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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.

Exposure triangle

Auto Bracketing

June 7th, 2011

Andrei

Autobracketing is a feature of some more advanced cameras, particularly single-lens reflex cameras (SLR), where the camera will take several successive shots (usually three) with slightly different settings. Later, the best-looking pictures can be picked from the batch. When the photographer achieves the same result by changing the camera settings between each shot, this is simply called bracketing.

Auto Bracketing

Aperture Priority

May 23rd, 2011

Andrei

Aperture priority, often abbreviated Av (for Aperture value) or A on a camera mode dial, is a setting that allows the user to choose a specific aperture value while the camera selects a shutter speed to match. The camera will ensure proper exposure. This is different from manual mode, where the user must decide both values, shutter priority where the user picks a shutter speed with the camera selecting the aperture to match, or program mode where the camera selects both.

Aperture Priority Selector

Aperture

May 16th, 2011

Andrei

Aperture refers to the size of the opening in the lens that determines the amount of light falling onto the 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.

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.

Aperture values

AE Lock

May 14th, 2011

Andrei

Auto Exposure Lock (sometimes called AEL) is an excellent method of gaining added control over exposure, without losing the speed and convenience of automation. In fact, since most photographers today use some form of auto exposure the majority of the time, an understanding of how AE Lock operates can add a new dimension to their photography. This function offers something close to manual control over the exposure.

AE Lock Button on Canon

RAW Images

March 23rd, 2011

Andrei

A RAW file contains the original image information as it comes off the sensor. No in-camera processing is applied. This will be done afterwards on your PC with special software.

TIFF file only retains 8 bits per channel of information but it will take up twice the storage space because it has three 8 bit color channels compared to one 12 bit RAW channel. JPEG resolve this issue by compression, at the cost of image quality. So RAW offers the best of both as it preserves the original color bit depth and image quality and saves storage space compared to TIFF. Some cameras offer nearly lossless compressed RAW.

JPEG

March 21st, 2011

Andrei

The most commonly used digital image format is JPEG. It is universally compatible with viewers browsers, and image editing software, it allows photographic images to be compressed up to 10 to 20 times compared to the uncompressed original with very little visible loss in image quality.

High and Low Quality JPEG

Interpolation

March 7th, 2011

Andrei

Interpolation is an imaging method to increase or decrease the number of pixels in a digital image. Many compact cameras uses interpolation to produce a larger image than the sensor captured or to create digital zoom. Almost all image editing software support one or more methods of interpolation, the quality of the final image and the amount of jaggies depends on the complexity of the algorithm.

Interpolation

Histogram

March 4th, 2011

Andrei

As we’ve talked in the previous topics about sensors and pixels, each photo is composed of a number of pixels. Each of these pixels have a certain level of brightness, ranging from 0 (black) to 255 (white). A histogram is a graphical representation of these levels of brightness for the entire photo.