The Nikon SU-4 is a dedicated optical slave for Nikon flashes. Its release predates D-TTL, i-TTL and CLS (Creative Lighting System from Nikon). However, if the the master flash (the built-in pop-up or a shoe-mounted one) and the slave unit are both capable of firing in legacy TTL mode, the SU-4 will provide fully automatic TTL wireless flash exposure without the pre-flashes of CLS which trouble some photographers, especially those who shoot portraits. In A-mode, the SU-4 would detect the firing of the master flash and trigger the slave mounted on it. Then, it will send a “stop” signal upon the master flash cease firing. This mechanism allows the SU-4 works with non-TTL auto masters, as generic as a SB-E mounted on an EM! The user manual of SU-4 provides an extensive chart of compatible flashes/modes which I found extremely useful.
Most Nikon DSLR users interested in the SU-4 would be using it in the M-mode instead. This mode allows the built-in flash of an entry-level body like my D40 to be used as a master to trigger a CLS-slave only flash like the SB-600. However, the system is fully manual which means both power of the master and the slave will needed to be determined manually either by a flash meter or simply trail-and-error by viewing the resulted image and its histogram on the camera’s LCD. The setup procedures are as follow:
- From the camera menu, set the built-in flash to manual.
- Mount the SB-600 to the SU-4 and set it to manual mode also.
- The exposure on the camera should also be in manual. If you do not want any ambient light to affect the image, simply set the shutter speed to the highest sync speed. Otherwise, you may want a slower shutter speed but watch for camera shake and motion blur of the subject.
- The power setting of the master, the power setting of the slave and the aperture on the lens are the three inter-related factors that affect the final exposure.
Here are a few more tips. As a start to determine the final exposure, you may want to start with f5.6 or f8 on the lens. This should give optimal image quality on most lenses and allows adequate amount of depth-of-field. If the image is still underexpose even the flashes are at their maximum power output, you can simply open up the aperture to allow more light in. Since you are shooting digital, you may also want to crank up the ISO instead of openning up the aperture. If you want to minimize the effect of the master flash on the subject, simply set it to minimum power output, the sensor on the SU-4 will still be able to pick up the firing of the master even it is too dim to really affect the main subject. On the other side, the master can also be used as a fill-light with the slave being the main. Simply varying the output the master will give different ratios on the main subject.
Moreover, you can also use an older Nikon SB as the slave. Old powerful pro-flashes like the SB-24 and SB-28 are good candidates since they give as much power as a modern day SB-900. Some pro-flashes have the SU-4 unit built-in, these are, to my knowledge, SB-26, SB-80DX, SB-800, SB-900 and SB-910. In that case, you can trigger these pro-flashes with the built-in flash of an entry-level body. This setup procedures and tips above would also applies.
Below is a typical setup of a Nikon SB-600 mounted on a SU-4. The pair is then attached to a lightstand and a shoot-through umbrella is used to diffuse the light. This setup was used to shoot the Sigma XQ 39-80mm lens in the second photo. I blocked the built-in flash light with my hand so that it won’t land on the front element of the Sigma lens and held the camera, a D40 in this case, single-handed with its super fast 1/500 seconds sync speed.
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