Archived posting to the Leica Users Group, 1998/06/08[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
Pascal wrote: > > On 06-06-1998 16:32 Chris Bitmead wrote: > > >It is a fact of life that no 50mm f1.4 lens will measure up > >to an 80 f1.4 lens. > > How is that? Could you explain this, Chris? Well the main thing I would say is, don't worry about why, but just look at the performance of lenses from different manufacturers. There seems to always be quite an increase in quality from 50 to 85mm. But if you want to think about it, when a lens is being shot wide open, each part of the image is coming through every part of the front element. Like the nose in a portrait isn't just entering the centre of the front element, but the entire area. Therefore if you picture a wide angle lens, the light has to be defracted at very severe angles between the front element and the film in order for all those points of light from one side of the lens to the other, to all meet in the same point. The defraction in a long lens is much more mild. The longer the lens, the milder the defraction. There are other issues the wider a lens gets. Like with the rear element tending to be closer to the film plane, the centre tends to be more illuminated. Film plane flatness tends to be more of a problem. And there is more distortion with the designers wrestling to keep a larger angle of view square and rectalinear.