Camera Settings
Exposure
Exposure is the total amount of light allowed to fall on the image sensor. It is affected mostly by aperture, shutter speed and ISO settings. You can also control the exposure by adjusting the EV (Exposure Value) settings on your camera. By selecting a negative EV value, less light will reach the image sensor and the photo will be darker. By selecting a positive EV value, more light will reach the image sensor and the photo will be lighter.
Areas of a photo where information is lost due to extreme brightness are described as having “blown-out highlights” or “flared highlights”. In digital images this information loss is often irreversible. Areas of a photo where information is lost due to extreme darkness are described as “crushed blacks”. Digital capture tends to be more tolerant of underexposure, allowing better recovery of shadow detail.
Some cameras allow you to set “blinkies” warning to visually display areas of blown out highlights on your LCD display. If you see blinkies appearing in an area that is important to you, use your camera’s exposure compensation control to lower the exposure until the blinkies go away.
Proper exposure is critically determined by a relationship of aperture and shutter speed. When you change one, you must compensate the other in order to maintain a balance.
Histogram
Another useful tool in evaluating your image’s exposure is the histogram display. A histogram is a graphical representation of the distribution of digital data (brightness values) in an image, derived by counting the number of pixels and displaying them in a two dimensional graph. The histogram provides a compact summarisation of the distribution of data in an image and allows you to evaluate the distribution of tones in the image.
A histogram illustrates the distribution of 256 possible levels of brightness in all the colour channels together and in each one individually. The histogram’s horizontal axis represents the range of brightness from zero (0) (the shadows) on the left side of the graph to 255 (the highlights) on the right. The vertical axis represents an increasing amount of digital information from bottom to top. The more pixels to the right of the histogram, the brighter the image will be. The more pixels to the left, the darker the image will be. An ideal histogram will have the majority of pixels around the centre with low trails touching either side. If the histogram trails don’t touch the extremes, the image doesn’t contain true highlights or shadows.
Using the Histogram
Histograms come into play in two places: in capture and in image processing. By consulting your histogram after taking a photo you can determine whether your exposure is satisfactory and the image data contains enough information in the highlights, midtomes and shadows. if not, you can adjust your exposure for a second shot. You can adjust your exposure for an underexposed photo by opening your aperture, changing to a lower shutter speed or selecting a positive exposure value. For the opposite situation, when your photo is overexposed, you can close your aperture; use a higher shutter speed or select a negative exposure value. With practice, you can learn to trust the histogram better than trusting the image displayed on your camera’s LCD screen.
A reminder: it is always better to err towards the slightly underexposed image because overexposed image might have clipped areas with no data to recover in them.
Aperture
Aperture is the diameter of the opening (meaning the diameter of the lens’ iris) in the lens that allows light to pass through. It controls how much light enters the camera. Aperture settings are expressed as f-stops (for example 4, 5.6, 8, 11, 16, 22) with the highest and lowest f-stop numbers depending upon the specific lens.
Aperture influences the depth of field, which is the range of focus. Depth of field is the appearance of objects to be in focus within a specific distance range. The smaller the aperture (higher f-stop), the more depth of field and more of the image will be in focus. The bigger the aperture (lower f-stop), the less depth of field and less of the image will be in focus. This can be a bit confusing but let’s try and work it out: (include illustration)
Many point-and-shoot cameras use a high f-stop as a default, thus everything in the photo (foreground and background) is in focus.
In cameras that have an option for aperture priority, you can use a small f-stop to blur a distracting background behind your subject and to redirect the viewer’s attention to your main subject.
Shutter Speed
Shutter speed is how long the camera allows light to reach the image sensor. Shutter speed is expressed in fractions of a second (for example: 1/250th of a second). The faster the shutter speed, the less light reaches the image sensor. Slow shutter speeds allow more light to reach the image sensor but there is an issue of holding the camera steady enough for a sharp image. Underwater, when photographing fast moving subjects you should set your camera on shutter priority, select a fast shutter speed and let the camera adjust the aperture accordingly.
ISO
ISO value is taken from film photography but in digital photography it indicates how sensitive the image sensor is to light. ISO values range between 50 to 800 or even 1600 in some cameras. The higher the ISO, the more sensitive the image sensor is and the lighter the image will be, which is good for low light conditions. On the other hand, high ISO settings will cause a grain effect in your image thus lowering its quality. In order to lower the amount of noise in your image, you should select a low ISO setting. Usually ISO 100 is a good default to work with underwater (you might need to disable the Automatic ISO setting on your camera).
Colour Loss
As mentioned on the Shooting Techniques page, light and colour get absorbed underwater. There are a few options you can use to bring back the lack of colour underwater:
- Use a colour correction filter.
- Use a strobe.
- Control the white balance setting on your camera.
- Use a preset for underwater scenes.
White Balance
White balance settings compensate for different lighting situations. You can manually adjust the white balance underwater by using a white slate to set as the white values. Follow manufacturer’s instructions on setting white balance manually.
Underwater Scene Preset
Some cameras have predefined scene presets and one of those settings might be for an underwater scene. While on this preset, the camera compensates for the bluish-greenish hues underwater by adding reds to the image in order to achieve more natural tones.
Bracketing
Bracketing means taking the same photo at different settings that let in more or less light. Generally, you take three photos: one with either one f-stop or shutter speed less than ideal, one with ideal settings and one with one f-stop or shutter speed more than the ideal settings. Bracketing allows you to compare the same image under different settings.
Another way is to move the strobe either closer to or further away from the subject or adjusting the strobe’s power. Lower power settings come in handy if you need to open the lens to a wider aperture.
Contrast
Contrast is a measure of the difference between the brightness of the lightest and darkest areas in an image. A photo with very low contrast is called flat. The opposite will be a photo with very high contrast. The histogram of a flat image will be centred in the middle with no definite highlights or shadows. A high contrast image will have spikes at either end of the histogram with very little detail in the mid tones and no gradient transition in between. We can adjust flat images using the levels dialog box in Photoshop. Many cameras add contrast to images by default. The user can adjust this setting. If you are shooting Raw, the camera’s default settings do not matter as you can control them later when editing the image.