Focus shift and optimised for focus.

[AndySig]

I had never encountered the terms "Focus Shift" and Optimised For Focus (at Fx) until I started visiting this forum.

I can obviously guess at what they mean but could somebody explain please what the real definition is.

The second question that occurs is that are these things only factors when one is shooting wide open?

The third one is, are they only significant factors in RF photography?

Fourth: are they actually noticeable to ordinary mortals i.e. to people who can't tell which shade of black paint was used on a Leica 3 made in the first half of nineteen fifty whenever?

 

[ItsReallyDarren]

jonmanjiro pretty much sums it up quite well.

Focus shift will show up in lenses, some more than others and even then its probably not noticeable because depth of field will cover it. Having said that, if you specifically set out to get focus shift in lenses reported with focus shift you can probably get it under certain circumstances.

Shooting wide open at the minimum focusing distance on a tripod with an M8 doing an aperture by aperture shot will usually get you focus shifting. And again, depth of field will usually cover the difference on most lenses, RF or SLR.

 

[morback]

A nice article about this topic.

 

[ssmc]

A couple other things to consider:

- film is somewhat less critical of focus shift than digital because a) it's not flat, and b) the emulsion has a finite (sometimes considerable) thickness, and c) people did not routinely examine their images at enormous (for 135-format) enlargements, which they do now by pressing CTRL+ALT+0 (or whatever); so long as the plane of focus lay somewhere within the thickness of the emulsion you were (or are) good

- longer lenses that exhibit focus shift are much more of a problem than short/wide ones, as depth-of-field is inversely proportional to the square of the focal length; since rangefinders are much better with medium-wide lenses than with teles (or long lenses), it is less often a problem, though as noted the Sonnar 50/1.5 is probably the most notorious recent example. With an SLR you can (usually) stop the lens down and see if the focus has shifted noticeably, but the image gets darker and (with a few exceptions) modern cameras have crappy low-mag viewfinders which makes seeing it difficult. With a RF you won't find out at all until after the fact, unless you are familiar with the lens in question.

HTH,
Scott

 

[Roger Hicks]

From http://www.rogerandfrances.com/subscription/ps firstlook sonnar 50.html (review of 50/1.5 Sonnar) where you will also find quite a lot more information and a series of pictures showing what happens.

As with many very fast lenses, the point of sharpest focus of the Sonnar shifts as you stop down. In fact, it gets steadily further away, so that at the closest focusing distance (90cm/3 feet) it recedes several centimetres when stopping down from f/1.5 to f/5.6. Of course you have increasing depth of field to compensate for this, but the overall effect is that as you stop down, depth of field increases at the normal rate, but in effect, it increases much faster behind the focused point. Dr. Nasse at Zeiss explained this when we picked up the lens in May 2007, and it has worked for us ever since: the trick is simply to focus on the nearest important point, rather than relying on the usual rules for d-o-f.

To borrow a brilliant graphic representation, whereas with a lens without focus shift the depth of field could be represented thus: _____[]_______, where ____ is depth of field and [] is the focused point, with the Sonnar it is more like []____________. Actually, as the pictures of the Britannica below show, this is a bit of an exaggeration, and a more accurate representation might be _[]______ for the Sonnar and __[]____ for a lens without focus shift.

Cheers,

R.