Some delays, such as the response time of sensors or the jitter and rise times of flash circuits are electronic and so small that we can generally ignore them. Cameras, on the other hand, have mechanical components that can introduce highly significant delays. The most serious are found in reflex cameras that require a mirror to be swung out of the image path. Also, the focal-plane shutters of most 35mm cameras are inherently slow. Typically these delays are in the region of 75 - 100 milliseconds for 35 mm cameras and around 200 milliseconds in medium format, although the Nikon F70, a modern, state-of-the-art 35mm camera has a delay of 230 milliseconds. Some modern Canon cameras, on the other hand, can have delays of 10 milliseconds or less because they use a half-silvered mirror that does not require moving.

Older cameras allowed the mirror to be locked up prior to firing in order to eliminate vibration when using long lenses. Regrettably, this feature is increasingly uncommon on modern cameras, although in fact it often provides little benefit in reducing response times. The inbuilt programming to control the firing sequence has a delay included regardless of whether the mirror is up or down. Thus a Nikon F2 with MD1 motordrive gains only 17 milliseconds by locking the mirror up, still requiring 47 milliseconds for the shutter to open. Similarly, the Rollei 6006 medium-format camera reduces the delay from 200 to 80 milliseconds by locking up the mirror manually, despite the fact that the leaf shutter built into the lenses itself takes only two or three milliseconds to open. TC Nature have a way to by-pass the in-built delay, so making the Rollei the fastest camera available for high-speed applications.

It is often extremely helpful to know how long your camera takes to respond. It is not difficult to make such a measurement. All you will need is a photo-sensor and flash, a long ruler, a calculator and some black & white or Polaroid film to give immediate results.

A ball bearing (or similar solid object) falling under the force of gravity accelerates at a constant rate, this provides a fixed and unvarying basis for comparison. Set the ruler vertical and focus the camera so the scale is visible. Arrange the photo-sensor close to the ruler at distance (x m) from the top so the beam will be broken by a falling object, thus firing the camera. The resulting picture will show the dropped object somewhere below the level of the beam, at a distance (D m) from the top. The response time of your camera (T sec) can be calculated from the formula:

Somewhat surprisingly, the camera often proves to be the least critical part of a high-speed system. For many shots it is possible to shoot in total darkness leaving the shutter open on the 'B' or 'T' setting until the flash has fired. Indeed, even a simple pinhole camera would work in this situation provided there were sufficient light available. The open shutter technique eliminates all camera delays, the strobe being fired as soon as the sensor is tripped. This makes it the chosen arrangement in many high-speed situations.

Unfortunately, there are also plenty of occasions when one cannot work in the dark with an open shutter. Hummingbirds and butterflies, for example, are only active in sunshine. To capture subjects of this sort you have two choices. Either use a special high-speed accessory shutter in front of your camera lens or arrange the shot in such a way as to accommodate the delay of you camera. The latter solution is the simplest. A hummingbird close to a feeder is usually traveling very slowly even though its wings are beating rapidly. If the sensor is placed close by, the bird will still be in frame when the camera contacts finally trip the strobe. Likewise, an object (or liquid) thrown into the air will hang stationary for a brief moment before starting to fall. You can often exploit this phenomenon to get pseudo- or semi-high-speed images.

If the need for a fast response time cannot be avoided you should consider using an accessory shutter in front of the normal camera lens, the internal camera shutter being left open on the 'B' setting.

Vinten Uniblitz shutters (see below), designed for scientific applications, open in a few milliseconds, but with some ingenuity it is also possible to adapt a secondhand large-format Compur shutter to perform equally quickly for a fraction of the price.

There are many occasions when it is not convenient, or even possible, to place the sensor at the spot where the picture will be taken. With subjects that are predictable, and particularly with falling objects, it can be very helpful to specify how long after sensing the flash should fire. For example, in photographing the stages through which a falling milk drop forms a corona, one has only to increase the interval after detection by a few milliseconds for each frame in order to create a detailed sequence.

Similarly, with wildlife it is often easier to predict an animal's take-off point for a jump than guess the final landing position. Using a delay of about 400 milliseconds from launch, we were recently able to get consistent shots of nocturnal flying squirrels in mid-air even though the individual flight paths varied considerably.

Another application for which Delay timers are useful (and there are many) is when you want to obtain several images of an event in quick succession on a single frame. Delay timers can be used to specify the intervals between successive strobes going off.

It is not difficult to build your own delay timers. Dr. Loren Winters provides full instructions on his websites, but they are also available commercially from Woods Electronics and the Kapture Group.

Vinten Associates - (Uniblitz Shutters)

Woods Electronics Inc. (Shutter Beam and ADDjust a Sec)

Kapture Group (High-Speed Systems)
http://www.kapturegroup.com/