, for example).

You can easily check what your own D-lenses report. Flash is not necessary unless you want to see the TTL BL exposure. I just put a long measuring tape on the floor, end up at a closed door. Moving the tripod along the tape (known actual distance), I focus on the door frame and take the picture (make notes matching image file name to actual distance, to avoid confusion later). Exif data shows "Subject Distance", in meters (3.28 feet in one meter). This may be a little variable each time with some lenses (focus seeking, watch the lens rotation as you half press the shutter again). Technically, the distance is measured to the plane of the sensor, and DSLR camera bodies have a mark one side or the other of the rear edge of the top LCD that shows this plane (a circle with a line through it, something like Θ). Focus distances are measured to that mark. You could use a plumb bob string to the tape below, but an inch or two really won't matter much for this. I just set the center column of the tripod right before the tape mark, by eye, close enough at 5 or 10 feet. The lenses do not report it that precisely or that accurately.

Bounce flash is far better lighting anyway (when possible), and bypasses D-lens issues.

If TTL is a choice (SB-600, SB-800, SB-900), it is good stuff. Not always right either of course (because it does meter preflash reflections from the scene), but is consistent, predictable, no unknown programmers messing around mysteriously. Easily predicted and compensated. We do have to know to watch #1-4 at top of this page.

If TTL BL is the only choice, we can compensate results. Or Spot Metering does override TTL BL flash mode to instead be TTL mode, which ignores D-lens distance. However, it is still Spot metering, not simple point&shoot, instead requiring some thought about the spot selected. Spot metering is not general purpose, it does NOT meter the scene, it meters only the spot, and it is an advanced technique requiring more thought.

On cameras with commander, then FV Lock is a feature (for other purposes, to minimize subject blinking for example). But FV Lock can improve the worst TTL BL issues (sometimes giving different and substantially better results), and so also might do a lot to minimize this inaccurate D-lens data problem.

But the bottom line in every case is always Flash Compensation. If you don't like the results, simply fix it. It's pretty easy.

Next is Distant Backgrounds or White Walls are opposite extremes