Color bias, limited visibility, and low ambient light
Sea water, even when it is tropical clear, is an unforgiving light filter. By the time you are 4 meters deep the ambient light is greatly diminished and most of the red wavelengths are gone. Electronic Flash units or movie floods bring out the wonderful colors of the underwater world – but only very close to the camera. The artificial light is attenuated both on the way to the subject and on the way back so you’ll be lucky if you get nice colors more than two meters from the camera. This is not very useful for a sphere image because the immediate foreground would be beautiful the rest of the sphere would be dark and dismal. Using a pair of big strobes raises additional issues of positioning and rotating the whole rig. So as far as lighting goes I only use ambient light and limit my sphere taking to less than 4 meters – generally less than 3 metres.
When you are in the water your brain corrects for the universal blue cast and colors look wonderful – but your unprocessed images look blue and washed out. The solution is to do your photography in less than 4 meters, shoot in RAW format and correct the images during post processing.
I set the white balance to “shade” or “cloudy” rather than auto-white as this gives a more consistent color balance to start with. Having something white in one of the images helps calibrate the white balance in post processing .
I set the camera on AV and depending on the time of day, water clarity and depth I set the ISO between 200 to 600. The object is to keep the shutter speed above 1/100 sec in the darkest part of the panorama with an F stop between f11 and f16. This can result in individual frames that are darker or lighter than the others but I adjust them to the same relative values during post processing. My stitching program - AutoPano Giga - does a good job of blending it all together.
Focus is a problem underwater. The low contrast – or complete lack of contrast when the camera is pointing towards deeper water – may prevent the camera from focusing – and triggering. My solution is to either shoot a scene where I am sure there is something for the camera to focus on in every direction or to auto-focus the camera on the closest prime object and then set it on manual focus to shoot the sphere. Despite the excellent depth of focus of the fisheye lens, focus is still a problem when the sphere includes something very close to the camera as well as subjects several meters from the camera. If I want to photograph an object very close to the camera I generally make sure there is something for the camera to focus on in every direction and leave it on auto. Autofocus is necessary when, for example, something swims very close to the camera during an automatic sequence. If I am going to leave the lens on autofocus I check to be sure it is actually taking a photo in the most difficult direction. Originally I used my Canon 7D in the 19 point AF mode but like many 7Ds mine developed a focus error in this mode. I found a fix for this but even so, the AF point expansion mode gets consistently sharper focus – providing there is something to focus on.
This is Part 2 of the 360° underwater panorama tutorial series. Read Part 1. A new installment will be published every Friday, so watch this blog!