By Steve Miller

Aperture is one of the three major camera-set variables that affects your image's exposure. Along with shutter speed and ISO, aperture affects the amount of light that reaches the camera's imaging sensor. 

Think of the aperture like a pinhole. As the hole gets larger more light gets through. It's the same with your aperture. 

The lens opening reduces to half its size with each change in f/stop. Larger f/stop numbers let in less light, typically requiring more artificial light (strobe power). Image © KoeppiK / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) (modified text)

Aperture is represented by an f/number. Each change of f/number is called an f/stop or just stop. The only confusing bit is that a small f/number is a large aperture, and vice versa. When we say "large," "wide," or "wide open" aperture, we're referring to the range from f/4 to f/6.3. When we say "small" or "narrow" aperture we're usually talking about f/8 (small) up to f/22 (smaller) or even f/32 (smallest).

Each lens has a maximum aperture value or value range. The maximum aperture your lens offers determines how bright your scene appears to both your eye and your sensor- thus affecting how easily the autofocus does it’s job. We often call this how "fast" a lens is.

A lens that is f/1.2 can be triple the price of a "slower" f/4 lens in the same focal range. Even when shooting at smaller apertures the maximum aperture still affects the cameras performance - and your ability to see the image - since the lens only "stops down" a split second before capture.  Topside photographers rarely have issue achieving a focus lock quickly. Underwater the lack of overall contrast and light (the keys to fast autofocus) can cause your camera to struggle when focusing. 

.

(Left) A landscape shot at an aperture of f/11 has a deep depth of field where the foreground and background are all in focus. (Right) A shot of a turtle with very shallow depth of field shot wide open at f/4.5.

The most significant effect of aperture is brightness but it also affects depth of field. Depth of field refers to how much of your image's foreground and background is in focus. A very large aperture like f/2.8 will produce a shallow depth of field. You may be most familiar with this effect in portraits where a person's face is in focus but the background behind them is blurry. Small apertures will have deeper depths of field. Imagine a landscape where rivers, trees, and mountains are all in focus.

Small apertures improve photo sharpness in two ways: by increasing depth of field and by reducing aberrations. Photos taken at smaller apertures (higher f/numbers) will be sharper than photos taken at larger apertures. Depending on the distance to subject, lens, and conditions, the effect may be more or less noticeable.

When to Change Aperture

If you are using a camera with manual settings for the first time, you are finding a very wide range of options for aperture, shutter speed, and ISO compared to cameras that only work in automatic modes. 

(Links) Eine Landschaftsaufnahme bei einer Blende von f/11 hat eine große Schärfentiefe, bei der Vorder- und Hintergrund ganz scharf sind. (Rechts) Eine Aufnahme einer Schildkröte mit sehr geringer Schärfentiefe, aufgenommen bei einer Offenblende von f/4,5.

CFWA and Depth of Field

The above scenario has worked great for me for years, but it is predicated on the idea that you are using a super wide angle lens- or even better, a fisheye.  Super wide angle lenses share a wonderful attribute: they have incredible depth of field inherently built in to the optics.  Once your focus locks and you make your capture, the subject and everything farther away from that point will be sharp. However, anything closer than the focus point will be softer. This may be the case at any aperture value depending on your distance to the subject.

Smaller apertures (larger f/numbers) are highly desirable for improved edge sharpness when shooting CFWA and reefscapes. So once you have balanced the background water color properly, aim for an aperture of f/8 or smaller whenever you can.

A small aperture is critical when shooting super macro, like with this tiny pygmy seahorse. Your depth of field is only millimeters when working at close distances with a long macro lens in the 90-100mm range. Photo © 2020 Steve Miller

Macro Photography

When we move to longer focal lengths (and in our case mostly we are talking about macro lenses) the depth of field can become razor thin. Small apertures are your friend here. When the surge is rocking you back and forth and you are trying to lock focus on the eye of a tiny subject, misses are common. Many beautiful macro images miss the perfect focus point by a millimeter and the softness makes the image suffer.

Think of f/8 as a minimum when shooting macro underwater. If you're shooting a DSLR or mirrorless camera with dual strobes, then aim for the f/16 to f/22 range. Focus on the eye of your subject and bracket the aperture to acheive the perfect bokeh effect (that dreamy softness separating your subject from the background).

Aperture Priority Mode

If you're shooting a compact camera without a full manual (M) camera mode, then Aperture Priority (A or Av) mode can be your friend. It's important to keep an eye on the shutter speeds the camera selects. It may start shooting at too slow of a speed, leaving you with a card full of motion blur. Check whether the camera's menu options allow you to set a minimum shutter speed when shooting in Aperture Priority.

Most compact point-and-shoot cameras benefit underwater from turning on the macro focus mode, even when shooting wide angle. This mode typically forces the camera to shoot at smaller apertures resulting in an image with better color balance, especially when shooting with strobes.

Aperture priority mode set to f/8 with TTL strobes allows you to capture a wide scene in focus with a compact point-and-shoot camera and fisheye lens. This image was shot with the Olympus Tough TG-6 and FCON-T02 fisheye lens. Photo © 2020 Steve Miller

Conclusion

Aperture value is tremendously important to the exposure and sharpness of your underwater photos. Understanding aperture will take your photos from average to awesome! Remember that large numbers mean small apertures. As a general rule of thumb, start small and go from there.