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Home >> Articles >> Photography >> Tips for UW Strobe Use

Tips for UW Strobe Use

Photography is about capturing light, and to be able to make good photos, it is important to have good light. In underwater photography, the use of artificial lighting, in the form of strobes, is one of the most effective forms of lighting. The following represents several tips and tricks for using strobes.

(Please note: these tips were written in around the year 2000, and are mainly associated with film photography. Although the general thrust is the same, I plan to update this article shortly.)

Adjusting Guide Numbers for Different Speed Films

One of the common questions people ask when considering Guide Numbers is how they adjust them for different film speeds. Given that Guide Numbers are calculated based on ISO100, the general process is to calculate as if for ISO 100, and then convert depending on the speed.

However, it is also easy to apply a simple conversion factor, as the following table depicts

  • ISO50 - Adjustment Factor = 0.71
  • ISO100 - Adjustment Factor = 1.00
  • ISO200 - Adjustment Factor = 1.40
  • ISO400 - Adjustment Factor = 2.00

This adjustment factor is based on ISO 100 is the base (therefore its adjustment factor is 1.0). Multiply by the adjustment factor to get the Guide Number with the corresponding speed film.

For example, if you are using a Sea & Sea YS-60/N strobe, which has a guide number of 22m (surface, ISO100), with Fuji Velvia (ISO50), then the surface Guide Number would be 15.62. Underwater this would be between 5.2m (temperate waters) and 7.81 (tropical, clear waters).

Adjusted Guide Numbers can then be used as normal with the appropriate film.

 

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When to Use TTL

TTL is an enhancement to many camera systems that theoretically controls the strobes by shutting them off when a sensor near the lens detects that the correct exposure has been obtained. Commonly thought of as “Through The Lens” metering (but more correctly being Thyristor-Thyristor-Logic), TTL has the promise of simplifying UW photography by automating the process of determining and controlling correct exposure.

Theoretically.

In practice, TTL metering makes the determination of “correct” exposure by measuring the light in a part of the centre of the picture area. Unless your primary subject covers a reasonable area of the centre of the frame, the TTL system may expose the negative space rather than the subject itself. This would mean that the subject may be under or over exposed.

TTL takes into account many factors when determining exposure - light, film speed, aperture and shutter speed, meaning that bracketing for correct exposure (a highly recommended practice) can only be achieved through tricking the camera and changing the film speed.

So when should you use TTL? Simply put, when the subject (of average reflectance) occupies a reasonable portion of the centre of the frame.

Personally, there are two situations where I use TTL

  • When taking macro shots, TTL generally works well. For some darker subjects or more reflective ones, you might want to go manual and bracket, or bracket by changing exposure.
  • When I need to reel shots off fast. In this instance, TTL can provide acceptable exposure on most shots if shooting large-ish subjects, such as sharks and other big fish. Set the camera to A (aperture priority), determine strobe-to-subject distance, set the aperture, and shoot away on TTL.

In other photography (wide angle, normal angle), I prefer to have control for the exposure, and shoot manual modes (M90 on a Nik V), and strobes on full power or half power manual.