Archived posting to the Leica Users Group, 2009/12/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Ah!! Duh. OK, now it makes sense :-) I will check later on today. Basically, due to the somewhat imprecise and tolerance of various factor, the focus is "optimized" for one aperture. In some lens design, the focus remain constant even when the aperture changes, whereas some the focus changes a bit, which usually is hidden by the DoF or the film unflatness etc. Gotcha. Sorry for understanding it earlier. Thanks. On Mon, Dec 7, 2009 at 10:39 AM, Peter Klein <pklein at threshinc.com> wrote: > Richard: ?Remember, an RF is not an SLR. ?The rangefinder has no idea what > aperture you are at, and its mechanism is independent of the lens' > characteristics. > > The reason why I said "don't refocus" is that human error/inaccuracy can > slightly change the place the lens is focused on each time, muddying the > results. > > Focus shift is that property whereby if the lens is focused on Point A > wide open, and you stop down without changing the focus setting, the lens > will now focus on Point B, which is usually a bit behind Point A. ?As you > stop down, depth of field increases, and at some point it compensates for > focus shift. Usually at f/5.6 or 8. > > But this makes the intermediate stops a problem. ?With some lenses at say, > f/2, 2.8 or 4, you may find that you focused on someone's eyes, but in the > picture their eyes are sligthly blurred and their ears are in sharp focus. > ?But if you'd shot wide open, the eyes would have been in focus. > > The Noctilux has shift one can see even on film. ?The 35/1.4 'Lux Asph has > shift we can see on the M8. Sean Reid has found that most fast aspherical > Voigtlander/Cosina lenses don't have significant focus shift. But their > standard spherical designs do, such as their 28/2. ?I'm interested in the > 50/1.1, and it's a spherical design, so I'm asking. > -- // richard m: richard @imagecraft.com // w: http://www.imagecraft.com/pub/Portfolio09/ blog: http://rfman.wordpress.com // book: http://www.blurb.com/bookstore/detail/745963