Auto focus/Manual focus switch mechanism on the focus ring
The ability to switch quickly between auto focus (AF) and manual focus (MF) is a great asset in many types of shooting, especially sports and nature photography. This is easily accomplished with Tamron’s ingenious AF/MF system. By simply sliding the focus ring back and forth between the two positions you can select the AF or MF focusing position. Tamron lenses provide smooth, precise manual focusing in the manual focus mode by turning their wide, welltextured focusing rings.
Current lenses with this mechanism:
In photography an aberration is understood to be an image defect resulting from the divergence between the actual object picture and the ideal optical picture.
- chromatic Aberration (the variation of a lens's refractive index with wavelength)
- Spherical Aberration
- Field curvature
Analogue photography refers to conventional photography based on photosensitive films rather than sensors as in digital photography.
In contrast to conventional photography, digital photography uses electronic photosensitive sensors. These store an image in the form of an electric charge which is subsequently processed by means of a computer processor.
Modern CCD Sensor
The angle of view describes the angle with which lenses capture an image with sufficiently low image defects.
In the diagram below the angle of view is α. For the angle α/2, tan α/2 = d / f
d = focal length for the corner
f= focal length
Example: If the focal length is equal to the format diagonal, then tan α = 0.5 and therefore the angle is 53 °. This is a standard lens for the respective format.
Lenses can be roughly sorted into the following categories:
Telephoto lenses α < 20 °
long focal length 20 ° < α < 40 °
Normal lens 40 ° < α < 55 °
Wide-angle lens α > 55 °
wide-angle lens Telezoom
The evaluation of a lens can only occur when taking into account the format. For example, a normal lens for a certain format can also be used as a long focal length lens for a smaller format. This is particularly relevant in digital photography, because here sensors are used which are smaller than the 35mm format. Thus, for example, the angle of view of a focal length of 300mm with 24x36mm negative film corresponds to an angle of view of 480mm with a sensor of the size 15,2x22,7mm!
The equivalent focal length for 35mm would be apx. 1.6x longer, because the diagonal format is also apx. 1.6x longer. It seems as though the focal length is longer, but in reality merely the angle of view with small digital (APS C) sensors becomes smaller.
The following focal length comparison was shot using a 28mm focal length setting and from the same position. The first picture was taken with a full format sensor and the second with an APS C sized sensor. The angle of view of the image shot with the APS C sensor corresponds to a focal length of apx. 50mm when converted to full format (although the focal length has physically remained constant).
28mm full format
28mm APS-C Sensor (full format equivalent angle of view is apx. 50mm)
Anomalous Dispersion glass is a special optical glass that delivers an abnormally large partial dispersion ratio (amount of dispersion at a given wavelength range within visible light) relative to a specific wavelength zone. By combining AD glass elements with elements made of normal glass with different dispersion characteristics, dispersion factors of a specific wavelength can be controlled, resulting in effective compensation of on-axis chromatic aberration on telephoto lenses, or lateral chromatic aberration often associated with wide-angle lenses of conventional optical configuration.
The difference in partial dispersion factors between normal
optical glass and AD glass elements (schematic diagram).
Current Tamron lenses with AD glass:
- AF 18-250mm F/3,5-6,3 Di II LD Aspherical [ IF] MACRO (Model A18)
- AF 18-270mm F/3,5 -6,3 Di II VC LD Aspherical [ IF] Macro (Model B003)
- AF 28-300mm F/3,5-6,3 XR Di LD Aspherical [ IF] MACRO (Model A061)
- AF 28-300mm F/3,5-6,3 XR Di VC LD Aspherical [ IF] MACRO (Model A20)
The aperture (derived from the Latin word aperire meaning "open") is a device which determines the width of the path for incoming light. The opening is mostly often adjusted with the help of overlapping diaphragm blades which are aligned in a circular shape. The more these blades are moved into one another, the narrower the resulting light path and the less light can enter the system and vice-versa.
The aperture controls the level of lighting and together with the exposure time the exposure of the film or chip is regulated. Apart from the exposure time and the viewfinder the aperture is an important technical means of photographic creativity because it influences the depth of field.
The aperture is expressed as a number without dimension derived by the relation between focal length and the width of the lens opening (more exactly: the focal length divided by the diameter of the entrance pupil). The luminosity of the lens corresponds to the smallest f number, so of the biggest relative opening. The f number of mechanical cameras is adjusted on the lens ring, as opposed to electronic cameras where the adjustment is made per control elements on the camera body.
APS-C was originally a film format of the size 25,1x16,7mm (the crop format of the original negative format APS). Nowadays it is often given as a dimension format for image sensors of digital SLRs, because these have a similar scale. Tamron has a series of lenses designed exclusively for this format called Di II.
The green field in the following picture shows the size of an APS-C sensor in comparison to a 35mm conventional picture film (full-format sensors have the same size as 35mm). The white circle marks the image circle of Tamron Di II lenses.
ASL lenses use one or more aspherical lenses.
The following Tamron lenses have aspherical elements:
Di II Lenses
SP AF10-24mm F/3.5-4.5 Di II LD Aspherical [ IF](Model B001)
SP AF 17-50mm F/2,8 XR Di II LD Aspherical [ IF] (Model A16)
SP AF 17-50mm F/2,8 XR Di II VC LD Aspherical [ IF] (Model B005)
AF 18-200mm F/3,5-6,3 XR Di II LD Aspherical [ IF] MACRO (Model A14)
AF 18-250mm F/3,5-6,3 Di II LD Aspherical [ IF] MACRO (Model A18)
AF 18-270mm F/3,5 -6,3 Di II VC LD Aspherical [ IF] Macro (Model B003)
SP AF 28-75mm F/2,8 XR Di LD Aspherical [ IF] MACRO (Model A09)
AF 28-200mm F/3,8-5,6 XR Di Aspherical [ IF] MACRO (Model A031)
AF 28-300mm F/3,5-6,3 XR Di LD Aspherical [ IF] MACRO (Model A061)
AF 28-300mm F/3,5-6,3 XR Di VC LD Aspherical [ IF] MACRO (Model A20)
As a rule conventional lens elements have a constant radius (spherical), i.e. one can imagine that they are cut from a ball (sphere). This simple lens form, however, has disadvantages in terms of image defects. Thanks to innovative production technologies the trend is increasingly going over to the use of aspherical lenses. These lenses have, a surface which deviates from the ball form. Especially the correction of spherical aberration and coma can be strongly improved with these asphericals. Moreover, the distortion is compensated by means of aspherical lenses.
ASL lenses use one or more aspherical lenses.
The aspherical is manufactured by compounding a special plastic on the glass surface of the element. Tamron has perfected this cutting-edge technology for serial production and has integrated aspherical elements in a number of lenses, particularly high-power-zooms.
Tamron Aspherical Lens elements
Schematic diagram, compensation effect with an aspherical element
Astigmatism is a form of aberration in photography. The term is derived from Greek where "a" means without and "stigma" means spot. When incident beams of light fall in at an angle and should meet at a point on the image plane but are distorted to (elliptical) discs.
The reason lies in the different focal lengths between the so-called meridional and the sagittal ray bundle.
Astigmatism is minimized in Tamron lenses through the use of aspherical lens elements and the skilful arrangement and combination of glass elements and their deflection properties as well as an optimized aperture.
Available Light means refers to photography without any additional lighting i.e. a flash in spite of unfavorable lighting conditions (e.g., at dusk or indoors). Instead, the camera sensitivity/speed is increased or lenses with with image stabilizer are used ( i.e., Tamron
As shown in the two pictures below, the atmosphere of an image can change considerably or can even be destroyed completely if artificial lighting is used.
The left picture was photographed with a flash, the right without.