Pseudo TTL flash technique


I include a description of a specialized flash technique, which, as far as I am aware of, has not been described anywhere in the literature yet. It is taken from Geiger (1993): The Influence of Monospecific and Dispecific Diets on the Growth of Bowerbankia imbricata (Bryozoa - Ctenostomata). Diplomarbeit Universität Basel. 83 pp.: 22-24, and has been published in Geiger, D. L. 1998. A new photographic lighting method for macrophotography: the pseudo-TTL flash technique. Club Conchylia Informationen 30(4-6): 13-15.

Photography:

One interesting photographic technique has been developed while taking pictures of Bowerbankia imbricata. When taking photographs of small objects such as bryozoans through the water problems often arise for the following reasons:

  • Low light levels on the film plane result because a) the object should be photographed in full format, i.e. at large magnifications; b) the object should be photographed with as much depth of field as possible, respecting the maximum advantageous f-stop; and c) the light source should be colour-balanced corresponding to the film used, i.e. a strong blue filter (Kodak Wratten 80A or 80 B) has to be used with cold light source and a day-light film.
  • A short shutter speed is necessary because the object should be frozen in movement and the Schwarzschild-effects caused by long exposure should be shunned, but the object should be photographed on a fine grain film with high colour saturation and colour accuracy, i.e. a low speed film with a sensitivity of less than ISO 100 has to be used.
  • In aquatic biology the water imposes another problem, which is reflection of light on the air-water or air-glass-water interface, which is dependent upon the incident angle of the light source and must be termed in practice a random factor.

The only escape out of this combination of abstruse conditions is flash photography, as flashes are more or less colour-balanced with 5,000-6,000 K and are of a short duration, typically between 1/1,000 and 1/10,000 s; both parameters depend to some minor extent on the degree the capacitor is discharged. Additionally, many system cameras have automatic flash exposure control (TTL). However, these ordinarily available TTL-flashes are of rather low power and a combination of two flashes of equal power increases the combined guide number (GN) by only one f-stop, which corresponds to a factor of sqrt 2 (generalized formula: sqrt(sum of (GNi2)) ). There are very powerful studio-flashes with a GN of 100 and more, but those are not TTL. Calculating the correct f-stop by hand is virtually impossible, taking into account extension tubes, the nonlinear function of the power of the flash to distance to the object in macro-photography, the reflection on the water surface through which the picture should be taken, and realizing that on a macroscope no f-stop values are indicated on the diaphragm.

The solution is a combined pseudo-TTL-flash-technique. The concept is to give the baseline illumination with the non-TTL but high-power studio flash and modulate the remaining light necessary using a low-power but TTL-flash resulting in an overall high-power-TTL-flash photograph using the best parts of both flash systems. Sometimes the power of the studio-flash must be lowered to allow the TTL-flash to modulate. This technique was successfully used with the following equipment: Wild M420 Zoom Macroscope, Elichrome 500 studio-flash with cold light adapter, Olympus T-32 TTL-flash, Olympus OM-4 camera. The Elichrome was connected to the camera body by a non-TTL, 2-pole synchro cable, and the T-32 was connected by means of a TTL-cable. With such a set-up the camera body gives a correct TTL-ok feedback, i.e. ok, over- and under-exposed, and, therefore, corrections of the illumination can be made whenever necessary. For dark and light background (non neutral gray) the reading of the light-meter can be adjusted with the correction, respectively changing the film speed. Focusing light can be provided by the studio-flash, as most have a built-in light bulb, or a conventional cold light source can be used. This should be set at as low a light level as possible, because then the light intensity of the unbalanced light source can be neglected, particularly with slow films below 100 ISO. Otherwise a blue filter (Wratten gelatine filters) should be utilized.

Some flashes, notably those used in underwater photography, can be set in the so called slave-mode, where the flash is triggered by the light emitted by another flash, not by an electric current through a synchro-cable. Successful experiments were conducted with a Subtronic HST 2000 and an Aquatron SII triggered by a T-32 connected to the OM-4 body, and also with an additional T-32 or T-28.

It is expected that this technique will work with any system with the following specifications:

  • TTL capable camera body
  • camera body that measures the light on the film plane during the exposure (backwards oriented photo cell under the mirror)
  • at least one TTL-flash connected TTL to the body
  • any other flash linked by any means to the system above.

However, the geometry of the light sources and the lens still has to be taken into account. The scattering angle of an ordinary SLR-camera flash is generally 60°, but those of studio flashes or underwater flashes can be as high as 100°. Furthermore, the collecting angle of the lens must be considered, which becomes narrower the longer the extension tube is. Knowing these two parameters and the law 'angle of incident light = angle of reflected light', suitable positions of the light sources can be chosen; otherwise reflections on the surface of the water become inevitable. Blinds can be useful. A very flat angle of the incident light does not result in strong shadows as the light beam is bent when passing the air-water interface, creating a more vertical illumination of the object. Failure rate using the above technique is very low, and some results obtained with a Kodakchrome 64 colour reversal film are shown on Plate 1.