Non-TTL Flash Automation

Photo #1

When all is said and done, my favorite "legacy" Nikon speedlight is the SB-26 (Photo #1) because it has a built-in SU-4 optical slave, fully adjustable manual exposure in 1/3 stop increments, and Non-TTL exposure automation in one-stop aperture increments. For the purposes of this posting we shall define Non-TTL (N-TTL) exposure automation as the ability to deliver proper flash exposure within a given range by varying the duration of the flash pulse through a sensor built into the flash itself.

Because there is no direct communication between the camera and the speedlight, the ISO and aperture settings of the speedlight must be set to match those of the camera. In use, one should definitely NOT chose the camera's shutter priority exposure setting because N-TTL relies on a constant F-Stop in order to work. 

Photo #2

If you look at the cropped image at the right (Photo #2), you can see where the sensor is located. It's mounted on the flash body so that it always faced forward, even when the flash head is rotated or tilted. Because the circuitry is completely contained within the unit, N-TTL flash can fully function with any camera when triggered by a PC cable or a standard hotshoe, depending on the flash. The SB-26, like the legacy flashes introduced after the SB-24, has a supplementary PC connector on the side, should the PC cable be your only triggering option.

Here’s how it works. Assume that both the speedlight and the camera are set to ISO 200 and an aperture of F 5.6, and that the two are attached. Let's make a theoretical test to see how N-TTL exposure automation works.

• Imagine that the flash/camera combination is placed five feet from a blank white wall. Take a photo. Based on a ISO and aperture settings, the flash will provide enough light to properly expose the wall with a burst of light, which in our hypothetically example might last 1/2000^th of a second. 

• Next, imagine moving the flash to a distance of ten feet from the wall. The inverse square law tells us that when we double the distance, we must increase the amount of light by a factor of four. When a photo is taken, the N-TTL sensor will measure the light bouncing back from the wall and increase the flash duration to 1/500 of a second, quadrupling the light output.

• The opposite is also true. If the distance is reduced to 2.5 feet, the flash duration would be cut to one-quarter, or 1/8,000 of a second.

One important footnote in the N-TTL story is the inclusion of Thyristor Circuitry. In early units, any  excess flash power that wasn't needed to expose the film was "dumped" in a quench tube and lost forever. The power was essentially wasted, meaning that every shot, no matter how close, was at full discharge. The introduction of the thyristor allowed any excess power to be re-cycled within the unit, allowing for a much faster recycle time and longer battery life.

N-TTL Automation In Action: I tested the consistency of N-TTL exposure by mounting a 60mm 2.8 Macro Nikkor on a D600 body, ISO set to 200, aperture set to F 7.1*, shutter speed 1/160 of a second. I mounted an SB-26 in the hotshoe, set it to F 5.6 at ISO 200, and set the camera to manual focus since I would be photographing a blank white wall and there wouldn't be anything to focus on. Four exposures were made at distances of three, six, nine, and twelve feet. The camera and flash settings were not changed between shots. The composite (Photo #3) was made from the histogram displays from the respective images with three feet at the extreme left, and twelve at the extreme right.

3 feet                                                 6 feet                                         9 feet                                             12 feet

You can see from the "white" histogram (the one at the top), that the exposure is very consistent from shot to shot. Each time the flash moves farther from the wall, the sensor allows more light to escape. When enough light for a proper exposure has been dispensed, the excess power is shunted away from the flash tub and returned for use during the next flash.

This series of photos demonstrates (to my satisfaction, at least), the N-TTL flash automation is capable of providing consistent exposures over a variety of distances. This neither proves nor disproves the accuracy of the exposure, only that given a constant set of exposure conditions, the discharge accurately dispenses the correct amount of light over the four tested distances. By adjusting the size of the lens aperture, the results can be fine tuned to taste. But at least we know that Non Through The Lens exposure automation is capable of accurately compensating for changes in flash to subject distances.

*Several old timers recommended selecting a lens aperture 2/3 of a stop smaller than the one suggested by the flash. This would, in theory, underexpose your flash images. I'm willing to take this on faith.

Nikon SB-30 Micro* Speedlight

http://www.kenrockwell.com/nikon/sb30.htm

While checking e-Bay for Nikon speedlights, for found a Nikon SB-30 for $30.00. Too great a coincidence to not purchase it. And since it was sold by a brick an mortar store in the midwest, how could I go wrong?

Ken Rockwell, a reliable and "to the point" photo blogger, started a writeup on this flash unit, but the last time I checked, it was still under construction. He did make some wonderful photos of the unit, which I'm including here, along with links to his article. Granted, I can see why this flash wouldn't be Priority One on his, or anyone's, blogging to-do list, but having purchased one and played with it a fair amount, I can think of a number of uses for it. Incidentally, for a posting from DPreview, click here.

The unit uses a single CR-123 battery, which contributes to its small size and rapid recycle time. However, the batteries are expensive when compared to AA cells. It does not offer iTTL metering as does the Nikon SB-400, my other tiny flash. But it offers far more exposure control features than you might expect. For starters:

http://www.kenrockwell.com/nikon/sb30.htm

TTL Exposure Metering: If you're a digital photographer and have a D1x, or D100, or even a Fuji S1 or S2, you're in luck. These cameras can utilize the speedlight's basic TTL metering. I suspect that if you're reading this post you don't have any of these DSLR cameras, but if you do/did, you've got TTL exposure automation with your flash.

Non TTL Flash Exposure Metering: The unit has its own sensor that meters out four measured bursts of light that will provide proper exposure for four basic aperture settings at ISO 100 and four more at ISO 400. Need something in between? You can easily extrapolate what you need for ISO 200, and maybe even ISO 800. While a full post on non-TTL flash automation is justified, it suffices to say that for fill purposes, the SB-30's non-TTL metering mode should work well, or well enough.

+/- 1/2 Stop Exposure Compensation: This is an interesting tweak, but rather limited.

3 Manual Output Levels: You can set the flash to manually discharge a "full" discharge, plus 1/8 and 1/32 power discharges. This could be very handy in situations where a precise (and small) burst of light is all that's needed.

Built-In IR Panel: Like the Nikon SB-50 DX, there is an infra-red panel you can use to cover the flash tube for use as a wireless optical trigger. Unlike the SB-50, there is some white lite leakage, a definite distraction when photographing anything with functional eyes.

SU-4 Style Optical Slave Modes: The orange M and A allow for firing the unit as a optical slave, but only at full power in the M(anual) mode. Haven't quite figured out the A(uto) mode, but just having the slave feature is cool enough.

So where does this unit fit in? First off, it's small size (it could probably fit in a cigarette box) makes it a great fill light for a compact camera that can take advantage of the speedlight's manual controls. Used in either the non-TTL or the manual mode, you can arrange for a fast fill light solution, or a more precise flick of fill light, depending on what you need. It sits very close to the lens axis, but not close enough to cast a shadow when your wide angle sports a petal lens hood.

I'm pretty impressed with the flash, and plan to use it for filling some upcoming outdoor events. We'll find out whether I picked a winner or a loser!

*Micro is my descriptor. When compared to the SB-910, it is REALLY small! 

February 10 Addendum: I used the flash with the IR Panel in place to trigger a wall-bounced SB-800 in SU-4 Mode. Using a 60mm 2.4 Macro on a Fuji X-E1, I made this selfie (Photo #1):

Photo #1

Incidentally, the shot is full width, with the cropping affecting only the top and bottom edges.

Photo #2

As a subject, I didn't find the white light leak particularly annoying. In fact, I really don't remember seeing it. But in this cropped photo (Photo #2), you can  see the distinct two-spot "signature" made by the SB-30, but you have to look very carefully to find it. While this would be an easy post processing fix, it could give you some grief if the reflections show up on eyeglasses. Just the same, this feature proved very convenient!