Focus shift - please explain

[Austerby]

For some time now I've been reading comments here and elsewhere about the focus shift phenomenom exhibited by some lenses - examples given as the Zeiss 50mm f1.5 c-Sonnar, the Voigtlander 28mm f2 Ultron and the Voigtlander 40mm f1.4 Nokton.

I have all three lenses but am yet to detect this effect and am unclear as to its impact, whether serious or not. It may be that I don't take many close-ups at widest aperture with these lenses, or that I'm blaming out-of-focus images on my own inadequacies.

Can someone please describe a test for me to conduct so I can understand exactly under what circumstances a focus shift occurs and what to do about it?

(I have an M8 and an M3 so can conduct the test with both digital and film with the same lenses.)

Edit - is back focusing the same as focus shift or something else entirely?>

 

[Gabriel M.A.]

http://www.google.com/search?hl=en&source=hp&q=focus+shift+tests&aq=f&oq=&aqi=

 

[ampguy]

my explanation

my explanation

focus shift is when your point of focus changes when aperture is changed.

If you start focus wide open, and stop down, your point of focus should not only stay in focus, but should be in focus relative to the total DOF for that distance, aperture, and lens (coc).

Back focusing just means focus is off, not necessarily shifted.

For some time now I've been reading comments here and elsewhere about the focus shift phenomenom exhibited by some lenses - examples given as the Zeiss 50mm f1.5 c-Sonnar, the Voigtlander 28mm f2 Ultron and the Voigtlander 40mm f1.4 Nokton.

I have all three lenses but am yet to detect this effect and am unclear as to its impact, whether serious or not. It may be that I don't take many close-ups at widest aperture with these lenses, or that I'm blaming out-of-focus images on my own inadequacies.

Can someone please describe a test for me to conduct so I can understand exactly under what circumstances a focus shift occurs and what to do about it?

(I have an M8 and an M3 so can conduct the test with both digital and film with the same lenses.)

Edit - is back focusing the same as focus shift or something else entirely?>

 

[Roger Hicks]

There's quite a detailed discussion, and a 'worked example' (with illustrations) about a quarter of the way down this review: http://www.rogerandfrances.com/subscription/ps firstlook sonnar 50.html

Cheers,

R.

 

[Austerby]

Roger, thank you: I did look for an explanation in your pages but didn't realise it was hidden in the lens review. I now remember having read this but I've not put it into practice.

I shall take my cameras to the nearest bookshelf, place them on a tripod, set focus and change the aperture then review the results.