Please help me understand focus shift.

[keithdunlop]

I am looking at possibly acquiring a Voigtlander 28mm f/2.0 Ultron for my M6 and I keep reading about focus shift problems. I understand what focus shift is and generally what causes it. What I guess I'm having trouble understanding is why it's such a big deal.

For example, let's say a particular lens exhibits focus shift when stopped down to f/4.0. Can't you simply get around the problem by focusing at the shooting aperture? If I know in advance that an f/2.0 lens focus shifts at f/4.0, then why not just either shoot wide open, or stop down prior to focusing.

Am I missing something?

 

[mfogiel]

Yes, this is a rangefinder forum, so when you focus with a rangefinder, it does not matter what aperture you have selected, this can only be done on a slr camera. As for the Ultron, if you shoot film, then the focus shift is not a big deal, you should just remember when shooting close objects, to focus on the nearest element that you want in focus.
A test of the lens is here:
http://www.rangefinderforum.com/forums/showthread.php?t=75314&highlight=ultron+focus+shift+test

 

[Ezzie]

I never experience focus shift, even on my CV 35/1.4 Ultron. Probably because I pre-visualise what I want from a scene and therefore set aperture before focusing. In the event that exposure needs to be adjusted, I adjust shutter speed rather than aperture.

 

[sevo]

I never experience focus shift, even on my CV 35/1.4 Ultron. Probably because I pre-visualise what I want from a scene and therefore set aperture before focusing.

That will only help you on SLRs with DOF preview, provided that you use the latter.

 

[keithdunlop]

when you focus with a rangefinder, it does not matter what aperture you have selected, this can only be done on a slr camera.

That's what I thought -- thanks!

 

[keithdunlop]

http://www.rangefinderforum.com/forums/showthread.php?t=75314&highlight=ultron+focus+shift+test[/URL]

That's extremely helpful -- thanks for publishing the test! I ordered a copy of the lens and will do my own comparison to see if there is sample variation, but now I know what to look for.