By Ambassador Steve Miller

Aperture, Shutter speed, and ISO make up the big three of exposure controls. But what does this strange initialism do? Should you just stick with Auto?

It turns out that with a few key points in mind, you can be using your camera's ISO setting to its best advantage underwater. So let's dive in.

Be prepared to use ISO speeds of 1600 and higher to capture streaming light rays in the cenotes. The trick is to open up your aperture then juggle shutter speed and ISO to use the fastest shutter speed and lowest ISO possible. 

What is ISO

ISO is the measurement of the camera's sensitivity to light. Setting the camera at a higher ISO makes it more sensitive to light. Effectively it controls how much light is hitting the camera's sensor, making the image overall brighter (higher ISO) or darker (lower ISO).

Aperture, shutter speed, and ISO all change the amount of light. The difference in between "stops" or "f/stops" is the same regardless of which setting you change. For example, going from ISO 100 to 200 is effectively the same difference as going from aperture f/8 to f/5.6 or shutter speed 1/250 to 1/125. But in camera settings you never gain more light without a trade-off. With ISO, higher sensitivity means less fine detail in your image.

Higher ISO settings will increase the graininess of your photo. This was taken at ISO 1600 using the 2008 model Canon EOS 50D. Cameras have gotten a lot better since then and 1600 isn't considered particularly high anymore. You probably wouldn't see the same graininess if this photo was re-taken with a new model.

Where to start

A look through the average metadata of 100,000 underwater images shows that ISO 160 works about 90% of the time.  A lower ISO is always better because higher ISO speeds increase the noise or graininess in the photo.

The brighter and more tropical your diving is, the better these low numbers will serve you.  We use ISO as the baseline for the exposure. Starting at ISO 160, increase it when you are no longer able to work within a reasonable shutter speed and aperture range. Desirable shutter speed range is 1/60 to 1/250 of a second to prevent motion blur. The desirable aperture range is dependent on your subject, lens, and depth of field. Generally smaller apertures produce sharper photos and greater depth of field.

About Auto ISO

Auto ISO is the ISO equivalent of Aperture Priority or Shutter Priority as an Auto setting.  The camera chooses the appropriate ISO based on the light in the scene.  It is seldom used underwater, but can be very helpful if you are shooting in extremely dark conditions, or when the light levels in your scene vary widely (think manta ray night dives with artificial light arrays).

Like aperture priority and shutter priority modes, auto ISO works beautifully with natural light photography, but may not produce the desired effect when using flash. The camera's default reaction is to brighten the scene and lessen the need for flash fill. This is exactly the opposite of what we want to do underwater and will result in a dull, monotone photo.

I remember going to Bonne Terre Mine with a camera in my very early days of underwater photography. I came out of the water at the end of the day frustrated that I could barely get a photo of anything in the flooded mine shaft. It was weeks  later when a colleague was talking about shooting at a high ISO that it clicked with me that I had been shooting at ISO 200 in the mine and needed higher sensitivity to be able to use a decent shutter speed.

- Jean Rydberg

ISO in Low Light Environments

If you've been following camera trends you know that today's cameras are capable of incredibly high ISO speeds. Cameras can practically shoot in the dark! In the olds days, ISO 1000 would mean your image looked like a sand painting from all the grain. Now we can shoot with some cameras at ISO 24000 with very low noise.

Higher ISOs can be critical in underwater scenes where you want to capture the environment but have very little available light. Think caves, caverns, shipwrecks, and swim throughs.

Capturing the "smoke" coming out of this pipe underwater in Bonne Terre Mine was a big deal on our afternoon dive. Unfortunately it was impossible to get a sharp photo using ISO 200 as the camera required a shutter speed of 1/8 and wide open aperture of f/2.8 to make anything show up in the photo. ISO 1600 would have been more appropriate. - Jean Rydberg

ISO with Fast Moving Subjects

When you add or subtract stop of light from one of your three inputs - shutter, aperture, or ISO - you can compensate it in either of the other two.  When we shoot fast moving objects like dolphins, we know that we will have motion blur with any shutter speed slower than 1/500. ISO is your baseline to allow the camera shoot at the shutter speed that you know you need.  If your light reading for fast objects selects a shutter speed of 1/125, raise you ISO until that shutter speed is what you need. 

Pro Tip: Dolphins may need shutter speeds up to 1/600 to 1/700. Most slower sharks will be pretty sharp in the 1/125 to 1/250 range.

ISO with Macro vs. Wide Angle

Camera technology has become so advanced in low light photography that most people can’t notice any increase in noise when going from ISO 100 to ISO 400. So in wide angle photography we set our ISO to match the ambient light. A good rule of thumb is ISO160 for sunny tropical diving and ISO 400 for deeper dives, cloudy water, or overcast weather.

You'll need to bump up your ISO a little bit to give yourself enough light to shoot at faster shutter speeds. These dolphins were taken with available light at ISO 400 and shutter speed 1/640.

With macro photography and dual strobes we can set the ISO as low as it goes. It won't necessarily make a difference in clarity but you will have more light than you need in these tiny scenes when you are shooting at very small apertures of f/16 to f/32. 

ISO and Flash Use

It's important to avoid the mistake of using too high of an ISO with strobes. If you shoot ISO 800 on a sun drenched reef and your strobes are set to TTL, the resulting images will be monotone blue or green. This is because when the scene is read by the TTL system, the camera thinks there's so much ambient light that all but the smallest blip of flash will overexpose the scene. So your strobes will be firing at a very low power and the colors on your subject will be washed out and lacking vibrancy.

Lower ISO speeds are critical when using TTL strobes in sunny conditions underwater. ISO 400 is about as high as you want to go to still get the blast of strobe light that will add color back to the reefscape.

ISO in Video

High range ISO lets us record without video lights in places previously impossible. Although we sacrifice color and color contrast, natural behaviors of the marine life aren’t altered by the presence of an artificial lighting source. 

If you choose to use lights, you can balance the ISO the same way you would for stills. More light allows you to shoot lower ISOs for increased sharpness and clarity.

Auto ISO can be helpful when panning through changing light, allowing you to continue shooting without having to stop and make an adjustment.

Conclusion

You probably won't (shouldn't) be changing ISO on every shot, but is an important part of your exposure toolbox. Take some time to really understand the basics and it will become automatic to you.