Focus Shift

[LeicaFoReVer]

I read an article in a photo magazine about the focus shifts in the lenses when shot in wide apertures. To be honest I wasnt aware of that issue before that detailed and I was surprised that it is still not understood perfectly in the literature.

It says (and shows by a test) that the focus you adjusted before you shot with wide apertures like 2 or larger apertures, shifts in the resulting image due to the affect of light waves from the outer periphery of the lens glass. I could not explain it good but I am sure you are already aware of the concept.

So what I wanted to learn is how much is that of an issue in fast RF lenses (compared to SLR lenses), like noctilux or Canon-Nikon fast lenses. The test was considering only Canon 50mm and 85mm 1.2 for SLR bodies.

 

[David William White]

Sounds like *someone* was having a problem focusing!

On an SLR, you always focus wide open, and through the lens, so I don't get that. Is the article online? Can you cite the reference?

 

[hans voralberg]

How can you have focus shift on SLR lens when you're focusing what you're seeing? The point of focus on the focusing screen can't "shift" from the one landed on the film.

 

[LeicaFoReVer]

Sounds like *someone* was having a problem focusing!

On an SLR, you always focus wide open, and through the lens, so I don't get that. Is the article online? Can you cite the reference?

Well it is on photography techniques magazine, they dont have online article though,

http://www.phototechmag.com/current.htm

 

[LeicaFoReVer]

How can you have focus shift on SLR lens when you're focusing what you're seeing? The point of focus on the focusing screen can't "shift" from the one landed on the film.

Well dont ask me 😀 It is what he says...and he says it is a well known issue,

According to test the focus can shift up to 1.5 inches! in some lenses!

 

[Roberto]

For example Noctilux has a very big focus shift, aspherical lenses have less...

I found this great article on line, but unfortunately it is italian only..
http://www.luciolepri.it/lc2/marcocavina/articoli_fotografici/TEST_Leica_M_50mm/00_pag.htm

Maybe Marco is reading us and can translate... 🙂
Rob.

 

[David William White]

I coudn't get to the article Chambers wrote (you need a password), but I found some information on his blog about this supposed effect. Kind of have to read between the lines, but what he is saying is that if he uses his auto-focus at his widest aperture and then stops down one or two stops and uses his auto-focus again, the point of focus selected by the camera is slightly different.

He claims that spherical aberrations in the fastest lenses causes a "halo" effect which confuses digital auto-focus systems. Tough nuggies for dSLR auto-focus junkies, I guess.

Now: "focus shift" is a bonafide concept known especially to people that shoot IR film. Lenses focus infrared closer than visible. Could it be that the near-IR focusing aid in digital SLR systems are the culprit?

Does this have anything to do with non-auto-focus systems?

 

[Roberto]

Let me try to use Marco's images to explain what I understood from the article and to make a long story short:

Moving from f1,0 to f2,0 and f2,8 the focus has a backward shift as you can see looking at the "OLIO FIAT" writing on the right hand side of the wheel.

The area focused with the rangefinder get ini focus again only at f5,6..

Rob.

 

[ferider]

Focus shift can be an issue for both RF and SLR lenses. When closing aperture of a lens, due to spherical aberrations, the effective focal length changes.

With an RF camera the lens' focus mechanism is typically optimized for a given aperture.

On an SLR, when focusing wide open, and then later (auto-)closing down when taking the photo but not changing the focus, the lens might shift focus. Some autofocus systems automatically correct for shift, like in the Hexar AF.

When a focus/lens system correctly focuses wide open, the additional DOF when closing the aperture often covers focus shift.

Some lenses do this more than others. Sonnars shift noticably. Other lenses known for this are the Noctilux, some of the Leica Summilux lenses, etc.

Typically it is easy to predict once you have "learned" your lens.

Apparently, digital sensors are more sensitive than film to focus shift.

Roland.

 

[David William White]

All I can say is this concept will be a tough sell to anyone who regularly uses a view camera or an enlarger. It is not a basic optical effect.

 

[Roberto]

What were these shot with?

Per Marco Cavina's web page (link is above in my post):

Leica MP n° 2.890.952
Leica Noctilux-M 50mm f/1,0 n° 3.533.952

 

[john_s]

All I can say is this concept will be a tough sell to anyone who regularly uses a view camera or an enlarger. It is not a basic optical effect.

Whether it be "basic" or not, it happens. It is far worse with wide aperture lenses. The most dramatic example with which I'm familiar is the Zeiss 50mm f1.5 Sonnar-C, where the effect is huge. Depth of field comes nowhere close to making the effect insignificant.

I think I'm right in saying that Ansel Adams in "The Camera" or another book describes it as somehing to be aware of even in LF lenses. I will have a look when I get home.

 

[john_s]

In the real world of lenses there are all sorts of effects outside the basic lens equation that we learned about at school. There are all sorts of compromises made in the design and manufacture of lenses. On Erwin Puts's web site there is diagram showing the rays of light converging to focus at the film surface. It is not a pretty sight! There is no best point of focus: a compromise is chosen in the design of the lens/body system, so that the performance at one setting is not absolutely optimised at the expense of performance at other settings. This is worse with wide aperture lenses.

There are elaborate designs that minimise the problem, like the Summilux ASPH 50mm, but they are expensive and heavy as well.

 

[LeicaFoReVer]

Well guys thank you very much for the input. It is definetely interesting and you see how some of us did not know the issue at depth.

So we learned that the aspherical concept is for minimizing this effect of spherical abbreviations, and it can be less in film cameras, and some autofcus systems take this into consideration while other not.

For sure it is a bigger problem in the spherical and fast lenses. That was my idea to open this thread, as we all love the fast RF lenses 🙂 Although I dont have one yet...

At the end it is not difficult to understand if you know the basic optics...All lenses have to the abbreviations otherwise how to scale down the real world scale thing into smaller media...

Exactly as it was said you have to be familiar with the lenses you have for taking precautions. Some people I remember were focusing to the nose at widest aperture know the fact that the focus will move to the eye...

If any of us can find an internet site in english please post here...

 

[LeicaFoReVer]

Well, where the focus initially set to? To the olio writing?

Let me try to use Marco's images to explain what I understood from the article and to make a long story short:

Moving from f1,0 to f2,0 and f2,8 the focus has a backward shift as you can see looking at the "OLIO FIAT" writing on the right hand side of the wheel.

The area focused with the rangefinder get ini focus again only at f5,6..

Rob.

 

[ferider]

If any of us can find an internet site in english please post here...

Here is a nice intro with a good visual example:

http://diglloyd.com/diglloyd/free/FocusShift/index.html

I once "measured" for the ZM C-Sonnar here:

http://ferider.smugmug.com/gallery/2867043_GcEYX

I agree that in general ASPH show less shift. But not all of them (the Summilux ASPH 35/1.4 seems to be frustrating for some M8 users ...).

The Summilux 50/1.4 ASPH not only has aspherical elements, but also floating elements and apo-chromatic glass. The floating element helps, too.

Note that there are other aberrations that can be just as bad in practice: field curvature, for instance.

Cheers,

Roland.

 

[LeicaFoReVer]

Thanks Ferider...

I am a little bit confused though :bang:, does that thing happen when you focus at wider aperture and shoot at smaller apertures 2-8? In that case why we dont see the change in focus in the viewfinder? because it is too little? Or it changes after projected on film/sensor?

Or it can happen with those lenses even if you focus and shoot in the same aperture?

Well if it first one, he suggest focusing and shooting in the same aperture but how can you be sure for the change you dont see through viewfinder...

I think I am confused 😀

 

[LeicaFoReVer]

Here is a nice intro with a good visual example:

http://diglloyd.com/diglloyd/free/FocusShift/index.html

I once "measured" for the ZM C-Sonnar here:

http://ferider.smugmug.com/gallery/2867043_GcEYX

I agree that in general ASPH show less shift. But not all of them (the Summilux ASPH 35/1.4 seems to be frustrating for some M8 users ...).

The Summilux 50/1.4 ASPH not only has aspherical elements, but also floating elements and apo-chromatic glass. The floating element helps, too.

Note that there are other aberrations that can be just as bad in practice: field curvature, for instance.

Cheers,

Roland.

In your test I am not sure I understood where the shift is...could you explain where you focused and with which aperture?

Cheers,
Aykut

 

[Roberto]

Well, where the focus initially set to? To the olio writing?

The focus point was the wheel, but it's easier to look at the writing in my opinion.. and they shall be at the same distance (anyway you have to look at the wheel if you want to have the exact focus point)

 

[David William White]

I must admit to complete ignorance of this matter. I don't own any auto-focus SLR's and my fastest rangefinder lens is f/2. If true, this must be a real bummer to anyone who spends those really big bucks on a lens!