focusing with nikon F3 - DOF preview or not?

[ampguy]

I just noticed that wtih my non-AI 43-86 1st gen zoom (and possibly other lenses), the focus changes when I press the DOF preview button.

Shouldn't the point in focus look the same, at the same distance wide open, and stopped down?

Haven't pulled out a tape measure yet, but it seems that wide open is more accurate, and stopped down, the point of focus is a bit closer.

For those of you with this system, do you focus wide open, and stop down for exposure (or DOF viewing), or do you actually focus with stop down button?

 

[Chris101]

I shoot my 1st gen. 43-86 (Ai'd) on an FG which does not have stopped preview, so of course, I focus wide open. I have not noticed a problem with focus shifting. I will look for it in the uture and report back if I notice anything.

 

[Chriscrawfordphoto]

Never tried the lens you're using, but some lenses do focus shift...notice all the complaints about this on the modern Zeiss 50/1.5 Sonnar for Leica-M mount. When stopped down, you may not notice the shift because depth of field may cover it...if you stop down enough (eg. f11 or 16)

 

[uhligfd]

Stopping down with the preview button darkens the pic in the finder. That is all I notice, maybe i can decide on the limits of DOF that way, but not on the critical main focus at all.

Have you noticed any focus shift from full open to stopped down in any of your pics? Maybe it is only the relative darkness that fools your eyes?

 

[ampguy]

thanks all, I have not noticed the focus shift in photos, but have not done a lot of wide open close tests. Yes, stopping down does darken, but there is still a shift of to get say a contrasty line in focus at min. focus ~1-1.5m.

Would be interesting to know what those Sonnar 1.5's looked like in an SLR!

 

[nzhang]

The early 105mm F2.5 lens is Sonnar formula, and later Guass version is aimed to correct this focus shift issue.

 

[ampguy]

Thanks nzhang, very interesting info. I think it is ok to assume that at min. focus, focus at frontmost section of available DOF, while focusing further from min, there is a weighting towards front focus, so to really test focus shift with an SLR wide open, and stopped down, one needs to test only at min. distance.

Or are there other reasons here?