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All you need to know about lenses,
As most 25mm lenses are well corrected (minimal distortions) and designed for the much larger and now defunct 1 format camera, all the worst bits at the edge of the image circle, are actually not within the area of the picture, cast onto a much smaller 1/2 or 1/3 CCD.
How can this be? Well the basic principle for any complex lens design (Binoculers and Telescopes being perfect examples) is that the size of the front optic will dictate the lens' light gathering capabilities. If you look at the 25 mm lens as our example, this would normally have a maximum aperture of f1.4, but by screwing the range extender on the back, the front lens diameter still remains the same, but the light gathering ability will now become an f2 lens. In practice, if you bench test (in an optical laboratory) a quality 25mm lens fitted with a x2 range extender, against a prime 50mm lens, with both optics set to around f 5.6 under well lit conditions, the chances are you wouldnt see a great deal of difference.
Some lenses, such as wide angle or ultra wide angles, and selected pinhole optics, are really just not suitable for use with a range extender.
The wide angle conversion lenses, and close up adaptors screwed onto the front of a lens, can also significantly affect the overall quality of a lens' performance, but it must be stressed that whilst different manufacturers produce variations in quality (both in terms of materials and production methods), Doktor Jon is aware that even with a world class manufacturer, some lenses in the range can produce a less than satisfactory performance.
During tests many years ago, three ultra wide lenses (manual iris, direct drive and auto iris) from a leading manufacturer, were bench tested using the same camera - the differance in optical performance was quite simply ..... staggering! The manual iris was superb, direct drive acceptably average, and the auto iris ... absolutely awful.
In practice, some zoom lens performances (particulalrly smaller format) can be quite stunningly unattractive, particularly if the lens is forced to operate at or near to maximum aperture, under very low light conditions.
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