Using External Flash with the Nikon N90

July 11, 2010 @ 8:43PM

Using External Flash with the Nikon N90

A Quick Look at the Creative Lighting System

By Simon Stafford

The D90 offers full i-TTL flash exposure control with four external Nikon Speedlights (flash units) that are compatible with the CLS:

SB-400 with a GN of 98 feet (30 m) at ISO 200
SB-600 with a GN of 138 feet (42 m) at ISO 200 with the flash head set to 35mm
SB-800 with a GN of 175 feet (53 m) at ISO 200 with the flash head set to 35mm
SB-900 with a GN of 157 feet (48 m) at ISO 200 with the flash head set to 35mm (and with Normal light distribution selected)

All models can either be attached to the camera directly or via the dedicated Nikon TTL remote flash cords: SC-28, SC-29, or the now discontinued SC-17. The SB-R200, which has a GN of 49 feet (14 m) at ISO 200, can only be controlled as part of the Nikon Advanced Wireless Control (AWL) flash system via either the SU-800 commander unit, or the SB-800 and SB-900 Speedlights when used as a master flash unit.

These four Speedlights offer further versatility since their flash heads can be tilted and – in the case of the SB-600, SB-800, and SB-900 – swiveled for bounce flash. Unlike earlier Nikon Speedlights, which cancelled monitor pre-flashes if the flash head was tilted or swiveled for bounce flash, the SB-400, SB-600, SB-800, and SB-900 emit pre-flashes regardless of the flash head orientation. The three latter units also have an adjustable auto zoom-head (SB-600: 24–85mm, SB-800: 24–105mm, SB-900: 12–200mm) that controls the angle of coverage, and a wide-angle diffuser to allow them to illuminate an even wider field of view. With the diffuser, the SB-600 and SB-800 can cover down to 14mm, while the SB-900 can cover down to 12mm.

Tip

Hint: The coverage of the SB-600 and SB-800 is set to correspond to the field of view of a lens with one of the focal lengths within the range of its zoom head. However, this is based on the assumption that the Speedlight is attached to a camera with an FX-format (35mm, or full frame) sensor (when used with the D90, the SB-900 automatically adjusts its coverage to match the DX format). The DX-format of the D90 corresponds to a reduction in the angle of view for the same focal length when used on the FX-format, so the flash will illuminate a greater area with the D90 than is necessary. Consequently, this will restrict the potential shooting range and squander flash power. In this situation, use the following table to adjust the zoom head position and thus maximize the performance of the flash unit:

Table 1.

Focal length of lens (mm)	Zoom head position (mm)
1 - Available on SB-800 only.
14	20
18	24
20	28
24	35
28	50
35	50
50	70
70	85
85	105¹

Automatic (Non-TTL) Flash with the SB-800 and SB-900 Speedlights

When using the SB-800 and SB-900 external Speedlights with the D90, there are two additional non-TTL flash modes available; these are selected on the Speedlight:

Auto Aperture (AA): In this mode, the SB-900 and SB-800 read the ISO sensitivity setting, lens aperture, and the command to fire the flash from the camera automatically. The AA flash mode can be used in Aperture-priority or Manual exposure mode. Thereafter, the flash output level is determined using a sensor on the front panel of the Speedlight to monitor the flash exposure and as soon as this sensor detects that the flash output has been sufficient, the flash pulse is quenched. If, between exposures, you decide to alter the focal length or change the lens aperture, the Speedlight will adjust its output accordingly to maintain a correct flash exposure. The problem with this option is that the sensor does not necessarily “see” the same scene as the lens does, which can lead to inaccuracies in flash exposure.

Automatic (A): This is the only automatic, non-TTL flash mode available with the SB-800 and SB-900 Speedlights. It can be used in Aperture-priority or Manual exposure mode. Similar to in the AA mode, a sensor on the front of the SB-800, SB-900, or DX-type Speedlight monitors flash levels and shuts off the flash when the Speedlight calculates that sufficient light has been emitted. However, the lens aperture and ISO sensitivity values must be set manually on the Speedlight to ensure that the subject is within the flash shooting range. As with the AA mode, the sensor does not necessarily “see” the same scene as the lens does, which can lead to inaccuracies in flash exposure.

Manual Flash Exposure Control

In Manual flash mode, you set the output of the Speedlight (built-in or external) to a fixed level. It is necessary to calculate the correct lens aperture as determined by the flash-to-subject distance and the Guide Number (GN) of the Speedlight.

For example, at its base sensitivity of ISO 200, the built-in Speedlight of the D90 has a guide number (GN) of 59 ft (18 m). The output level of the built-in Speedlight is determined by CS-e2, where a value between 1/1 (full output) to 1/128 can be selected. Since there is only one specific exposure value for any given level of sensitivity (ISO) at a particular flash-to-subject distance, it is necessary to calculate the lens aperture required to record a proper exposure.

Use the following equation:

Aperture = GN / Distance

Therefore, with the built-in Speedlight of the D90 set to 1/1 (full output) at a flash-to-subject distance of exactly 10.5 feet (3.2 m), the lens aperture required to obtain a correct exposure of the subject will be f/5.6. (5.6 = 59/10.5). Similar calculations will have to be performed when using an external Speedlight in Manual flash mode. Check the Guide Number (GN) for the particular Speedlight model and ensure that you conduct the calculations using the same unit of distance throughout.