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January 7, 2009 |
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Created and Maintained by: The Photoimaging Information Council |
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by Alfred DeBat |
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Point-and-shoot digital cameras and interchangeable-lens digital single-lens reflex (dSLR) cameras use different nomenclature to describe lens focal lengths. The lens focal length usually indicates the field or angle of view produced by the lens. In the film SLR camera world, 50mm or 55mm focal lengths are considered “normal” lenses. Here’s an approximate comparison: A moderate wide-angle lens is 35mm, shorter focal lengths are wide-angle (24mm to 28mm) and super wide-angle (17mm to 20mm). Lenses longer than 50mm are considered telephoto. Depending on focal length, there are short telephotos (75mm to 140mm), long telephotos (200mm to 300mm), and super-long telephoto lenses, which can range from 400mm to more than 1,000mm.  Tamron AF55-200mm F/4-5.6 Di II LD Macro lens
On digital SLR cameras, the physical size of the sensor array determines how much the lens “sees”. It is the same for point-and-shoot models and digital cameras with non-detachable lenses. Because there are no standard sensor chip sizes, manufacturers list the digital camera lens focal lengths by the equivalent view produced by 35mm film camera lenses. In other words, when a point-and-shoot digital camera is listed with a 38-117mm zoom lens, it produces views equivalent to that focal-length range compared to a 35mm film camera. The actual focal length of a digital camera’s zoom lens could be 5-12mm, but it doesn’t matter because the sensor chip is so much smaller than a 35mm-format negative.  Pentax DA 16-45mm lens
The system based upon 35mm film camera focal lengths works well and allows buyers to decide what they can expect from a particular camera. The trouble arises when selecting interchangeable-lens dSLRs where lenses are listed by the actual zoom focal lengths. These focal length references have no relationship to 35mm film camera views. This can be a particular problem for photographers who have a collection of film SLR lenses, and want to use their older lenses on new dSLRs. The lenses will fit the camera lens mount, but not produce their listed focal length views. Interchangeable-lens digital SLR brands include Canon, Fujifilm, Nikon, Olympus, Panasonic, Pentax, Samsung, Sigma, Sony, Tamron, and others. The film SLR lenses will capture the correct view, only if the digital camera sensor chip is the same size as a 35mm-format negative (36-by-24mm). Unfortunately, only one popular camera maker now produces a dSLR with full-size sensor array – and that’s Canon. Therefore, if you have Canon EOS SLR lenses, they will operate correctly on the expensive Canon EOS-1Ds Mark II dSLR.  Canon EF-S 17-55mm f/2.8 IS USM
Now let’s see what you get when you purchase one of the new “digital” lenses for the current crop of dSLRs. The chip is approximately half the size of a full-frame 35mm negative. So there is a focal-length conversion factor listed in the dSLR specifications – typically a factor of 1.5. The “digital” zoom lens frequently sold with these cameras is advertised as an18-55mm zoom, but it actually produces images equivalent to a 27-82mm zoom lens. That may not matter to you, but if you were expecting the super wide-angle view that 18mm film camera lenses produce, you’re not going to get it. For example, if you own a family of SLR lenses from your film SLR camera shooting days, what happens when you put a 70-250mm zoom on a dSLR body with a 1.5 focal-length factor? You’ll now have a zoom range equivalent to 105-375mm long telephoto. (You multiply the focal lengths by the conversion factor for the equivalent lens view.) That zoom focal length might be great for sports and wildlife photography, but it’s not the best for general picture taking. By the way, if you can mount a new dSLR camera lens on a suitable SLR film camera, you’ll find that the lens image does not cover the entire 35mm-format negative, because the lens is designed only to produce a complete image for the much smaller sensor array.  Nikon 70-300mm f/4.5-5.6G ED-IF AF-S VR Zoom-Nikkor
A dSLR focal-length conversion factor number is seldom prominently advertised and consumers usually have to go online to find the information buried in the individual camera’s technical specification sheets. (In current dSLR models, the focal length conversion factor can range from 1.3 to 1.7.) You can check the conversion factor by reviewing interchangeable-lens dSLR specs at camera manufacturers websites, including Canon, Fuji, Nikon, Olympus, Panasonic, Pentax, Samsung, Sigma, Sony, and Tamron. If you are interested in wide-angle photography, a couple of digital cameras can produce images in the wide-angle equivalent range of 23mm and 24mm lenses, which are great focal lengths for indoor picture taking in tight spaces. Digital point-and-shoot cameras with built-in wide-angle lenses include three EasyShare Kodak models: The dual-lens, 23mm, plus 39-117mm zoom Kodak V570 and Kodak V705 models, or the 24-140mm zoom lens Kodak P880 camera.
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