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Old Sep 11, 2003, 10:10 PM   #1
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Default Focal length - Analog vs. Digital

I am confused about some terms used in digital photography.

For example, my Nikon Coolpix 5400 has a 4X Zoom Nikkor ED lens (F=5.8-24mm). How do I equate that to the focal length descriptions on analog camera lenses? The specs on the camera indicate that it has a 35-mm camera format equivalence of 28-116mm.

I am trying to use a program called PhotoStitch to create wide angle pictures from separate pictures shot using the camera's panorama assist feature. The program asks for the 35mm film equivalent Focal Length of the pictures to be "merged". How do I determine what the 35mm film camera equivalence of a 14.5mm digital focal length is? Why are the lenses measured differently?
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Old Sep 12, 2003, 1:35 AM   #2
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They have diffrent ratings because they use sensors that are usally smaller than the area exposed on film. If your camera is 28-116mm equivalent, then that is what the equivalence is. When your camera is at wide angle, it is equivalent to a 28mm slr film camera lens. When you are at your full 4x telephoto, it is equivalent to a 116mm film slr lens. Just think of your camera as having a variable slr lens with a focal length range of 28mm-116mm. What the equivalent focal length is depends on how much zoom you used.
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Old Sep 13, 2003, 3:35 PM   #3
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So... is it all strictly a matter of doing the math?

If I divide the wide angle 35mm equivalent (28mm) by the wide angle digital focal length (5.8mm), I get a result of 4.83. The digital pictures were shot at a focal length of 14.5mm. Can I rely on simply multiplying the digital focal length (14.5mm) by 4.83 to get the 35mm equivalent (70mm) and use that information on the PhotStitch dialog box?

Thanks.
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Old Sep 13, 2003, 4:16 PM   #4
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Exactly.

Some times the manufacturers round up or down for the 35mm Equivalent Figures, so it's not unusual to see the exact same lens specifications from one manufacturer, being called a slightly different 35mm equivalent from another manfucacturer -- even though the lens and sensor sizes are identical.

But, your figure of 4.83 (multipled by the actual focal length used) should be close enough to determine the 35mm equivalent needed by the software.
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Old Sep 17, 2003, 6:22 PM   #5
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BTW, that 4.83 factor *isn't* the same with every digital camera...it's dependent on the size of the CCD. For instance, my camera with a 5.9-59mm (38-380mm 35mm equivalent) has a 6.44 factor.
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