Can anyone please explain regarding the issue of focus shift I keep hearing about in the Noctilux?

-- Simon Wong (drsimonwong@hotmail.com), August 17, 2001

Answers

Simon,

You can go to this site and read a detailed review on the Noctilux, including the theory on the focus shift. Basically, (according to the article from Erwin Puts), the focus plane shifts slightly when closing the aperture. All of the math is in the article.

http://www.imx.nl/photosite/leica/mseries/testm/M10-50.html

-- Al Smith (smith58@msn.com), August 17, 2001.

As I understand it, the focus shift in lenses as they are stopped down is due primarily to un(der)-corrected sperical abberations. Due to the large spherical surface(s) of the Noct, the focal length of the lens actually changes as the lens is stopped down, and less of the glass is being used to generate the image. In most modern designs, these abberations are very slight, and generally are more than compensated for by DOF. However, at f1.0, there is precious little DOF, so the lens must be right-on to focus properly here. In the Noct, the problem then crops up as the lens is stopped down to f1.4 which is still very wide with a slim DOF. This pattern holds through about f4, until near f5.6 where DOF has sufficiently begun to compensate. In most normal photographic situations you won't notice this as a focus problem because some point of the image is in perfect focus - it just won't be the point you were focussing on.

As an aside this problem is very well known in LF camera use, as many older lens designs, such as triplets, are still in use. The lenses are of significantly longer focal length, significantly more under- corrected for spherical abberation, and hence exhibit even more extreme focus shifts. With these older lenses, you focus, adjust movements, and compose wide open, but then must re-focus your prime subject after you have stopped the lens down -- not an easy task while trying to adjust the dim image on a dimmer groundglass under a dark cloth at f64!

-- Jack Flesher (jbflesher@msn.com), August 17, 2001.