How much light is let in, and what affects exposure?
Exposure can be defined as the amount of light your camera lets in for any given shot. In the old days, "the exposure" was another term for your photo that was produced by your camera.
Alternatively, you can say it's the amount of light that hits your camera sensor. Exposure is one of the most important things for an underwater photographer to master.
What is a good exposure?
When people talk about having a good exposure, they mean that their photo is not too dark and not too bright.
But what is a good exposure? A good exposure can be defined as a an exposure that gives your photo the tonal values and colors that you imagined when you took the shot. Many people will tell you correct exposures are defined by a histogram. In the end, a good exposure is defined only by your judgment. A histogram can be very helpful in telling you what kind of exposure your photo has.
The definition of a proper exposure is an exposure that matches the exactly what the photographer's vision was. Even the best camera will not automatically expose a photo in the way a creative photographer imagined the scene, which is why photographers often take control of the exposure.
Factors affecting exposure
Shutter speed, aperture, and ISO all affect your exposure. All of these are explained further on in this chapter. If you are using a strobe, then the strobe power is also a 4th (and major) factor in your exposure.
The slower the shutter speed, the larger the aperture (smaller F-stop number), and the higher the ISO, the brighter the exposure will be.
When the camera shutter is opened, light enters through the aperture of the lens. The resulting signals are then amplified electronically if your ISO is greater than 100.
Base ISO is the ISO at which your camera will have the optimal noise, dynamic range, and color sensitivity. Assuming we are shooting at base ISO (ISO 100 on most cameras, 200 on the Nikon D300 and some other dSLRs), we will look at how shutter speed and aperture affect UW photography in the next few pages of this chapter.
Talking about "stops"
Before we talk about shutter speed and aperture, we need to talk about "stops". A "stop" is a relative term. Making a photo one "stop" brighter means you are letting in twice as much light. Making a photo a stop darker means you are letting in half as much light. People will often say "go up one stop", or "go down one top", or "make it one stop brighter". Changing either the shutter speed, ISO, aperture, or strobe power, depending on the circumstance, can accomplish this. More examples of this will follow.
Exposure Value (EV)
EV stands for "Exposure value", which is the amount of brightness in a photograph.
Let's continue on to understand aperture.