Archived posting to the Leica Users Group, 2012/03/30
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]On Sat, Mar 31, 2012 at 6:00 AM, Richard Man <richard at richardmanphoto.com>wrote: > I think the proper terms is back focus. > "Back focus" gets used all over the internet for everything from out of spec equipment to focus shift. Steve's lens is not adjusted to factory specs. > If the focus point changes as you change the aperture, then it's focus > shift and you are stuck. You can then decide to calibrate to an aperture > that makes the most sense to you, usually at wide open and hope the DoF > will cover for any errors in narrower aperture. > If you have genuine focus problems wide open the problem is not focus shift. The focus point of the Noctilux shifts substantially when stopping down. As Leica adjusts this lens when correctly focused the plane of optimum sharpness for a Noctilux is slightly (very slightly) in front of the point you focus on. Then as you stop down the focus point shifts back, mostly within the plane of sharpness offered by increased depth of field. You can get Leica to adjust the lens to focus correctly at f1 but then focus shift means that your focus will be off until f5.6. I got mine adjusted this way because I always carry an f2 50mm lens when I carry my Nocti, so it basically gets used at f1 or not at all. Your lens and camera(s) need to be adjusted to factory specifications and tested together. You need specialised equipment to do this. > I believe this is one of the reasons why the Nocti /0.95 was created. > And because some of the special glass in the f1 ran out and more couldn't easily be manufactured. > You should test out whether it's focus shift. Just put the camera on a > tripod and photograph a yard stick at an angle. Take multiple photos > without changing the focus but only the aperture. > This is good advice, but the first thing to do is to do some really critical wide open focusing to see where the point of optimum sharpness is relative to the focus point. If this is off, then there's no point worrying about focus shift; you know it's there. Marty