Welcome back dear nascent underwater photographers! Got your camera? Did you figure out how to find the proper settings for shutter speed, ISO and aperture? Well, then you are well on your way to becoming a capable underwater photographer, without even having gotten wet for one minute! This land-based thinking and learning you are doing now will really pay off once you go underwater and know what the buttons on your camera actually do!
Shutter Speed and Sunlight
You now know how shutter speed influences the sharpness of your image. If you choose a too-slow shutter speed value, then you allow light to reach the sensor of your camera while your photo subject moves from one point to the next, and that blurs it out. If your shutter speed is fast, you freeze one brief moment in time. This principle is explained in this graphic of two ghost pipefish, one shot at 1/200 seconds (sharp), and one at a slower speed of 1/40 seconds (blurry):
But things are different when you are using a strobe which illuminates all of your scene. This most often occurs in macro photography. The shutter might stay open for longer, but all of the light hits your photo subject at the instant of time when the strobe fires – which is faster than 1/40 or 1/200 seconds. No matter how much longer after the firing of the strobe the shutter is open – almost all the light which will reach the sensor comes from the strobe, and that light has already arrived. Shutter speed only has a secondary importance in such a photographic situation.
As so often in life, the situation is still a bit more complicated: it’s not binary. There are cases where both ambient (sun) light and a strobe contribute to the light reaching your sensor. This occurs more often, but not exclusively, in wide-angle shots. In this shot of a shark, the strobe light only illuminated the center of the image. Most of the light still came from the sun – a slower shutter speed would have made the shot brighter, but also blurrier.
Photographic greetings from the Philippines! If you have any questions or suggestions, email me at firstname.lastname@example.org