TTL seems not to work on D7000 with sigma ring flash for dental photography

Question

Cameras: D100 D7000, lens Nikon 105mm macro, Sigma ring flash Photos taken: facial head shots from approx 6 feet away from subject (f10) and dental intra-oral photos including frontal, right and left laterals taken with a mirror, maxillary and mandibular occlusals taken into a mirror. (all f36)

With the D100, body was set to aperture priority, TTL did not work due to differences in communication protocols, but it still did work in aperture priority. Had to manually boost flash EV to +1.0 for facials, +0.3 for laterals and 0.7 for occlusals. Photos were all generally acceptable.

Sigma ring was replaced with a new sigma ring and D7000 use initiated. TTL is supposed to work but does not seem to work. In aperture priority, everything is dark using the same settings as above. I called multiple dental photography equipment suppliers to see how they sell and recommend their equipment to be used. They all indicate their ring flashes which include the sigma flash, should work in aperture priority and they recommend their clients only use aperture priority.

I contacted Sigma and they said the body has to be set to manual mode because the D7000 will use the flash as a fill flash and not the main flash even though the flash on the body remains closed. In manual mode, settings above do not work, as they are still too dark. I have varied the ISO, the EV, the white balance, and changed the metering to matrix. The photos start to look surreal when the ISO or EV is more than the settings outlined above. To me the metering from the body is not being communicated to the flash, so I assumed the problem was with the new flash.

I sent the flash into Sigma and they said nothing is wrong with it, but do not have the means to test it with way a dentist would use it. Everybody is pointing the finger to someone or something else, but I am no closer to a solution. Does anyone have any thoughts or more importantly a differential way of testing the equipment to establish the source of the problem?

Answer 1

TL;DR Try P mode on the camera.

You need to learn how flash works. Putting everything on auto is the easiest way to lose control over exposure with flash. If you're shooting in completely consistent circumstances all the time (which seems likely), you'll probably do better having everything on M mode, finding out what works best, and then locking down the settings.

The auto-exposure system in the dSLR has to make a decision about how you want to balance the flash illumination against the ambient (all the light that isn't from the flash) light. There is no single right choice here--you can do everything from all flash/no ambient to no flash/all ambient and everything in between. But the camera manufacturers have decided that most folks will be using A or S modes with flash for fill. So that is how the flash will behave on the auto modes: most of the light coming from the ambient, and only a small amount of flash to "fill in" the shadows. This is most typically used in a daylight portrait scenario.

The P (Programmable Auto) mode acts a little differently. The flash will behave as fill in well-lit situations, but in lower-light situations the behavior will then shift to using the flash as the main source of illumination--like a P&S built-in flash. This is more likely to be the easiest setting for any circumstances if you don't want to step in and take control of the flash and ambient exposures explicitly through the exposure values.

M mode on the camera allows you to completely control where you want to set the balance between the flash and the ambient. ISO and aperture affect both the ambient and the flash illumination. And shutter speed only affects the ambient (if you're below your max. sync speed), so you do have a control to independently adjust the balance (shutter speed. See: dragging the shutter on neilvn.com)

Flash is also controlled by two other things: the distance of the light from the subject (the farther away, the lower the light level gets), and the flash power setting. This is why most of us who are into lighting will use an off-camera flash (to be able to adjust placement and angle as well as add modifiers), and M mode on the flash for precision and consistency of the power setting.

TTL is metering-based. The camera tells the flash to send out a preflash burst of light of a known power/brightness level, meters it, and then adjusts the flash's power level to where the autoexposure system thinks is a good exposure. Like using the A or S modes on the camera, it is fast and convenient, but may not be exactly where you want the exposure to sit. Just as we use M mode on the camera for precision and consistency, we use M mode on the flash, too.

See: Neil van Niekerk's Flash Photography Techniques and the Strobist.