Understanding focus shift

[Csacwp]

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I’m confused by an aspect of focus shift, so please correct me if I am wrong:

 

If I focus wide open on some lenses and then stop down before taking the photo, depending on the lens there might be focus shift.

 

But what about if I focus while the lens is already stopped down? Would there still be focus shift?

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[wattsy]

If you are using an M camera and the rangefinder then you will likely be focussing with the lens already at the appropriate F stop – it doesn't make sense to focus wide-open and then stop down for the photograph (as traditionally occurs when using an SLR with an auto or manual diaphragm). That doesn't mean that focus shift cannot be an issue because M lenses with distinct focus shift will be optimally calibrated only for a certain part of the aperture range.

Edited February 6, 2018 by wattsy

[jdlaing]

I’m confused by an aspect of focus shift, so please correct me if I am wrong:

If I focus wide open on some lenses and then stop down before taking the photo, depending on the lens there might be focus shift.

But what about if I focus while the lens is already stopped down? Would there still be focus shift?

No.

 

Focus shift is most common in lenses that are stopped down by the camera as in Dslr and mirrorless where the lens is focused at a wider aperture and when the shutter button is pressed the camera stops the lens down.

 

The last thing I do before pressing the shutter is focus. Moving the aperture ring after focusing it is too easy, for me, to accidently move the focus ring.

 

I think focus shift in a rangefinder lens is a misnomer.

[01af]

If you're focusing by looking through the stopped-down lens then there won't be any focus shift. This is possible with SLR viewfinders and—preferably, as SLR finders will get dark upon stopping down–with electronic viewfinders.

 

When focusing at one aperture and shooting at another (which is the usual case with SLR finders), or when focsing via rangefinder, then focus shift may occur.

[UliWer]

Focus shift means that the level of focus and maximum sharpness is not the same for all f-stops of your lens - it shifts forward or backward when you change the f-stop.

 

When you focus directly through the lens as you do with a SLR-camera or with Liveview the focus and sharpness you see in your finder or on your display is identical with the the focus and sharpness of your lens and on your photo - even if the focus shifts in the lens, you may see it in the finder (if the finder and your eyes are good enough), as long as you don't change the f-stop after focussing.

 

Rangefinders are different. You don't look through your lens to see where it focusses but just through a rangefinder which is independant of your lens. What the rangefinder indicates should be identical with your lenses view and it can be identical - better than your eyes. But the exactness of the rangefinder depends on how the lenses mechanism to activate the rangefinder was calibrated. If the lens shifts a lot for different f-stops your lens-rangefinder-combination may be calibrated for maximum opening or for say f/2.8.

 

Now when you follow your rangefinder when it is calibrated for f/1.4 and you stop down to f/2.8 the focus of your lens will differ from what your rangefinder shows. Same vice versa.

 

One has to find out, whether a lens has considerable focus shift, and how the lens activating the rangefinder is calibrated by testing. With new lenses from Leica the exact focus you see in the rangefinder should be the focus of maximum aperture. With older lenses it may be exact stopped down two values.

 

Focus shift is not generally a problem. Tiny faults with focussing or little movements of the body etc may have much more effect than focus shift. Highly opened older lenses (first version of 35mm Summilux asph,  Noctilux 1:1) had obvious focus shift. Newer versions of these lens-types still have some, but it doesn't really make so much difference.    

 

    

 

 

 

 

[Michael Geschlecht]

Hello Everybody,

 

In "M" cameras: Turning the focusing mount moves the cam in the lens mount along the surface of the roller in the camera body.

 

Because this cam varies in thickness along its surface: It moves the roller from closer to the film/sensor plane to further from the film/sensor plane, & back again, as the lens is focused.

 

The mechanism connected to this moving roller activates the rangefinder mechanism that appears as the moving rectangle in the range/viewfinder window.

 

There is no connection between this roller system in the camera body interacting with the cam at the base of the focusing mount of the lens:

 

And the aperture ring on the lens in Leitz/Leica "M" lenses. Or their Leitz Screw Mount predecessors.

 

Whether the aperture is set before or after focusing has no effect on focus shift.

 

Focus shift is the change of the position of the "plane of best focus", relative to the film/sensor surface, when: The lens has been focused to a specific distance on the lens barrel (ie: 1.5 meters.) and: Then the actual "plane of best focus" moves as the aperture is opened or closed.

 

Best Regards,

 

Michael

Edited February 6, 2018 by Michael Geschlecht

[pico]

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Just to clarify what Michael wrote: there is a part of the lens that pushes against the rangefinder roller in the camera. It pushes straight although with some lenses we might imagine that the lens's focusing surface turns. It is tempting to think it does turn with a curving surface to accommodate focus differences/shift. That wouldn't take care of shift that occurs at different ƒ-stops. (Well, at least I went through such confusion long ago.)

 

Michael's native language is English. Mine is _english.

 

Thank you, Michael.