View Full Version : Digital Photography Glossary

10-10-2007, 11:31
Here are some terms that are used in Digital Photography. I'll try and provide some of the key elements in Digital photography with the idea that if people who are interested and take the time to read will get a better understanding of how to use these wizbang digital marvels.

Feel free to ask questions, or request explanation of terms that I've not covered.

Exposure Control
Aperture - Aperture is used to control the exposure, and the depth of field. It is the size of the opening in the lens, and is adjustable. It acts much like a pupil. Aperture values are referred to as f-stops. The lower the number, the larger the opening, and the more light that reaches the film or sensor plane. Larger apertures result in less depth of field. Small apertures (larger number) results in more depth of field, and less light reaching the film or sensor plane. Here is a list of relative aperture setting in full stops.

Shutter/Shutter speed - Shutter speed is the amount of time the sensor or film plane is exposed to light. Shutter speed controls exposure, and can be used to freeze action, and to prevent camera shake from impacting an image. Shutter speed in most digital rangefinder camera's (PnS) is a misnomer as most have no shutter. Most DSLR's use a shutter, but non-DSLR digital camera's generally do not. Instead they turn the sensor on, and then off to control the shutter speed, or exposure speed. Shutter speeds are represented in seconds, or fractions of a second. Here is a list of shutter speeds in full stops in seconds:

ISO/ASA values - ISO or ASA values is a holdout from film days, but in digital the refers to the sensitivity of a sensor. The common default, or normal ISO value for most sensors is ISO 100, but some go as low as ISO 50, and others may start at ISO 200. Each sensor has a base value (the lowest value), and at that setting will produce the highest quality image, but it will require more light than higher settings. When the ISO value is increased it will require less light to capture the image. When the ISO value is increased, the sensor is amplified. The unwanted side effect in increasing or amplifying the sensor value is noise (similar to grain in film). This is like turning up a guitar amplifier. You turn it to 11, and you hear a loud hum. In digital camera's that hum is noise, and in both cases it is undesirable, but not avoidable. Common ISO values in full stops are:
ISO 50,100,200,400,800,1600,3200,6400

Photography is ALL about light. The exposure is how much light hits the film or sensor plane, and is controlled by the Aperture, and shutter speed. The amount of light required will be dependent on the ISO setting or Sensor Sensitivity.

There is a LOT more to photography than the four terms listed above, but if you understand Exposure settings and control, that is the foundation to learning how to control the camera, and capture better images.

Metering - Camera's use a reflective meter to determine the amount of light in a scene, and determine the proper exposure value. Reflective meters can be fooled, but today's cameras use sophisticated and time tested algorithms to determine the correct EV or Exposure Value. There are generally three types of metering available in most cameras, Evaluative or Matrix Metering, Center weighted metering, and spot metering. The main differences between the types of metering is the area that is getting metered. For more information on what your camera uses, read the manual!

Maunal Mode - In this mode the user manually sets the aperture, shutter, and ISO values. This is a good mode to use UW, or in any environment were one is using flash as a main light source.

Aperture Priority - In this mode the user sets the aperture, and the camera selects the shutter speed based on the ISO setting and aperture. This is generally used where DOF is the over-riding priority. Landscapes is a good example. The downside of A mode is that if one is not paying attention, the camera may select a shutter speed that is too slow to hand hold.

Shutter Priority (S/TV) - In this mode the user sets the Shutter speed, and the camera selects that aperture based on the ISO setting and Shutter selected. This is a generally used for shooting moving subjects, like sports or action. The downside of S mode is the if one is not paying attention, the camera may not be able to choose an aperture within the range of the exposure reading.

Program Mode - In this mode, the camera pretends that the photographer has no brains, and chooses the aperture, and shutter based on the ISO value set. The downside of this mode is that the camera may not choose the right shutter speed, or aperture for the subject being photographed.

All those other Modes - There is really only THREE major things that impact exposure, and if you've read above, you should know what they are. All the other modes are just smoke and mirrors. For example a "Sports" mode is going to select the highest shutter for a given exposure based on the fastest aperture setting and ISO selected. Landscape mode is going to select the maximum DOF for the scene based on... well your guess is as good as mine. That is the downside of all these modes. They are designed for those that don't want to understand how to use a camera, but the decisions they make may not be documented, or desirable.

Exposure Compensation - In camera meters are not always accurate. Exposure Compensation is a way to adjust the exposure for a given scene without changing the aperture, shutter or ISO settings. Exposure Compensation is not found in all camera's, but is available in most prosumer, and DSLR models. The settings are generally from +2/-1 stops in either 1/3 or 1/2 stop increments. Based on the Histogram, if the photographer determines that the image is underexposed by 1 stop, setting exposure compensation to +1 will lower the metering EV by one stop, and double the amount of light hitting the sensor. It does this based on the mode selected. If shooting in A mode with the aperture set to F8, if the initial settings were ISO 100 @ 1/250 @ F8, then adding one stop of exposure compensation would result in a setting of 1/125 @F8 to increase the exposure by one stop. This is a handy tool for fine tuning exposure in any mode. When shooting in manual mode the setting will not be adjusted by the camera, rather the metering display will be adjusted to impact the exposure by manually matching settings to the meter.

Histogram - The histogram is one of the most powerful tools that a digital photographer can use to fine tune exposure. A histogram is a visual bar graph representation of an image. The histogram values range from 0 to 256 going from black being zero, to white being 256, and every color in between. The height of each bar represents the number of pixels for a given tone or color. This is a great benefit to the photographer as it can be used to determine correct exposure, and adjust the exposure. There is no *correct* histogram, but generally seeing spikes at either the low or high end of the histogram means a loss of data. Depending on the lighting, and dynamic range of the camera, it may not be possible to achieve a good histogram, but that is useful as well. I would strongly suggest further reading on this subject as the histogram is the most useful tool available to digital photographers to ensure good exposure.

Dynamic Range - Dynamic range is a measure of the Exposure Values (EV) captured by the camera, or in a scene. The range of a camera sensor or film is limited. Fuji is king of the Dynamic range game, with the S5Pro boasting an impressive 11+ EV's of dynamic range. What does that mean? A scene may have a lot more Dynamic range than any camera can capture. In that case a decision must be made on if the image is worth shooting. An example of such a scene would be a mountain where the top is in bright sunlight, and the base is in dark shadows. Add some snow on top, and some dark timber in the shadows, and you have a dynamic range nightmare. That scene will have more dynamic range than any camera can capture. When shooting that image, one would have to decide where to place the exposure and what part of the image will fall outside of the limits of the camera. Most opt to clip shadows in favor of blowing out highlights. The histogram is the tool to use to determine if a scene falls outside of a camera's dynamic range. Spikes at both ends of the histogram tells the photographer this scene is well outside the camera's range.

Digital Terminology
White Balance

Optical Controls
Depth of Field
Barrel Distortion
pincushion Distortion
Focal Length