Tamron has the greatest zooming range of all and the Tamron 16-300mm f/3.5-6.3 Di II ([pounds sterling]430) features a nominal focal length
of 16mm at the widest setting (equivalent to 24mm when mounted on a cropped sensor DSLR), and extends out to 300mm.
The scope has a nominal focal length
of 650 mm (25.6 inches), but there are optional 0.73x and 0.60x focal reducers that deliver effective focal lengths of 470 mm (f/3.6) and 390 mm (f/3.0), respectively.
For example, a 32 wavelength wide lens aperture would have a nominal focal length
of 20 [Lambda], so a normalized path length error of 0.005 would correspond to 0.1 [Lambda], or 36 [degrees] of phase error.
This has tremendous advantages for longer lenses: a telephoto lens with a nominal focal length
of 200mm in the 35mm format has a working focal length of 320mm when attached to a digital SLR.
By measuring the difference between these foci, expressed as a percentage of the nominal focal length
, we can determine the amount of longitudinal spherical aberration.