Car Photography: Glossary for beguinners

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This part of the series will help you understand the most important terms regarding
technical details.

Autofocus

All digital cameras come with autofocus (AF). In autofocus mode the camera automatically focuses on the subject in the focus area in the center of the LCD/viewfinder. Many prosumer and all professional digital cameras allow you to select additional autofocus areas which are indicated on the LCD/viewfinder.

Buffer

After the sensor is exposed, the image data will be processed in the camera and then written to the storage card. A buffer inside a digital camera consists of RAM memory which temporarily holds the image information before it is written out to storage card. This speeds up the "time between shots" and allows burst (continuous) shooting mode. The very first digital cameras didn’t have any buffer, so after you took the shot you HAD to wait for the image to be written to the storage card before you could take the next shot. Currently, most digital cameras have relatively large buffers which allow them to operate as quickly as a film camera while writing data to the storage card in the background (without interrupting your ability to shoot).

The location of the buffer within the camera system is normally not specified, but affects the number of images that can be shot in burst mode. The buffer memory is located either before or after the image processing.

EXIF

Besides information about the pixels of the image, most cameras store additional information such as the date and time the image was taken, aperture, shutterspeed, ISO, and most other camera settings. These data, also known as "metadata" are stored in a "header". A common type of header is the EXIF (Exchangeable Image File) header. EXIF is a standard for storing information created by JEIDA (Japan Electronic Industry Development Association) to encourage interoperability between imaging devices. EXIF data are very useful because you do not need to worry about remembering the settings you used when taking the image. Later you can then analyze on your computer which camera settings created the best results, so you can learn from your experience.

Lag Time

Lag time is the time between you pressing the shutter release button and the camera actually taking the shot. This delay varies quite a bit between camera models, and used to be the biggest drawback of digital photography. The latest digital cameras, especially the prosumer and professional SLR’s have virtually no lag times and react in the same way as conventional film cameras, even in burst mode.

Manual Focus

Manual focus disables the camera’s built-in automatic focus system so you can focus the lens by hand. Manual focus is useful for low light, macro or special effects photography. It is very important when the autofocus system is unable to get a good focus lock, e.g. in low light situations. Note that some digital cameras allow you to manually focus only to a few preset distances. Higher-end digital cameras allow focusing using the normal focus ring on the attached lens, just like in conventional photography.

Pixel Quality

The marketing race for "more megapixels" would like us to believe that "more is better". Unfortunately, it’s not that simple. The number of pixels is only one of many factors affecting image quality and more pixels is not always better. The quality of a pixel value can be described in terms of geometrical accuracy, color accuracy, dynamic range, noise, and artifacts. The quality of a pixel value depends on the number of photodetectors that were used to determine it, the quality of the lens and sensor combination, the size of the photodiode(s), the quality of the camera components, the level of sophistication of the in-camera imaging processing software, the image file format used to store it, etc. Different sensor and camera designs make different compromises.

Unfortunately there is no single standard objective quality number to compare image quality across different types of sensors and cameras. For instance, a 3 megapixel Foveon type sensor uses 9 million photodetectors in 3 million pixel locations. The resulting quality is higher than a 3 megapixel but lower than a 9 megapixel conventional image and it also depends on the ISO level you compare it at. Likewise, a 6 megapixel Fujifilm Super CCD image is based on measurements in 3 million pixel locations. The quality is higher than a 3 megapixel image but lower than a 6 megapixel image. A 6 megapixel digital compact image will be of lower quality than a 6 megapixel digital SLR image with larger pixels. To determine an "equivalent" resolution is tricky at best.

End of the day, the most important thing is that you are happy with the quality level that comes out of your camera for the purpose that you need it for.

Active Pixel Sensors (CMOS, JFET LBCAST) versus CCD Sensors

Similar to an array of buckets collecting rain water, digital camera sensors consist of an array of "pixels" collecting photons, the minute energy packets of which light consists. The number of photons collected in each pixel is converted into an electrical charge by the photodiode. This charge is then converted into a voltage, amplified, and converted to a digital value via the analog to digital converter, so that the camera can process the values into the final digital image.

In CCD (Charge-Coupled Device) sensors, the pixel measurements are processed sequentially by circuitry surrounding the sensor, while in APS (Active Pixel Sensors) the pixel measurements are processed simultaneously by circuitry within the sensor pixels and on the sensor itself. Capturing images with CCD and APS sensors is similar to image generation on CRT and LCD monitors respectively.

The most common type of APS is the CMOS (Complementary Metal Oxide Semiconductor) sensor. CMOS sensors were initially used in low-end cameras but recent improvements have made them more and more popular in high-end cameras such as the Canon EOS D60 and 10D. Moreover, CMOS sensors are faster, smaller, and cheaper because they are more integrated (which makes them also more power-efficient), and are manufactured in existing computer chip plants. The earlier mentioned Foveon sensors are also based on CMOS technology. Nikon’s new JFET LBCAST sensor is an APS using JFET (Junction Field Effect Transistor) instead of CMOS transistors.

ISO - Sensitivity

Conventional film comes in different sensitivities (ASAs) for different purposes. The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or "fast" film is used which is more "grainy".

Likewise, digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal" setting for most cameras, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, or even 3,200 on high-end digital SLRs. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately that also amplifies the undesired noise. Incidentally, this creates more grainy pictures, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the (desired) music but also the (undesired) hiss and crackle or "noise". Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. And unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.

White Balance
Color Temperature

Most light sources are not 100% pure white but have a certain "color temperature", expressed in Kelvin. For instance, the midday sunlight will be much closer to white than the more yellow early morning or late afternoon sunlight. This diagram gives rough averages of some typical light sources

White Balance

Normally our eyes compensate for lighting conditions with different color temperatures. A digital camera needs to find a reference point which represents white. It will then calculate all the other colors based on this white point. For instance, if a halogen light illuminates a white wall, the wall will have a yellow cast, while in fact it should be white. So if the camera knows the wall is supposed to be white, it will then compensate all the other colors in the scene accordingly.

Most digital cameras feature automatic white balance whereby the camera looks at the overall color of the image and calculates the best-fit white balance. However these systems are often fooled especially if the scene is dominated by one color, say green, or if there is no natural white present in the scene as show in this example.

Most digital cameras also allow you to choose a white balance manually, typically sunlight, cloudy, fluorescent, incandescent etc. Prosumer and SLR digital cameras allow you to define your own white balance reference. Before making the actual shot, you can focus at an area in the scene which should be white or neutral gray, or at a white or gray target card. The camera will then use this reference when making the actual shot.

Aperture

Aperture refers to the size of the opening in the lens that determines the amount of light falling onto the film or sensor. The size of the opening is controlled by an adjustable diaphragm of overlapping blades similar to the pupils of our eyes. Aperture affects exposure and depth of field.

Just like successive shutterspeeds, successive apertures halve the amount of incoming light. To achieve this, the diaphragm reduces the aperture diameter by a factor 1.4 (square root of 2) so that the aperture surface is halved each successive step.

Because of basic optical principles, the absolute aperture sizes and diameters depend on the focal length. For instance, a 25mm aperture diameter on a 100mm lens has the same effect as a 50mm aperture diameter on a 200mm lens. If you divide the aperture diameter by the focal length, you will arrive at 1/4 in both cases, independent of the focal length. Expressing apertures as fractions of the focal length is more practical for photographers than using absolute aperture sizes. These "relative apertures" are called f-numbers or f-stops. On the lens barrel, the above 1/4 is written as f/4 or F4 or 1:4.

We just learned that the next aperture will have a diameter which is 1.4 times smaller, so the f-stop after f/4 will be f/4 x 1/1.4 or f/5.6. "Stopping down" the lens from f/4 to f/5.6 will halve the amount of incoming light, regardless of the focal length. You now understand the meaning of the f/numbers found on lenses:
F/1.4, F/2. F/2.8 F/4 F/5.6 F/ F/8 F/11 F/16 F/22 F/32 F/45 F/64

Because f-numbers are fractions of the focal length, "higher" f-numbers represent smaller apertures.

Maximum Aperture or Lens Speed

The "maximum aperture" of a lens is also called its "lens speed". Aperture and shutterspeed are interrelated via exposure. A lens with a large maximum aperture (e.g. f/2) is called a "fast" lens because the large aperture allows you to use high (fast) shutterspeeds and still receive sufficient exposure. Such lenses are ideal to shoot moving subjects in low light conditions.

Zoom lenses specify the maximum aperture at both the wide angle and tele ends, e.g. 28-100mm f/3.5-5.6. A specification like 28-100mm f/2.8 implies that the maximum aperture is f/2.8 throughout the zoom range. Such zoom lenses are more expensive and heavy.

Depth of Field

Depth of field (DOF) is a term which refers to the areas of the photograph both in front and behind the main focus point which remain "sharp" (in focus). Depth of field is affected by the aperture, subject distance, focal length, and film or sensor format.

A larger aperture (smaller f-number, e.g. f/2) has a shallow depth of field. Anything behind or in front of the main focus point will appear blurred. A smaller aperture (larger f-number, e.g. f/11) has a greater depth of field. Objects within a certain range behind or in front of the main focus point will also appear sharp.

Exposure

The exposure is the amount of light received by the film or sensor and is determined by how wide you open the lens diaphragm (aperture) and by how long you keep the film or sensor exposed (shutterspeed). The effect an exposure has depends on the sensitivity of the film or sensor.

The exposure generated by an aperture, shutterspeed, and sensitivity combination can be represented by its exposure value "EV". Zero EV is defined by the combination of an aperture of f/1 and a shutterspeed of 1s at ISO 100. Each time you halve the amount of light collected by the sensor (e.g. by doubling shutterspeed or by halving the aperture), the EV will increase by 1. For instance, 6 EV represents half the amount of light as 5 EV. High EVs will be used in bright conditions which require a low amount of light to be collected by the film or sensor to avoid overexposure.

Metering

The metering system in a digital camera measures the amount of light in the scene and calculates the best-fit exposure value based on the metering mode explained below. Automatic exposure is a standard feature in all digital cameras. All you have to do is select the metering mode, point the camera and press the shutter release. Most of the time, this will result in a correct exposure.

The metering method defines which information of the scene is used to calculate the exposure value and how it is determined. Metering modes depend on the camera and the brand, but are mostly variations of the following three types:

Matrix or Evaluative Metering

This is probably the most complex metering mode, offering the best exposure in most circumstances. Essentially, the scene is split up into a matrix of metering zones which are evaluated individually. The overall exposure is based on an algorithm specific to that camera, the details of which are closely guarded by the manufacturer. Often they are based on comparing the measurements to the exposure of typical scenes.

Center-weighted Average Metering

Probably the most common metering method implemented in nearly every digital camera and the default for those digital cameras which don’t offer metering mode selection. This method averages the exposure of the entire frame but gives extra weight to the center and is ideal for portraits.

Spot (Partial) Metering

Spot metering allows you to meter the subject in the center of the frame (or on some cameras at the selected AF point). Only a small area of the whole frame is metered and the exposure of the rest of the frame is ignored. This type of metering is useful for brightly backlit, macro, and moon shots.

Shutterspeed

The shutterspeed determines how long the film or sensor is exposed to light. Normally this is achieved by a mechanical shutter between the lens and the film or sensor which opens and closes for a time period determined by the shutterspeed. For instance, a shutter speed of 1/125s will expose the sensor for 1/125th of a second. Electronic shutters act in a similar way by switching on the light sensitive photodiodes of the sensor for as long as is required by the shutterspeed. Some digital cameras feature both electronic and mechanical shutters.

Shutterspeeds are expressed in fractions of seconds, typically as (approximate) multiples of 1/2, so that each higher shutterspeed halves the exposure by halving the exposure time: 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s, 1/8000s, etc. Long exposure shutterspeeds are expressed in seconds, e.g. 8s, 4s, 2s, 1s.

The optimal shutterspeed depends on the situation. A useful rule of thumb is to shoot with a shutterspeed above 1/(focal length) to avoid blurring due to camera shake. Below that speed a tripod or image stabilization is needed. If you want to "freeze" action, e.g. in sports photography, you will typically need shutterspeeds of 1/250s or more. But not all action shots need high shutterspeeds. For instance, keeping a moving car in the center of the viewfinder by panning your camera at the same speed of the car allows for lower shutterspeeds and has the benefit of creating a background with a motion blur.

Macro

In strict photographic terms, "macro" means the optical ability to produce a 1:1 or higher magnification of an object on the film or sensor. For instance if you photograph a flower with an actual diagonal of 21.6 mm so that it fills the 35mm film frame (43.3mm diagonal), the flower gets magnified with a ratio of 43.3 to 21.6 or 2:1, or with a magnification of 2X. Macro photography typically deals with magnifications between 1:1 and 50:1 (1X to 50X), while close up photography ranges from 1:1 to 1:10 (1X to 1/10X).

From the above it is easy to understand that digital cameras with sensors smaller than 35mm film have better macro capabilities. Indeed, a digital compact camera with a focal length multiplier of 4X can capture the above flower of 21.6mm diameter with a magnification of only 1:2 (close-up) instead of the 2:1 (macro) required with the 35mm camera. In other words, macro results are achieved with (easier) close-up photography.

On digital cameras there is often a Macro Focus mode which switches the auto focus system to attempt to focus on subjects much closer to the lens.

We measure macro ability (of cameras with non-interchangeable lenses) in our reviews as the ability of the lens to get the best possible frame coverage. So a camera which can fill the frame with a subject that is 20mm wide has better macro capabilities than one which can only capture a 40mm wide subject.

Generation after generation, Nikon Coolpix digital cameras delivered the ’best in class’ macro performance without add-on lenses.

Image Stabilization

Higher-end binoculars and zoom or telephoto lenses for SLR cameras often come with image stabilization. It is also available in digital video cameras with large zooms. Digital cameras with large zoom lenses also come with image stabilization or variants such as anti-shake.

Image stabilization helps to steady the image projected back into the camera by the use of a "floating" optical element-often connected to a fast spinning gyroscope-which helps to compensate for high frequency vibration (hand shake for example) at these long focal lengths. Canon EF SLR lenses with image stabilization have a IS suffix after their name, Nikon uses the VR "Vibration Reduction" suffix on their image stabilised Nikkor lenses.

Typically, image stabilization can help you take handheld shots almost two stops slower than with image stabilization off. For example if you would require a shutterspeed of 1/500s to shoot a particular scene, you should be able to shoot at only 1/125s (4 times slower) with image stabilization. This is very useful when shooting moving subjects in low light conditions by panning and/or when using long focal lengths.

Important footnote: The above "optical" image stabilization is different from the "digital" image stabilization found in some digital video cameras. "Digital" image stabilization only makes sense for digital video as it pixel shifts the image frames to create a more stable video image.

Picture Angle

The field of view is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically. Because the aspect ratio differs between formats, the more universal picture angle, measured along the diagonal of the scene is often used. A shorter focal length (such as a 28mm wide angle) produces a wider picture angle, while a longer focal length (such as a 200mm tele) produces a narrower picture angle. In 35mm photography, a 50mm lens is called a normal lens because it produces roughly the same picture angle as the human eye (about 46°).

Focal Length

The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the image is "in focus". The camera lens projects part of the scene onto the film or sensor. The field of view (FOV) is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically. Larger sensors or films have wider FOVs and can capture more of the scene. The FOV associated with a focal length is usually based on the 35mm film photography, given the popularity of this format over other formats.

Wide angle lenses (short focal length) capture more because they have a wider picture angle, while tele lenses (long focal length) have a narrower picture angle. Below are some typical focal lengths:

Typical focal lengths and their 35mm format designations:
< 20mm Super Wide Angle
24mm - 35mm Wide Angle
50mm Normal Lens
80mm - 300mm Tele
> 300mm Super Tele

A change in focal length allows you to come closer to the subject or to move away from it and has therefore an indirect effect on perspective. Some digital cameras suffer from barrel distortion at the wide angle end and from pincushion distortion at the tele end of their zoom ranges.

Optical Zoom (X times zoom)
Optical zoom = maximum focal lenght / minimum focal length

For instance, the optical zoom of a 28-280mm zoom lens is 280mm/28mm or 10X. This means that the size of a subject projected on the film or sensor surface will be ten times larger at maximum tele (280mm) than at maximum wide angle (28mm). Optical zoom should not be confused with digital zoom.

Now that are clear most of the camera related terms there is one more step before shooting some great shots with your camera. In part 3 there are few lightning and composition tips (let’s call them - basic).


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