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Lens Vignetting

January 25th, 2012

Andrei

In photography, lens vignetting is a reduction of an image’s brightness or saturation at the corners compared to the image center. The word vignette originally referred to a decorative border in a book. Later, the word came to be used for a photographic portrait which is clear in the center, and fades off at the edges.

Lens Vignetting

Lens vignetting is often an undesired effect caused by camera settings or lens limitations. However, it is sometimes purposely introduced for creative effect, such as to draw attention to the center of the frame. For this purpose vignetting filter can be used. A photographer may deliberately choose a lens which is known to produce vignetting to obtain the effect, or it may be introduced with the use of special filters or post-processing procedures.

Camera Field of View

January 10th, 2012

Andrei

The camera field of view is determined by the angle of view from the lens out to the scene and can be measured vertically or horizontally. Because the aspect ratio differs between formats, the more universal camera field of view is calculated along the diagonal of the scene. 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 (35mm camera sensor), a 50mm lens is called a normal lens because it produces approximately the same picture angle as the human eye (about 46°).

24mm field of view

70mm field of view

Perspective In Photography

January 7th, 2012

Andrei

If you photograph a subject with a tele lens and want it to have the same size on the sensor when photographing it with a wide angle lens, you would have to move closer to the subject. Because this would cause the perspective to change, lenses with different focal lengths are said to have a different perspective. Note however that changing the focal length without changing the subject distance will not change perspective, as shown in the next example.

A. Taken with a 24mm wide angle.

Macro

December 18th, 2011

Andrei

In photographic terms, “macro” means the optical ability to produce a 1:1 or higher magnification of an object on the sensor. For instance if you photograph a aquarium shrimp with an actual size of 21.6 mm so that it fills the 35mm sensor (43.3mm diagonal), the shrimp 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).

Macro photography (click to enlarge)

Distortion

September 10th, 2011

Andrei

Distortion is a deviation from rectilinear projection, a projection in which straight lines in a scene remain straight in an image. It is a form of optical aberration. Although distortion can be irregular or follow many patterns, the most commonly encountered distortions are radially symmetric, or approximately so, arising from the symmetry of a photographic lens. The radial distortion can usually be classified as one of two main types:

Distortion

Image Stabilization

September 3rd, 2011

Andrei

Image stabilization (IS) is a family of techniques used to reduce blurring associated with the motion of a camera during exposure. Specifically, it compensates for pan and tilt (angular movement, equivalent to yaw and pitch) of a camera or other imaging device. It is used in image-stabilized binoculars, still and video cameras, and astronomical telescopes. With still cameras, camera shake is particularly problematic at slow shutter speeds or with long focal length (telephoto) lenses.

Image Stabilization

Application in still photography

In photography, image stabilization can often permit the use of shutter speeds 2–4 stops slower (exposures 4–16 times longer), although even slower effective speeds have been reported. The rule of thumb to determine the slowest shutter speed possible for hand-holding without noticeable blur due to camera shake is to take the reciprocal of the 35mm equivalent focal length of the lens. For example, at a focal length of 125 mm on a 35mm camera, vibration or camera shake could affect sharpness if the shutter speed was slower than 1/125 second. As a result of the 3–4 stops slower shutter speeds allowed by IS, an image taken at 1/125 second speed with an ordinary lens could be taken at 1/15 or 1/8 second with an IS-equipped lens and produce almost the same quality. The sharpness obtainable at a given speed can increase dramatically. When calculating the effective focal length, it is important to take into account the image format a camera uses. For example, many digital SLR cameras use an image sensor that is 2/3, 5/8, or 1/2 the size of a 35mm film frame. This means that the 35 mm frame is 1.5, 1.6, or 2 times the size of the digital sensor. The latter values are referred to as the crop factor, field-of-view crop factor, focal-length multiplier, or format factor. On a 2x crop factor camera, for instance, a 50mm lens produces the same field of view as a 100mm lens used on a 35mm film camera, and can typically be handheld at 1/100 of a second.

Shutterspeed

August 25th, 2011

Andrei

The shutterspeed determines how long the sensor is exposed to light. Normally this is achieved by a mechanical shutter between the lens and the 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.

Shutterspeed

Metering

August 22nd, 2011

Andrei

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.

Metering modes

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:

Exposure Compensation

August 10th, 2011

Andrei

The camera’s metering system will sometimes determine the wrong exposure value needed to correctly expose the image. This can be corrected by the “EV Compensation” feature found in prosumer and professional cameras. Typically the EV compensation ranges from -2.0 EV to +2.0 EV with adjustments in steps of 0.5 or 0.3 EV. Some digital SLRs have wider EV compensation ranges, e.g. from -5.0 EV to +5.0 EV.

It is important to understand that increasing the EV compensation by 1 is equivalent to reducing EV by 1 and will therefore double the amount of light. For instance if the camera’s automatic mode determined you should be using an aperture of f/8 and a shutterspeed of 1/125s at ISO 100 (13 EV) and the resulting image appears underexposed (e.g. by looking at the histogram), applying a +1.0 EV exposure compensation will cause the camera to use a shutterspeed of 1/60s or an aperture of f/5.6 to allow for more light (12 EV).

Of course, as you become more familiar with your camera’s metering system, you can already apply an EV compensation before the shooting. For instance if your camera tends to clip highlights and you are shooting a scene with bright clouds, you may want to set the EV compensation to -0.3 or -0.7 EV.

Exposure

August 7th, 2011

Andrei

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

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.