focus shift basic question

[sparrow6224]

I have read as much about this as I could take, including on RFF, but still don't understand it. I have the VC 35/1.4, the Canon 50/1.4, a post-war Zeiss sonnar 50/1.5... etc. So I assume I'm in focus shift situations without knowing it.

Which leads to my main question: I understand that if I focus on some point at f/1.4 and then zip down to f/8 I might not be in focus anymore. But does it still appear I'm in focus?

Similarly, if a lens might suffer focus shift can one not just re-focus when stopping down? If not, it seems to me, then f/8 will never be in focus on such a lens, rendering it somewhat useless.

Thanks for your patient, non-contemptuous help on this.....

 

[Phil_F_NM]

In an SLR we don't notice focus shift as much because we get to see the results, for the most part. Usually stopping down will negate focus shift enough that it's unnoticeable. More so on film due to its depth but the advent of digital rangefinders and obsession with pixel peeping has given a few lenses a bad rap for focus shift.
In a nutshell, there will be shift in a few of your lenses from wide open down to f/5.6 but you really may not be able to see it without putting a ruler on a table and testing your lenses. If you take photos of three dimensional things like most of us do, it shouldn't matter. And if you already have "shifty" lenses but use them and like the way they draw an image then it's a moot point anyway.
Basically, I'm saying that in most cases it's much ado about nothing.

Phil Forrest

 

[kxl]

Focus shift becomes apparent when you have minimal DOF to work with, i.e., at apertures from f1.4 or f1.5 through about f4 AND at relatively close focusing distances. When you have stopped down to f8 and are shooting distant objects, the great DOF essentially hides any focus shift.

 

[sparrow6224]

Well that's good to hear, thanks Phil. I'm sure some intense photo-Spartans will disagree. However I have never found that if an experienced and intelligent photographer has said something doesn't matter much, that I then decided it mattered a lot. I have finally decided after much study that I don't give a crap about bokeh, one example.

That said, my question still stands: if I refocus the lens after stopping down, is "focus shift" still a factor? I assume not.

 

[craygc]

That said, my question still stands: if I refocus the lens after stopping down, is "focus shift" still a factor? I assume not.

If its a rangefinder it makes no difference what aperture you're at when you focus. There is an optimum aperture that alines with you focusing and, if focus shift is going to noticeably happen, then being at any other wide(ish) aperture is going to suffer from it to various degrees. For SLRs, as mentioned above, its a little bit of what you see is what you get except many SLR focus screens cannot show anything faster than f/2 ~ f/2.8 so you can still get the problem.

 

[Roger Hicks]

From http://www.rogerandfrances.com/subscription/ps firstlook sonnar 50.html (Sonnar review)

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.

There is more, plus an illustration of how it works, at the link given.

Cheers,

R.

 

[sparrow6224]

I'm beginning to get it... I think. A little.

Meanwhile, which 35/50mm fast lenses are regarded as least subject to focus shift? Are tele lenses (50-100, that is, not real tele) with wide apertures more given to the problem than wider lenses?

 

[denizg7]

From http://www.rogerandfrances.com/subscription/ps firstlook sonnar 50.html (Sonnar review)

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.

There is more, plus an illustration of how it works, at the link given.

Cheers,

R.

this helped me a lot thanks Roge...


by the way when are you going to review new lenses?? thanks

 

[sparrow6224]

I think after reading Roger's page, which is quite good, that my confusion in part comes from my uninformed yet seemingly accurate belief that I want to focus on the nearest point, that I want at best to eliminate and at least to minimize out of focus area/items in the foreground, and that the depth of field whatever it is will extend behind my point of focus. This basic way of seeing, which I can't now recall a source for, seems to be in keeping with Roger's instructive page. The image that leads the aperture examples (Encyclopedia volumes) -- with its oof volumes in the foreground -- is the kind of image that drives me nuts. My eye objects to it. Obviously I see that it is there for instructive purposes.