Flash Basic Functions

Flash Basic Functions

This page reviews some features of a typical flash that are important to Coolpix camera users. Other advanced features of higher end flashes will be discussed elsewhere.

Guide Number (aka GN)

Each flash has its guide number, GN, for exposure calculation. The GN of a flash is defined at ISO 100 as the product of distance (i.e., flash to subject distance) and aperture:

This means that the product of the flash to subject distance and aperture being used is equal to the GN of the flash. Normally, you will see two GN values, one in feet and the other in meter. Therefore, you should choose the right GN in calculations. For example, suppose a flash has a GN 90 in feet and we intend to photograph a subject 20 feet away from the flash. Since 90 = 20×aperture, we have aperture=90/20=4.5. Hence, we should use F4.5 to take this shot in order to have a correct exposure.

Keep in mind that the GN of a flash is usually given at ISO 100. If a different ISO is being used on your camera, the aperture should be adjusted accordingly. For example, if the camera uses ISO 200, which is one stop faster, the aperture F4.5 calculated above should be reduced by one stop, which is about F6.3. Note that this F6.3 is computed as the square root of 4.5×4.5×2. Similarly, if the camera uses ISO 400, the aperture should be F9. In general, suppose the calculated aperture from the given GN is Fx.y at ISO 100. If the camera is adjusted to use a new ISO value k, the new aperture is calculated as follows:

The manuals of many newer flashes have started to provide guide numbers for various focal lengths. This can help more accurately determine the aperture to be used. The following table is taken from Nikon SB-50DX manual. As you can see, a wider angle setting has a smaller GN.

Focal Length 14mm 24mm 28mm 35mm 50mm
GN (m/ft) 12/39 18/59 20/66 22/72 26/85

Suppose we wish to take a photograph using 28mm and a SB-50DX at ISO 100. If the subject-flash distance is about 8 feet, then the aperture to be used is F8.0 = 66 (28mm's GN) / 8 = 8.25.

Flash Modes

The most important feature of an external flash is the number of available modes. While there are so many modes available on a high end Nikon TTL-compatible flashes, only three of them are important to a Coolpix user. They are TTL, Auto and Manual. The TTL mode means flash exposure is controlled by the camera; the Auto mode means the camera only triggers the flash which uses its own flash sensor to cut off flash power when scene illumination becomes sufficient; and the Manual mode means you do all the calculations for flash exposure. See here for the details of these modes.

A Nikon TTL-compatible flash does not have to support all three modes listed above. Some cheap ones can only do TTL without the Auto and Manual modes (e.g., Sunpak Auto 355AF). With such a flash, you use the variable power feature of the camera to fine tune flash exposure. Many Nikon TTL-compatible flashes support the Auto and Manual modes to some degree. Those good ones may provide you with a range of apertures to use. For example, the powerful Sunpak PZ5000AF allows you to use apertures from F1.3 to F13 in 1/3 stop increment. Some Nikon TTL-compatible flashes only provide a few apertures. For example, Nikon's SB22s has only four aperture settings (e.g., F2.8, F4, F5.6 and F8 at ISO 100). Note that the popular Nikon SB-50DX does not have the Auto mode! The Manual mode is the most basic one because we must calculate the aperture from the subject-flash distance and vice versa. Some flashes have a table relating aperture, subject-flash distance and ISO sensitivity for a photographer to look up, while some others may have a calculator built-in for a photographer to compute the subject-flash distance from a chosen aperture.

Bounce and Swivel Flash Head

A bounce and swivel flash head is very useful to creative flash photography. The following images, from left to right, show the flash head of the Nikon SB-80DX in straight, bounce and swivel (and bounce) positions, respectively.

Not all flashes have bounce and swivel flash heads. However, many flashes do have bounce flash heads. The left image below shows a Nikon SB22s. Its flash head is fixed; however, its flash tube can be rotated up to vertical position, and down a little. The SB-50DX is another example. Its flash head, similar to that of the SB-80DX, can bounce but cannot swivel. You have to check your flash manual to see the bounce and swivel ranges of the flash head.

Some Nikon higher end flashes have a built-in bounce card. See the right image above. One can pull this bounce card out so that when the flash illumination is bounced, the bounce card will create a highlight in the subject's eyes.

Angle of Coverage

Most flashes can cover the angle of view of 63 degree that is approximately equal to the angle of view of a 35mm lens (in 35mm film format). With a flash that can only go as wide as 35mm, it will not be able to completely illuminate the scene if a 28mm or wider lens is being used. The following image is an example in which the focal length of the camera lens produces an angle of view wider than the flash angle of coverage. It is clear that the corners are not properly illuminated by the flash light. Some better flashes may be able to go down to 28mm, while newer Nikon flashes such as SB-50DX and SB-80DX covers as wide as 24mm!

To reach a wider coverage, some flashes include a wide flash adapter. A wide flash adapter is a diffuser capable of dispersing the flash light to a wider area. If a wide flash adapter is available, we normally pull it out (below left) and use it to cover a portion or the whole area of the flash tube (below right). With a flash wide adapter, the coverage of a flash can be significantly wider. For example, the SB-80DX and SB-50DX can go as wide as 14mm.

The telephoto end is not as problematic as the wide end because if the flash cannot cover a very narrow area that matches a telephoto lens, the flash illumination is wasted. Some flash manufacturers may have telephoto flash adapters available to produce a narrower angle of coverage for telephoto lenses.

It is very important to remember one fact. The maximum flash output is a constant. If the angle of coverage is wider, this constant flash illumination will be distributed to fill a wider view volume, and, hence, cannot reach far enough. On the other hand, if the angle of coverage is narrower, the view volume is also narrower, and, the constant flash illumination will be able to reach farther. In summary, a wider (resp., narrower) angle of coverage produces a shallower (resp., deeper) illumination zone.

Power Zoom

Based on the discussion in the above section, it would be much better to match the angle of coverage with the angle of view of the lens so that the scene would be illuminated properly. Most flashes have the power zoom capability. With this capability, the camera can communicate the focal length being used to the flash for the flash zoom motor to move the flash head to match the focal length. Unfortunately, as of this writing, none of the Coolpix cameras can communicate the focal length being used to the flash. As a result, either you always set the flash angle of coverage to the widest or manually adjust the zoom setting on the flash. If you choose to use manual zoom setting, you have to be very careful because flashes use 35mm equivalent focal lengths. In other words, you have to convert the 5700 focal length to its 35mm equivalent. Worse, without shooting an image, you perhaps do not know what the focal length is. Refer to Lens Overview and Optical Zoom for more details, including focal length conversion.

AF Assist Illuminator

Many flashes have AF assist illuminators. When the scene illumination is low, the camera may activate the AF assist illuminator of the flash to project a dark red pattern as an aid for auto focusing. This illuminator is usually behind a dark red cover on the front panel of a flash. The image below, from upper left and clock-wise, shows the AF assist illuminator of Nikon's SB-80DX, Sunpak PZ5000AF, and Nikon SB22s. The flash in the lower left corner is Sunpak DS20, which is not a Nikon TTL-compatible flash and has no AF assist illuminator (because the Sunpak DS20 has an ISO contact that is not sufficient for the camera to communicate the activation signal).

Unfortunately, as of this writing, none of the Coolpix cameras will active the AF assist illuminator of a Nikon TTL-compatible flash. This causes auto focus to fail frequently in low light/contrast environment even with an external flash.

Modeling Light

Some new flashes have modeling light (modeling illuminator in Nikon's word). Modeling light is used for checking scene illumination and shadow. When it is activated, the flash emits a reduced flash light repeatedly for a short period of time. In this way, a photographer will be able to check scene illumination and shadow. The image below shows the modeling illuminator button of a Nikon SB-80DX flash.

If your flash has a modeling illuminator, you can push the modeling illuminator button and at the same time lightly press the shutter release button to focus. It is likely that the camera can lock on your subjects correctly. The only problem is that this is a two-hand operation, one for the modeling illuminator button and the other for the shutter release button. Some practice may be needed.

Red-Eye Reduction

Do not look for this feature in your flash manual because it is a camera feature. You should select red-eye reduction with your camera. Some flashes permit the red-eye reduction and slow synchronization to be used together. Note that with a Coolpix camera, the red-eye reduction is performed by the camera rather than by the flash even though the flash has a red-eye reduction lamp.

Slow and Rear Curtain Synchronization

Do not look for these two features in your flash manual because they are camera features. Use your camera to make selection!