focus shift - please explain it

Freakscene said:
The difficulty with focus shift in a lens like the 50/1.5 Sonnar is that if the lens is adjusted for accurate focus at f1.5, as you stop down the depth-of-field increases, but it shifts backwards sufficiently that the point you focus on is in front of the plane of acceptable sharpness - and therefore unsharp. If you focus on an eye you end up with a sharp ear. The increase in depth-of-field in the 50/1.5 Sonnar does not overcome the focus shift until ~f4.5 as this test shows quite clearly: http://photo.net/leica-rangefinders-forum/00aKSW

Focus shift is not imaginary, or an invention, but it's also not fatal. Understanding it can facilitate you being able to better use of lenses that display it, and make photos that look like you want them to, which is the point after all.

Marty
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Ok, but if I focus on something at 1.5, take a shot, then step down to say 2.8, does refocussing correct it - or is this lens always out of focus - even if it appears in focus in the viewfinder - from F2~4?
 
Scheelings said:
Ok, but if I focus on something at 1.5, take a shot, then step down to say 2.8, does refocussing correct it - or is this lens always out of focus - even if it appears in focus in the viewfinder - from F2~4?
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What camera are you using? If you're using something that allows you to look through the lens at the selected aperture (e.g. an electronic viewfinder interchangeable lens camera, or an SLR with the depth of field preview engaged) then something that appears to have the center of focus on a particular point at f2.8 actually has the center of focus on that point at f2.8 (barring user error).

If you're using a rangefinder or an SLR that holds the aperture open while you focus and stops down only for the exposure, then something that appears to be have the center of focus on a particular point at f2.8 may actually have teh center of focus on a different point.
 
JHutchins said:
What camera are you using? If you're using something that allows you to look through the lens at the selected aperture (e.g. an electronic viewfinder interchangeable lens camera, or an SLR with the depth of field preview engaged) then something that appears to have the center of focus on a particular point at f2.8 actually has the center of focus on that point at f2.8 (barring user error).

If you're using a rangefinder or an SLR that holds the aperture open while you focus and stops down only for the exposure, then something that appears to be have the center of focus on a particular point at f2.8 may actually have teh center of focus on a different point.
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I'm using a Leica M8. So Actually I was weighing up whether to get the 1.5 Sonnar or the 2.0 Planar. Half a stop doesn't sound like much to give up if more of my shots are going to be as tack sharp as I would expect from Zeiss....

Maybe I need to research some planar vs sonnar threads.

Is there much difference with Bokeh?
 
Scheelings said:
I'm using a Leica M8. So Actually I was weighing up whether to get the 1.5 Sonnar or the 2.0 Planar. Half a stop doesn't sound like much to give up if more of my shots are going to be as tack sharp as I would expect from Zeiss....

Maybe I need to research some planar vs sonnar threads.

Is there much difference with Bokeh?
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So with a Sonnar on an M8 the center of sharp focus can and will be different from what your rangefinder tells you it is. This can be dealt with -- it's just a matter of having an idea of how your lens behaves at various apertures and distances.

But these are very different lenses and if you're considering the Sonnar just for the extra half stop then probably you'd be happier with the Planar. It is a sharper lens, it vignettes less, it is more neutral. The Sonnar imparts a color of its own, yes, the bokeh is significantly different from the Planar, also contrast, field illumination &c. Apologies for the obvious advice, but if you're on the fence you really should do a tag search on flickr for both lenses because the best reason to get a Sonnar is that you like the Sonnar look and to understand it you should see it.
 
What JHutchins said and to add that the Zeiss "C" designation lenses are a throw back to old and loved lens designs with warts and all. They are meant to be like that for good reason. If you want clinical looking images then the Planar is probably for you.
 
135format said:
What JHutchins said and to add that the Zeiss "C" designation lenses are a throw back to old and loved lens designs with warts and all.
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The C-Biogons 35/2.8 and 21/4.5 do not have many warts.

Due to their relatively symmetrical designs they have some vignetting and don't do well in the corners on digital sensors. On film they are darned close to flawless.
 
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