Those of us who grew up photographically on film were ingrained — no pun intended — in the intricacies of ASA and ISO (a.k.a. film speed) because it made a difference in picture quality, as well as what we could shoot.
Simply put, when we were dealing with film, “speed” really referred to the size of the silver halide crystals (grain) in the film. Small numbers (50, 64, 100) meant relatively small grain. Larger numbers (400, 800, 1600) meant larger grain. The size of the grain determined the light-gathering ability of the film. Small grain didn’t “see” as much light, so you would use it on bright days or with powerful strobes. Large grain, because it had more surface area, collected more light and could be used under low-light conditions. But there was a trade-off. Larger grain might allow you to shoot in lower light, but it also meant that when you enlarged your picture, you would start to see the grain. In the most basic terms, the pictures would get a little fuzzy or noisy. Small grain film was generally used for sharper images that could be blown up larger without degradation of the quality of the image.
Then along came digital photography with seemingly the same ISO numbers. Or are they? It certainly doesn’t seem so to me. I’ve been shooting digitally since 2005, have probably taken close to 40,000 underwater images and all I can tell you is that I can shoot at digital ISOs that would generally be unthinkable with film.
I was at the Rose Bowl (not underwater) for a UCLA evening football game last year and, since my only light source was the stadium lights, set my ISO at 1600. I then pushed it one additional stop in the camera, effectively making it 3200 ISO. Conventional wisdom would hold that my pictures would be so grainy that they’d not be viewable. Yet the images are clear and crisp even to the point of stopping motion on passes. Granted, not every shot was perfect and there were certainly some stinky ones. But, especially when you consider I was hand-holding the camera from my seat using a 300mm lens, the results were pretty impressive.
When we’re talking about film ISO, we’re talking about changing grain size. When we change ISO in a digital camera, what’s changing? There’s no “grain” on the sensor, so all that changes is the sensitivity of the sensor to lower levels of light. But it would certainly seem that the tradeoffs digitally are nowhere near as punitive as the tradeoffs with film.
The point of all of this is to encourage you to experiment with your digital ISO. Don’t be locked in to thinking that you always have to shoot at the lowest possible ISO to get the best quality. I frequently shoot images for this publication and others at digital ISOs of 400 or 800. Whatever differences there are seem negligible.
What advantage would you get from a higher digital ISO? For one thing, you can shoot at a smaller (higher number) f-stop. That will increases your depth of field but also, more importantly, builds in a little fudge factor for your focus.
You may also be able to shoot at a faster shutter speed. That will not only stop motion better (those pesky fish never seem to want to sit still) but will also reduce the effects of camera shake, which can be quite noticeable if you’re floating neutrally buoyant in the water column.
All of this means you’re giving yourself a better possibility of producing a good image simply by raising the digital ISO as it will produce a corresponding shift in the other areas as well. And isn’t that the point of taking pictures — trying to produce a pleasing image?
So I encourage you to experiment. Don’t forget that, unlike film, you can change digital ISO in your camera on the fly. Find a cooperative subject and shoot it at 100 ISO, and then at 200, 400, and 800 (with corresponding shutter and aperture changes). Look at all the images on your computer. See what you like and don’t like. See what changes happened in your camera as you made these shifts. Decide what you like and then try some more that way.
Digital cameras open up a whole new realm to photography, let alone underwater photography. Think outside the housing and discard some of your preconceived notions and try some new things. You may not only surprise yourself, but you may actually find your images improve.