Nikon D70 Flash "Blinkies"

Users of flash with the Nikon D70 digital SLR (DSLR) camera will probably understand what the title of this page means without having it explained to them. For the rest of you, I'm referring to the fact that using the built-in flash or an external Nikon iTTL flash (SB-800 or SB-600) will often cause subjects of photos to blink at just the wrong time, leading to an eyes-closed photo. With many subjects (my wife being one), the blink reflex is so strong that even when they try to suppress it, they can't. This makes for a frustrating photo shoot for all involved.

Those coming to the D70 from film cameras, including Nikon film cameras, are surprised to discover this oddity of the D70. What causes it? What can be done about it?

The problem is caused by the D70's preflash. This flash is emitted before the mirror is raised and is used by the iTTL system to measure the needed exposure. Then the mirror is raised, the shutter is opened and the flash is fired again to make the exposure. The delay between the time the preflash occurs seems to be just the right amount to "sync" with some people's blink reflex.

This is different from the way film TTL (through-the-lens) flash systems work. They measure the light being reflected from the film itself during the exposure flash. No preflash is needed, so the "blinkies" don't happen.

What to Do, What to Do...

Let's cut to the chase. Your options for fixing the blinkies are:

Don't use the iTTL mode
Do the preflash separately from the exposure

Both of these approaches work, and both are useful.

Flash Without iTTL Mode

It's nice to have the camera figure out the flash settings for you, but it isn't strictly necessary. People were using flash long before TTL flash systems. Some flashes (the SB-800, for example) have an automatic mode in which the flash unit itself measures the light returning from the scene and adjusts its output accordingly. Since the flash doesn't know exactly what the lens is seeing, this is a cruder method that is subject to more error than TTL flash. In many situations, though, it works just fine. When the subject is likely to be the nearest object seen by the flash and lens, it can work quite well. Shooting people at an event is one such case.

In situations where the distance to the subject doesn't change appreciably, another usable option is manual flash control. In this case, you manually set the output of the flash. This is the only non-iTTL option available with the D70 pop-up flash. Fortunately, being digital, the D70 provides instant feedback via the histogram display that allows you to set the flash output by trial and error. (You can calculate it, too, if you've a mind to, or measure with a separate flash meter.)

iTTL Without the Blinkies

If only you could change the amount of time that elapses between the preflash and the exposure flash, maybe you could catch the subject with their eyes open, right? Unfortunately, there is no way to reduce the delay the D70 imposes between preflash and exposure -- that's determined by the speed of the mirror action. But you can increase the delay. You do this by manually firing off the preflash. Then, when you and you subject are ready, you take the exposure -- the flash fires only once from the time you press the shutter release until the exposure is complete. No blinkies!

Manually firing the preflash is done by putting the D70 into FV Lock mode. Use Custom Setting 15 to select FV Lock. Once you do this and have your flash connected (or raised, in the case of the pop-up flash) and set to TTL mode, simply press the AE-L/AF-L button to fire the preflash. The preflash will occur and the camera will calculate the proper flash amount. From then on, until you press the AE-L/AF-L button again, each time you press the shutter release the flash will fire at the same level, without preflashing. Of course, shooting more than one shot with the same flash output is valid only if the flash-to-subject distance doesn't change between shots.

(There's another way of increasing the delay, by using rear-curtain sync, but since that requires leaving the shutter open a long time to make any significant difference in the delay, it's not too useful for this discussion.)

Why the Nikon D2X Costs Five Times What a D70 Does

Okay, there are a number of reasons, but one is because the mirror action is a lot faster.

When I upgraded to a Nikon D2X, I noticed immediately when I first used flash that the blinkies were simply gone. I didn't immediately understand why. The D2X still performs a preflash in iTTL mode. Eventually I came to understand that the sequence of events outlined above: preflash, mirror-up, exposure, were simply a lot faster on the D2X. Since the mirror is so fast, the delay between preflash and flash can be much shorter.

Being a geek at heart, I decided to measure the difference. I built a simple circuit to convert the light from the flash to an electrical signal that I could view on an oscilloscope. That allowed me to measure the delay between preflash and exposure flash using an SB-800 on-camera flash:

Here are some oscilloscope photos that show the measurements. Another thing they show is that the D70's preflash consists of two pulses while the D2X only uses one. I don't know why that is, but I'd be interested to hear an explanation.

In the photos, the horizontal axis is time, at either 20 milliseconds per division or 5 milliseconds per division, as noted on the photos. (There are 10 divisions across the graph.) The vertical axis is voltage, with the higher value being when the flash is off, the lower when it is on. So, the downward-pointing spikes represent the flashes of light from the flash.

D70 preflash-to-flash delay

D2x preflash-to-flash delay

To make a more precise measurement, I measured the D2X with a faster timebase:

D2X preflas-to-flash delay (expanded scale)

These measurements confirmed what I was seeing when I used the two cameras, and the measured D70 delay of a bit more than 1/8 second is enough to cause the "blinkies" with some subjects.

Some day when I have time, I plan to use the circuit to investigate the patterns of pulses the Nikon system uses to implement wireless flash. That should be interesting!

Jon Bloom
jbloom@arrl.org