The recent discussion about Erwin's reviews reminded me about his Noctilux review. While I appreciate all his efforts his review of the Noctilux leaves me hanging a bit. The most disturbing part is this focus shift or Blendendifferenz. He documents it, but then doesnt really translate his flat plane test chart numbers into real 3D world DOF consequences. What I would like to know is what does the focus shift mean to someone shooting at 1.5 meters and f2 for instance. Does that means I have to move the focus 2 centimeters closer? further away? or is the focus shift less than a centimeter?
Has anyone run these figures and can post the results here? Thanks...

-- Russell Brooks (russell@ebrooks.org), April 05, 2002

Answers

Theory is all well and good, and there might in fact be a focus shift, but people are using this lens at full aperture for real photography with success. I would think that a focus shift is less of a problem than a less than perfect rangefinder. On the LUG I have read several post about how everything was great when the user only had a 35mm lens, but upon getting a 50mm Noct' or a 75mm Summilux, it was clear that their rangefinder was not correct. Below are some shots from one person that posted these series on the LUG demonstrating the use of the Noctilux at f/1.0. If there is a focus shift, it doesn't show up here with these three dimensional subjects. Maybe it would be a problem for a flat test pattern shot, but only lens testers do those kinds of pictures. These photos are slow to load due to the resolution, but they show off the lens pretty well.

series 1 , series 2 , series 3

-- Al Smith (smith58@msn.com), April 05, 2002.

Russell,

As I understand it, the focus shift only applies when stopped down. You should have no problems between F1 and F4, but at F5.6 and smaller the plan of focus shifts. Can't remember the exact reason though.

The biggest problem is the limited depth of field wideopen up close with the Noctilux. Again no problem when you are centering your subject directly over the focus rectangle, but if you recompose your image after focus as I often do with my portraits, then the plane of focus will appear to be closer than your point of intended focus.

I over come this by focusing on my subjects eyes, recomposing and then 'tweeking' my focus ring to a point closer than the original focus corresponding to the depth of field marker distance. It is almost failsafe for me.

I love my Noctilux but only because I nail my focus everytime with this method.

Haven't noticed the focus shift effect much as I mainly shoot between F1 and F2.

Good luck

-- Simon Wong (drsimonwong@hotmail.com), April 06, 2002.

Why not defeat this problem by composing first and then focussing on the eyes?

-- Albert Knapp MD (albertknappmd@mac.com), April 06, 2002.

Albert, in the very process of moving the viewfinder back to the desired composition after focusing, a subtle change in camera to subject distance occurs. It's referred to as "cosine error" because the ratio of the two distances is equal to the cosine of the angle through which the camera is moved. There is an earlier thread or two on this.

Actually, it looks like we have two different topics going here, both under the heading of focus shift. Shift of focus, as I believe E. Puts used it, refers to the fact that the point at which the lens is focus drifts somewhat as the aperture is stopped down. Not much of an issue with a 50mm Summicron; but with the lens designer having to juggle so many variables to correct an f/1 lens, evidently some shift of focus had to be accepted.

-- Bob Fleischman (RFXMAIL@prodigy.net), April 06, 2002.

Al, thanks for showing your 3 series of NOCT- photos. They show nicely what a good photographer with a terrific lens/ camera combo can accomplish.

There are simply not enough photos around on this LEICA- site, IMO.

Thank you again for sharing.

Best regards

-- K. G. Wolf (k.g.wolf@web.de), April 08, 2002.

Thanks for the answers guys, but that's not it yet... Please read Erwin's article. There is no focus shift at f1. where it gets "ugly" is in the f1.4 through f4 range. And it's not a condition of focusing on in the center and recomposing. But Erwin does a better job of explaining it than I can.
What I am after is how much of a focus shift it there at something like f2 and close focus. If the focus shift is only 1/8th of an inch then I can live with it since the DoF at f2 and close focus is much more than that. Thanks...

-- Russell Brooks (russell@ebrooks.org), April 08, 2002.