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Lens Auto
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Aberration.

The inability of a lens to produce a true image, particularly at the edge of a photograph.

Usually, the more expensive the lens, the better its optical quality and the fewer aberrations.

Angle of View

The area of a scene that a lens can cover. The focal length of the lens determines the angle of view. A wide-angle (short-focal-length) lens includes more of a scene than a standard (normal-focal-length) lens or telephoto (long-focal-length) lens. Angle of view is basically the angle at which light rays can pass through the lens to produce an image on the film.

Aperture

The aperture is the opening formed by the blades of the iris or diaphragm in the lens, through which light passes to expose the film. Aperture size is usually given in f-numbers, the larger the number, the smaller the opening. Aperture size together with shutter speed determine the amount of light falling on the film (exposure). The aperture is sometimes called the "stop".

Aspherical lens

A lens with a curved, non-spherical surface. Used to reduce aberrations and achieve a more compact lens size. With a spherical lens, rays travelling from the lens periphery create the image before the ideal focal point and give a blurred image centre. With an aspherical lens, even the rays travelling from the lens periphery converge at the ideal focal point, thus producing a sharp image.

Chromatic aberration

The inability of a lens to bring all light wavelengths (particularly red & blue) into the same plane of focus, thus causing overall blur. Usually found in regular large-aperture telephoto and super-telephoto lenses. Not improved by reducing aperture size. Can be corrected with low dispersion (ED, LD SD) glass.

Colour temperature

A method of expressing the colour content and quality of light and measured in Kelvin (K). "Photographic daylight" has a colour temperature of about 5500K. Photographic tungsten lights have colour temperatures of 3200K to 3400K depending on their construction.

Depth of Field

The distance between the nearest and furthest objects in a photograph that are considered to be acceptably sharp. Dependant on aperture, focal length and focused distance. The smaller the aperture, the wider the lens and the further the focused distance, giving a greater depth of field and vice versa.

Electronic flash

Designed to provide light where the lighting on the scene is insufficient. Electronic flash requires high voltage, usually obtained through batteries and a voltage-multiplying circuit which discharge a brief, intensive burst. Generally considered to have the same photographic effect as daylight. Modern flash units have multiple TTL exposure control functions and auto focus control.

F-numbers or F-stops

Numbers on the lens aperture ring and the camera's LCD (where applicable) that indicate the size of lens aperture. The lower the number the larger the aperture. As the scale rises, each number is multiplied by a factor of 1.4. Standard numbers are 1.0,1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc., each change resulting in a doubling or halving of the amount of light transmitted by the lens to the film.

Film Speed

ISO stands for International Standards Organization and numbers such as ISO 100 or ISO 400 etc. give the sensitivity of film to light. The higher the number, the more sensitive or faster the film. Basically, the slower the film (low ISO No.) the sharper and clearer the photograph. Grainy effects can be achieved with fast films (high ISO No.).

Flash sync speed

Exposure time with a focal-plane shutter is measured from the moment the first curtain is released until the moment the second curtain is released. The instant the first curtain closes, the electrical contacts for X sync close and instantly fire the flash.

Focal Length

The distance from the film to the optical centre of the lens when the lens is focused on infinity. Focal length on most adjustable cameras is marked in millimetres on the lens mount. On 35mm-format cameras, lenses with a focal length of 50mm are called normal or standard lenses. Lenses of 35mm or less are called wide angle lenses and lenses of 85mm or more are called telephoto lenses. Lenses which allow varying focal lengths without changing focus are called zoom lenses.

Lens

One or more pieces of optical glass or similar material designed to collect and transfer rays of light to form a sharp image on film, paper or a projection screen. In practical photography, compound lenses made of a number of elements of different types of glass are used. This enables the manufacturer to correct most of the faults (aberrations) found in simple lenses and provide images that are sharp across the whole picture.

Lens Speed

The largest aperture(smallest F-stop) at which a lens can be set. Fast lenses transmit more light and have larger openings than slow lenses. Determined by the maximum aperture in relation to focal length. Lens speed is relative: a 400 mm lens with a maximum aperture of F/3.5 is considered extremely fast, while a 28mm F/3.5 lens is considered to be quite slow.

Perspective

Perspective is a two-dimensional representation of a three-dimensional scene. In photography this can be achieved by viewing 3-D objects from an angle rather than head-on. A photograph is also given perspective if there are objects in the foreground, middle distance and background, giving the whole scene "depth".

Single-Lens-Reflex (SLR) Camera

Light entering the camera through the lens is reflected up by a mirror behind the lens onto a ground glass screen above. This screen is viewed through the viewfinder and a glass pentaprism which turns the image the correct way up. Other camera functions such as light metering and flash control also operate through the camera lens.

Zoom Lens

A lens which can be adjusted to a wide range of focal lengths without a change in focus, thus an alternative for a number of individual lenses of various focal lengths. A difficult type of lens to design and manufacture, but very useful for the photographer who likes to travel light.

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Auto and Manual Focus Modes in Digital Cameras

One of the important phases of shooting a good photo is focusing on the photo's objects. Photos that are out of focus look bad and blurry. Sometimes shooting out of focus objects is done on purpose as an artistic effect but in most cases a sharp focused object is the photographer goal.

Most low-end pocket digital cameras do all the focus work for you. These are literally "point and shoot" cameras. Furthermore even if you wanted to manually focus using such cameras it would have been practically impossible since they do not include a real optical viewfinder. Using the LCD to check for focus is useless as the LCD is not good enough to truly let you know if an object is perfectly focused or not.

Digital SLR cameras on the other hand have an optical viewfinder that allows you to truly see the photo through the camera's lens. With digital SLR cameras you can see how well the objects in the photo are focused and correct the focus as needed.

Digital cameras support at least three focus modes:

Manual focus: In this mode the camera does not automatically focus on the objects in the photo. You will have to manually set the focus as desired. This is accomplished by either pushing some buttons that in turn change the focus or more commonly by rotating a focus ring on the lens clockwise or counterclockwise as needed. You can see the results of changing the focus by looking through the viewfinder. When you are happy with the focus you can press the shutter button to take the photo.

Single auto focus: In this mode the camera automatically focuses on the objects in the photo. The camera automatically focuses when you press the shutter button either all the way down to shoot a photo or half way down to lock the focus. In this mode the automatic focus process is carried out only once as soon as the shutter button is pressed. Once the focus is achieved it is locked and it is not changed until the next time the shutter button is pressed. If you keep the shutter button pressed half way down and then the camera moves or the object moves the focus might be lost. This mode is useful when shooting static objects. In most cases you will press the shutter button and hold it half way down. The camera will run the auto focus process. It will let you know when focus is achieved by displaying a green icon and by playing a short beep.

Continuous auto focus: In this mode the camera continuously focuses on the objects in the photo. Once the shutter button is pressed and as long as it is held half way down the camera continuously focuses on the objects in the photo. In this mode the camera continuously corrects the focus as the objects distance from the camera changes. This mode is useful when you shoot photos of moving objects such as a race car during a race or airplanes during an air show. You can hold the shutter button half way down and continuously move the camera to follow the object. The camera will continuously keep the object in focus. When you are ready to shoot the photo simply press the shutter button all the way down. One drawback of this mode is high power consumption as the camera continuously corrects the focus it uses the power hungry motors in the lens in order to move the optical components.

Like any other feature automatic and manual focus modes have their pros and cons. The first step to using them to your advantage is to understand how they work and what they were designed for. The next step is to experiment shoot photos using different focus modes and different types of objects and see how the camera behaves. Once you have done that you will be ready to instinctively use the best focus mode for each photo situation.

About the Author

Ziv Haparnas is an expert technology writer. Mr. Haparnas writes about practical technology issues. This article can be reprinted only if the resource box including the backlink is included. You can find more information on digital photo printing and photography in general on printrates.com - a site dedicated to digital printing

Nikon D300 SLR Auto Lens?

I have this auto and manual swithch on my digital lens is it bad for it to be left on auto when closed?
same as the VR (ON AND OFF)

Not at all. If you are not pressing the shutter button, the lens is not doing anything and therefore not using any battery power. That would be a factor for the VR switch, but for the A/M switch, it is a non-issue anyway.

Canon EOS 550D: Moving up the DSLR chain
The Canon EOS 550D is a tempting proposition if you are looking to move on from an entry-level DSLR to bigger things.

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