Please define focus shift

[bdolzani]

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If I may, to sort out my confusion. People say there's shift 'changing from one aperture to another'. This may be semantic, but what if you go directly to, for instance, 2.8? Will the focus be incorrect? If it is, then I feel you should say that 'at 2.8, or 4, the lens does not focus correctly according to the RF patch.' It shouldn't have anything to do with changing apertures. 

I don't own any summilux's, but I'm curious if I do get one. 

Further then, to solve this issue, I assume you need to learn how to focus at that aperture correctly with that lens, via live view (easiest), and adjust the patch slightly ahead or behind alignment?

thank you-

brian 

 

 

 

Edited September 27, 2022 by bdolzani

[bdolzani]

10 minutes ago, Al Brown said:

Focus shift happens when an object is in focus at maximum aperture with fast lenses and photographed with the lens stopped further down - the optimum plane of sharpnes/focus "moves". This is about as simple explanation as it gets. You would need to use live view stopped down to solve this problem on the fly.

Ok but you wouldn't know it at the time of looking through the RF, correct? That's my confusion with the language - it's not like you know it's in focus at 1.4 but not in focus at f4, until you view the photo. Because the patch is still 'perfectly' aligned at both apertures....

However if you are saying that the focus moves throughout the rest of the apertures, that would be clear to understand. And again then there's no real way to adjust for it if it's that unpredictable using the RF. 

[UliWer]

13 minutes ago, bdolzani said:

If it is, then I feel you should say that 'at 2.8, or 4, the lens does not focus correctly according to the RF patch.' It shouldn't have anything to do with changing apertures. 

Well, if we call it shift of focus, the shift occurs when you change the aperture. If you don't change it, it won't shift but only be on or off focus - according to the calibration of the lens.

The focus must not be exact with maximum aperture, it may be better with a smaller one. In former days lenses which are prone to focus shift were often calibrated at f/2.8 or f/4, as it was assumed that people would use these smaller f-stops more often than maximum aperture. Zeiss explained this expressively for the 1:1.5/50mm Sonnar and offered to calibrate it for maximum opening. Leica did never explain this as clearly but did the same.  

[bdolzani]

2 minutes ago, UliWer said:

Well, if we call it shift of focus, the shift occurs when you change the aperture. If you don't change it, it won't shift but only be on or off focus - according to the calibration of the lens.

 

Yes I feel 'focus shift' is a gentler way of saying 'focus is incorrect at 2.8 and 4'. No matter where you start. 

 

2 minutes ago, UliWer said:

The focus must not be exact with maximum aperture, it may be better with a smaller one. In former days lenses which are prone to focus shift were often calibrated at f/2.8 or f/4, as it was assumed that people would use these smaller f-stops more often than maximum aperture. Zeiss explained this expressively for the 1:1.5/50mm Sonnar and offered to calibrate it for maximum opening. Leica did never explain this as clearly but did the same.  

 

[UliWer]

1 minute ago, Al Brown said:

and with SLRs ... you can use "working aperture", i.e. focus stopped down.

Yes you can, but most time people use automatic aperture, focussing wide opened to have more light for the viewfinder, and it stops down only when you push the release button - causing focus shift. 

[bdolzani]

1 minute ago, UliWer said:

Yes you can, but most time people use automatic aperture, focussing wide opened to have more light for the viewfinder, and it stops down only when you push the release button - causing focus shift. 

This I understand. With SLR you are looking at the scene one way in real time (TTL), and shooting another. With rangefinder this does not seem the case, to me. 

[UliWer]

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5 minutes ago, bdolzani said:

Yes I feel 'focus shift' is a gentler way of saying 'focus is incorrect at 2.8 and 4'. No matter where you start. 

No, it isn't. As i said, focus may be absolutely correct at f/2.8, and way off at f/1.4, if your lens is calibrated at f/2.8. Due to optical laws the focus shifts when you change the aperture. 

Here is an explanation by Zeiss: https://www.zeiss.com/consumer-products/int/photography/content/manual-focusing-with-af-camera-system.html . Scroll down to the chapter about "Focus shift"

Edited September 27, 2022 by UliWer

[UliWer]

3 minutes ago, bdolzani said:

With SLR you are looking at the scene one way in real time (TTL), and shooting another. With rangefinder this does not seem the case, to me. 

Yes, but this doesn't tell you anything about the lenses aperture. If you use "working aperture", the result may be spot on or off. If you use automatic aperture focussing wide open, the aperture changes and may cause focus shift. 

[bdolzani]

3 minutes ago, UliWer said:

No, it isn't. As i said, focus may be absolutely correct at f/2.8, and way off at f/1.4, if your lens is calibrated at f/2.8. Due to optical laws the focus shifts when you change the aperture. 

Here is an explanation be Zeiss: https://www.zeiss.com/consumer-products/int/photography/content/manual-focusing-with-af-camera-system.html . Scroll down to the chapter about "Focus shift"

Ok I do understand it depends where/how the lens is calibrated. So I did misspeak. 

[bdolzani]

2 minutes ago, Al Brown said:

It depends where the lens is calibrated ONLY for RF lenses with cam. Not for all the others.

Right, I did mean RF lenses e.g. Leica. 

I have desires for summilux lenses yet shoot f8 and street/candid/documentary 95% percent of the time. So the idea of focus shift is not even in my world, really. I prefer most things in focus. 

[UliWer]

3 minutes ago, Al Brown said:

It depends where the lens is calibrated ONLY for RF lenses with cam. Not for all the others.

I can only once again recommend the article by Zeiss quoted above. It precisely describes the problem of focus shift for camera models like the Nikon D700 and the Canon 5D MkII - both are far away from rangefinders. 

[01af]

vor einer Stunde schrieb bdolzani:

... what if you go directly to, for instance, f/2.8? Will the focus be incorrect?

Depends on the focusing method ... and the lens, no less.

Edited September 27, 2022 by 01af

[jaeger]

Focus shift - your wife smacks your forehead while you looked away when she is talking to you.

[bdolzani]

5 minutes ago, Al Brown said:

There is virtually no focus shift with Summilux 35mm FLE. You're good there. Proven first hand for a long time.

I thought so but what made me start this topic was someone saying in the new 35 summilux thread that their FLE does have shift. 

[bdolzani]

5 minutes ago, jaeger said:

Focus shift - your wife smacks your forehead while you looked away when she is talking to you.

'Instant Calibration' - John Lennon's first draft...

[bdolzani]

1 minute ago, Al Brown said:

There is already a full thread on that.

I'll re-read but yes I was surprised they said that. 

[ibramr]

I wonder if focus shift could be defined as “the change in the point of focus as aperture changes and this change can not be compensated for by the corresponding change in the depth of field.”  Regards.  

Edited September 27, 2022 by ibramr

[UliWer]

Yes, this definition is helpful for practical photographic purposes. Though one has to keep in mind that the depth of focus just covers the fact that the point of exact focus changes by changing apertures. It does so in any lens. Though fortunately the change of exact focus is neglible for many lenses. 

[Olaf_ZG]

Very simple: it’s the reason why my image isn’t sharp, it surely couldn’t be me, so it must be focus shift 😎

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Hallo,

Look here: Leica M

[bdolzani]

1 minute ago, Olaf_ZG said:

Very simple: it’s the reason why my image isn’t sharp, it surely couldn’t be me, so it must be focus shift 😎

Or, 'when the lens is smarter than you'