> When I was in Bolivia last month, my 85/1.4D would not focus to > infinity (using my F5). This happened at 4,300 m altitude. Can the > thinnner air and its (very slightly) different optical properties > explain this? For details see IR/EO handbook vol 2: Atmospheric prop. of radiation (ed F G Smith) pub. Infrared Information Analysis Center / SPIE 1993 but in short - the visible band refractive index of air is sensitive to pressure, temperature and water vapour content (humidity). (Since maths and email don't work ... no maths). Will this produce a loss of infinity focus ... I don't know without analysing the lens design (and I don't have time for this). Some things to consider: 1) If the lens contains "ED" elements, these are often made of glasses which are temperature sensitive (nb this is a generic statement - individual glasses vary and I don't have access to a Nikon glass catalogue to check the specific materials). 2) Does the lens focus BEYOND infinity normally (in manual operation) - whilst this can be part of the mechanical tolerances in the design, it is is often an indication that the lens is temperature sensitive. 3) Atmospheric fluctuations can make things look out of focus - at 30C (86F) if you were focusing over an optical path "near" ground then this can be quite a problem. > At the time I did not have two Nikon bodies with me, so I could > not positively conclude if it was a lens oor body problem. > Temperature was around 30 centigrades. Now, back at sea level, > everything works fine. Have you tried it at 30C at sea level? Andy Shaw
