Through the Lens – 3. Choosing the settings.
Jon Arden asked the following question on social media, “How do you tie the aperture, speed and ISO together to get the correct exposure and which one should be compromised first? Read somewhere, always try and get lowest ISO and around f/8, then adjust speed as required. True or false? Assuming a generic large fish (shark, manta)”
Thanks Jon, this leads me onto the next piece of the jigsaw.
As can be seen from the illustrations in TTL-2, the aperture (f-stop) and lens focal length determine how much of the scene will be in focus in front and behind of the subject; to all intents, this is where the artistic content of the final image can be formed.
So, the first thing to consider when selecting the correct settings is what do we want in focus? This as we now know is down to the aperture and is therefore the very first we should set.
The shutter speed is then adjusted to gain the exposure required for everything our underwater strobe lights don’t illuminate, this is normally the background (in many cases blue water), the adjustment range here can be huge, from as slow as 1/10 and up to the cameras max flash synch speed of between 1/250 and 1/320 (camera model dependant).
Last but not least is the ISO setting, this increases or decreases the sensors sensitivity to the light striking it but adds electrical interference the higher we go, this interference shows as digital noise in the final image. We can adjust the ISO to gain smaller apertures and faster shutter speeds for given exposures, but at the cost of image degradation if we overstep the mark.
In summary, you were very close Jon, but f/8 although a great starting point is going to limit the artistic content of your work, not so much for Mantas and Sharks because of the shorter focal lengths wide angle lenses have but certainly in the world of close up macro shooting where f/8 on a 100mm lens at minimum focus distance is going to deliver a depth of field that could be described as “paper thin”.
If all of the above has fried your brain cells, here are a few real world examples to illustrate the thought process and theory. In the first image below (Wonderpus) I wanted to separate the animal from a really boring, dirty black sand background, I couldn’t get the camera low enough to shoot the subject with a plain water background. Instead I chose to open the aperture to f/5.6 and reduce the depth of field to such an extent that the background was rendered soft and totally out of focus. The tentacles can just about be made out behind the octopus but the leading eye is pin-sharp. I also shot this image at 1/320 to get as dark a background as I possibly could and actually lowered the ISO one click below its native setting of 200 to help in this department.
The second image is almost the polar opposite of the first. These Bumpheads were cruising the shallow reef slope minutes after the dawn sun had broken the horizon. Underwater it was almost dark, with very limited ambient light, I wanted to show the fish cruising the reef slope and also the detail of ripples at the surface whilst giving the impression of the early morning light. This required a huge depth of field; an aperture of f/16 sorted that out. A shutter speed of 1/5 was needed to get the ambient light exposure correct, this proved too slow, so I upped the ISO from 200 to 320 enabling a faster (1/20) shutter speed that didn’t result in motion blur for everything the strobe light didn’t reach.