[LRflex] Re: Converters

At 27/07/2007, you wrote:

Xavier, et al
Converters only work with telephoto lenses! If you fit a lens less than 100mm with a 2 or 3x converter, the angle of incidence of the light rays near the edges is simply too great for the converter to cope with. My Adaptall 2 Tamron is a set: the 135 and 2x converter work very well together. About 45 years ago I had a a lens that was, I think, about 75 or 80 mm, with matching 2 x converter, and it was awful! In those days, though, the converter was just a couplet; later ones are either 4 or 7 element, and work far, far better. The posts about the other lens (24mm, I think?) and not working well on a digital sensor is exactly the same problem! The angle of incidence to the corners of film is far less than to the edges of a sensor, because the lens is mounted so much closer to the sensor. With most digital cameras, if you want the best performance, you really need a lens designed for the closer distance. Because of the large angle of incidence, too, you get much more chromatic aberration than you would using the same lens with a 35 mm film camera. Another related point is that the sensor is not a planar device; the light must travel down short 'tubes' to the actual sensor material, and these vary in length depending on whether they are green, red, or blue elements. Consequently, the sensitivity changes towards the edges with an unsuitable lens, by an amount depending on the colour. 



G'Mornin' Keith!
You are, of course, right; when you say that converters are designed for tele lenses only.... the longer the focal length, the better. Yet, I have seen photos (not made by me) made by people using 21mm SA's and 2x converters with remarkably good results. Others w/a lenses seem to work OK, too ... but certainly not most of them.
(Some long time ago, there was an article in, I believe, LFI, showing w/a lens use with converters.)
Both Alex Hurst and I have tried the same 2x converter with our 90mm Summicrons, and results can only be described as disastrous! Yet, that same 2x, on the Telyt 400/f6.8 yields results hard to tell from the prime lens alone! 

All to do, as you say, with the angle with which the rays hit the sensor/film.


Cheers!

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David Young,
Logan Lake, CANADA

Wildlife Photographs: http://www.telyt.com/
Personal Web-pages: http://www3.telus.net/~telyt
Stock Photography at: http://tinyurl.com/2amll4

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