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What is Focal length?
Focal length is the distance between a lens’ nodal point and the camera's image plane (in millimeters). Its numerical value offers a sense of a lens’ angle-of-view characteristics (broad for wide angle or narrow for telephoto.) A focal length approximating the diagonal dimension of the camera’s image plane will render an angle-of-view with negligible magnification—similar to normal human vision. Focal lengths numerically lower than normal will render negative magnification, resulting in wider angles-of-view (wide angle), while those numerically greater than normal render positive magnification, producing narrower angles-of-view (telephoto.) Zoom lens focal lengths are expressed as a range in a pair of focal length values, leading with the numerically smaller of the two values, e.g., 18-200 mm. However, depending on the format of the camera to which the lens is attached, the relationship between focal length and angle-of-view can vary.
Considering camera format. An individual 35mm film segment is approximately 36 mm x 24 mm, as is a full-frame DSLR’s sensor. Both measure approximately 43.5 mm* diagonally. Thus, lenses ranging from 45 to 50mm and attached to these camera formats are considered normal. A typical APS-C digital SLR’s sensor is approximately 25.1 mm x 16.7 mm and 30.5 mm diagonally, requiring a 30mm lens to produce a normal angle-of-view. This differential is commonly referred to as crop factor—a multiplier that can be applied to a lens’ actual focal length to help gauge the effective focal length equivalent of lens attached to a camera with an image plane smaller than 36 mm x 24 mm. For example, an APS-C DSLR with a 300 mm lens attached will render an angle of view approximating that of a 465 mm lens on a 35mm film camera or full-frame DSLR.
*Focal lengths, image plane dimensions and calculations are approximate and for example purposes only.
Angle of View (Telephoto & Wideangle Photographic Lens)
The area size captured by a photographic lens can be expressed as a diagonal angular field called Angle of View. Generally speaking, a focal length range that provides a similar perspective to the human eye is considered to be somewhere between 40-60mm. With this established as a standard focal length, those with shorter focal lengths are called "wideangle" and those with longer focal lengths are called"telephoto". The shorter the focal length becomes, the wider the angle of view (wideangle), while the longer it becomes, the narrower the angle of view (telephoto).
* The relationship between focal length and angle of view is generally consistent regardless of the lens' focal length. However, there are cases where different angles of view are observed depending on the difference in the focusing system of a lens and shooting distance.
Perspective (Sense of distance)
A photographic lens provides a visual effect, making closely located subjects larger while remotely located subjects smaller. As the focal length becomes shorter in a wideangle lens, this perspective difference expands making closely located subjects even bigger and remotely located ones even smaller (exaggerated perspective).
In contrast, in a telephoto lens, as focal lengths become longer, less difference is observed between close and distant subjects, making it appear as if they are closer regardless of the distance between them(compressed perspective).
Depth of Field
When focused on a subject, there are areas in front of and behind the main subject where details are sharp. This area is referred to as depth of field. When the sharp image area is narrow, it is expressed as "shallow depth of field". When it is wide, it is expressed as "deep depth of field". The depth of field becomes shallower as the lens aperture goes toward a full open position (or the faster the lens' maximum aperture becomes). It becomes deeper when the aperture gets closer to the fully stopped down position (or the slower the lens' maximum aperture becomes). Also, a wideangle lens delivers deeper depth of field compared with a telephoto lens.
Zoom Lens vs. Fixed Focal Lens
Zoom lenses offer versatility
A zoom lens allows continuous shift of focal length without shifting the focus point. Because it delivers various angles of view, one lens can serve as multiples of fixed focal length lenses. Tamron offers a wide array of zoom lenses covering virtually all of the focal lengths required for normal shooting conditions, including those with fast apertures.
Standard Range Zoom
Standard zoom (28-80mm)
Telephoto zoom (80-210mm, 70-300mm, etc.)
Advanced Range Zoom
Wideangle zoom (10-24mm)
High-image-quality telephoto zoom (70-300mm)
Ultra telephoto zoom (200-500mm)
Standard zoom (28-105mm)
Ultra-high-power zoom (24-135mm, 28-200mm,
Wideangle zoom (17-35mm F/2.8-4)
High power zoom (28-75mm F/2.8, 28-105mm/2.8)
Telephoto zoom (70-210mm/2.8)
High image quality and compact fixed focal lens
A fixed focal length lens can deliver outstanding image quality and dimensional compactness because it can be designed with specific photographic needs and applications in mind. Tamron offers a line of fixed focal lens models applying the sophisticated technologies developed for making zoom lenses of various kinds.
Ultra wideangle lens (17mm*)
Wideangle lens (24mm*)
Macro lens (90mm/2.8 1:1)
Fast-aperture telephoto lens (300mm/2.8)
Catadioptric lens (500mm/8*)
* Available in manual focus model only
Macro Photography (Close-up photography)
Features of Macro lens
A macro lens is designed to capture a tiny subject as a bigger image, while suppressing the aberrations that tend to be more noticeable in closer focusing distances.
Macro ratio (Magnification ratio)
A macro magnification is expressed in 1: x, which is a ratio of the actual size of a subject  to the size of the subject image reproduced on the film plane [1/ x]. Therefore, the larger the x value becomes, the smaller the reproduced image on the film plane. For example, an image of a coin reproduced on film as the same size as the actual coin is 1:1 macro, while the same image reproduced at 1/2 of the original size is 1:2 macro. The macro ratio is also referred to as magnification ratio, and the maximum ratio of a lens' reproduction capability is designated as "maximum magnification ratio".
Macro photography with a zoom lens
As mentioned above, macro photography is a specific type of shooting that magnifies the size of a subject. Therefore, the same effect can be achieved by using a zoom lens or a telephoto lens to place some distance from the subject as long as the lens offers a sufficient magnification ratio. On Tamron lenses, a macro shoot-ing range is indicated on the lens barrel corresponding to the range that provides a magnification ratio of 1:4 or larger.
* This indication is limited to the lenses designated as "MACRO" on the barrel.
Technologies to attain the highest level of image quality
To suppress reflections and dispersion on the lens element surface that result in reduced light transmission or cause flare and ghost images, Tamron developed a proprietary BBAR (Broad-Band, Anti-Reflection) multiple-layer coating technique that also renders the best possible color balance. It is applied to a majority of Tamron lenses. A new BBAR coating, which successfully increases transmission of both longer and shorter wavelength ranges, has been developed and is applied to latest lenses, including the 28-200mm Super.
Most Tamron lenses, excluding just a few models, are supplied with a lens hood as a standard accessory. The Tamron lens hood is designed as an integral part of the optics to provide the maximum shading for each lens. This is the case even for those made for a zoom lens, where the wide-end of the focal length range inevitably becomes the benchmark for the optical design. For lenses with an Internal Focusing system that cover wideangle ranges, a flower-shaped lens hood is employed. This type of hood delivers the best possible shading effect even when shooting at telephoto because the edge of the hood is extended to the maximum length in the areas corresponding to the top and bottom of the image, while being cut back in the areas that correspond to both sides of the image in order to avoid vignetting at any of the four corners.
Effect of Lens Hood Style
Changes in Depth of Field
Depth of field varies depending on the
focal length lens used even when shot at the same aperture. The wider the focal
length, the more depth of field produced. Depth of field becomes shallower as
the focal length changes towards more telephoto. The out-of-focus background
Changes in Perspective
Capturing a subject the same size with different focal length lenses yields very different results. When using a wider lens, more of the background is depicted. In contrast, with a telephoto focal length, the background view is narrower and will appear closer to the subject (compressed).