Monday, December 18, 2006

Digital SLR optical systems glossary

Most DSLR cameras are able to use conventional 35mm lenses. However, such lenses are designed to create an image circle that covers a 35mm film frame and are therefore larger and heavier than necessary for sensors which are smaller than a 35mm film frame. "Digital" lenses as Canon Short Back Focus Lenses, Nikon DX Lenses, and Olympus 4/3" System are lighter because their image circles only cover the sensor area.
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 subject is "in focus". The camera lens projects part of the scene onto the film or sensor. 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. Larger sensors or films have wider field of views and can capture more of the scene. It’s associated with a focal length is usually based on the 35mm film photography, given the popularity of this format over other formats.
Focal lengths of digital cameras with a sensor smaller than the surface of a 35mm film can be converted to their 35mm equivalent using the focal length multiplier.
Many DSLR cameras have sensors smaller than the sensitive area of 35mm film. Typically the sensor diagonal is 1.5 times smaller than the diagonal of 35mm film. As a consequence, a sensor smaller than a 35mm film frame captures only the middle portion of the information projected by the lens into the 35mm film frame area, resulting in a "cropped field of view". A 35mm film camera would require a lens with a longer focal length to achieve the same field of view. The Focal Length Multiplier is equal to the diagonal of 35mm film (43.3mm) divided by the diagonal of the sensor. Let's now discuss two cases. So a 200mm lens on a digital SLR with FLM of 1.5X will have the field of view of a 300mm lens on a 35mm film camera which would be heavier and more expensive. Also, because the 35mm equivalent field of views is achieved with shorter focal lengths, depth of field is larger. This advantage on the tele end becomes a disadvantage on the wide ange end. For instance, a 19mm lens fitted onto a digital SLR with FLM of 1.5X will only generate the field of view of a 28mm lens fitted on a 35mm film camera.
Optical zoom is a division of maximum focal length on 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).
Zoom lenses, especially the lower end ones, can sometimes suffer from vignetting. The barrel or sides of the lens become visible, resulting in dark corners in the image as shown in this example. The use of converters can also result in vignetting.
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°).
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.
Prosumer cameras typically allow the zoom range to be extended via converters. Converters are add-on lens adapters which expand the picture angle or make it narrower. For instance, fitting a 0.8X wide angle converter on a 35mm lens will result in a 28mm picture angle. A 2.0X telephoto converter on a 100mm lens will give the picture angle of a 200mm lens. Converters often cannot be used across the whole range of a zoom lens and sometimes only at the end of the zoom range because they would introduce vignetting. Also, the internal flash may no longer work properly because the converter will cast a shadow and the flash sensor is covered by the converter.
Higher-end binoculars and zoom or telephoto lenses for SLR cameras often has the 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. It makes the CCD move so that it compensates for the camera movement as implemented in the Sony A-100. The sensor is mounted onto a platform which moves in the opposite way as the movement of the camera, which is determined by motion detectors. According to Sony and Konica Minolta, this "anti-shake" system gives you an additional 3 stops. For example if you would require a shutter speed of 1/1000s to shoot a particular scene, you should be able to shoot at only 1/125s (8 times slower) with anti-shake enabled. This is very useful when shooting moving subjects in low light conditions by panning or when using long focal lengths.
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 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 stabilized Nikkor lenses.
The width divided by the height of an image or aspect ratio is usually expressed as two integers, for example width/height = 1.5 is expressed as width to height in the ratio of 3:2.
Barrel distortion is a lens effect which causes images to be spherical or "inflated". Barrel distortion is associated with wide angle lenses and typically occurs at the wide end of a zoom lens. The use of converters often amplifies the effect. It is most visible in images with perfectly straight lines, especially when they are close to the edge of the image frame.
Chromatic aberration or "color fringing" is caused by the camera lens not focusing different wavelengths of light onto the exact same focal plane (the focal length for different wavelengths is different) and by the lens magnifying different wavelengths differently. The amount of chromatic aberration depends on the dispersion of the glass. Chromatic aberration is visible as color fringing around contrast edges and occurs more frequently around the edges of the image frame in wide angle shots. Special lens systems using two or more pieces of glass with different refractive indexes can reduce or eliminate this problem.
The term Circle of Confusion usually brings up around people's eyes. But this does not need to be the case as it is actually rather simple. Depth of field defines the distance range where things have an acceptable level of sharpness. Although sharpness is very subjective, it is in general based on an 8" x 10" print viewed from a one feet distance.
Depth of field is a term which refers to the areas of the photograph both in front and behind the main focus point which remains "sharp" in focus. It is affected by the aperture, subject distance, focal length, and film or sensor format.
If you photograph a subject with a tele lens and want it to have the same size on the film or 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 example below.
Pincushion distortion is a lens effect which causes images to be pinched at their center. It is associated with tele lenses and typically occurs at the tele end of a zoom lens. The use of converters often amplifies the effect. It is most visible in images with perfectly straight lines, especially when they are close to the edge of the image frame.
Subject distance is the distance between the camera lens and the main subject. Varying the subject distance will change perspective. Also, varying the subject distance with the same aperture will produce a different depth of field.
Digital compact cameras are fitted with lenses with short focal lengths to create 35mm equivalent field of views on their small sensor surfaces. Typically the sensor diagonal is 4 times smaller than the diameter of 35mm film. A 7mm lens fitted on such a camera will have the same field of view of a 7mm x 4 or 28mm lens on a 35mm film camera. Just like the digital lenses for digital SLRs, these lenses are designed to generate image circles to cover the smaller sensor. This allows these lenses to be much smaller and cheaper to manufacture. Because of the very small focal lengths used, the depth of field is much larger than digital SLRs or 35mm film cameras with the same field of view.
Most digital compact cameras have non-interchangeable zoom lenses which have been designed to work with a specific sensor size. Some prosumer models allow extending the zoom range via converters. Because of the small sensor sizes, the lenses used in digital compact cameras have to be of much higher optical quality than glass which would be "acceptable" on a 35mm camera. This is less of an issue with digital SLRs with because their sensors are much larger.


vlas1k said...

In my opinion Canon EF 50mm F 1.8 is good alternative to a kit lens in price and quality.

fototramp said...

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vlas1k said...

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