Why does focus shift matter?

[fursan]

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I have always wondered why the discussion of focus shift? back/front focus..yes

 

I select an aperture ( for dof/light whatever ) and focus or use hyperfocus and shoot away.

 

I am missing something?

 

Regards.

[j. borger]

I have always wondered why the discussion of focus shift?

 

I select an aperture ( for dof/light whatever ) and focus or use hyperfocus and shoot away.

 

Regards.

A healthy attitude .... as long as your pictures are not out of focus for unpredictable reasons do not worry or even read about it!

[cme4brain]

I have always wondered why the discussion of focus shift? back/front focus..yes

 

I select an aperture ( for dof/light whatever ) and focus or use hyperfocus and shoot away.

 

I am missing something?

 

Regards.

 

There will be lots of answers to this question, and it has been mentioned in several threads. I am an amateur, so I may not understand the problem as well as others. Leica lenses, like all lenses, are made to a series of compromises. Lens abberrations (chromatic abberration, spherical abberation, coma, etc) are minimized for a focus point and set aperture, and focus points and/or apertures other than the one that is optimized have more abberations. Fast lenses like the Noctilux are optimized for wide open shooting, and its smaller aperture performance (still excellent) is felt to be a good compromize to obtain the best performance wide open.

 

Focus shift is where changing the aperture to a smaller one will actually shift the plane of focus very slightly to a point other than what the lens barrel or rangefinder says the focus point is. This is apparently prominent in the Leica 35mm f/1.4 lens. LFI magazine has a great article on this. Stopping down the lens from maximum one stop or so will move the focus point (which changes due to the aperture) to some place other than where it should be. This (usually) one or two stop smaller aperture is not enough to have the increased DOF minimize this focus shift. At even smaller apertures, the DOF will make up for the focus shift. Back/forward focus seems to be an error in either the lens barrel moving of internal elements or the rangefinder gearing mechanism where you think the lens is focused on a particular point but it is actually focused forward or behind this point slightly. On the M8 where the focus plane is an absolutely flat image chip (and not a thick film emulsion), this error is easily seen, and more so because you can immediately view the picture realtime. With film that took hours or days to get back, you would not have remembered perhaps that you exactly focused on a particular spot and would chaulk up the focus error some other way. It seems that many Leica owners have sent their lenses and M8 bodies back to Solms for calibration. These focusing errors are less of a problem in non-autofocus SLR cameras as you view the image live and correct for any focus shift. I would imagine that DSLR's that autofocus at wide open also have this focus shift error, unless they autofocus at shooting aperture (which they do not).

 

I do not worry about this. There are many reasons for me not worrying about it. One is that my 48 year old eyes cannot absolutely within arc-minutes of error tell that the rangefinder patch is PERFECTLY aligned anyway. Also, there is inherent gear lash in any complicated focus mechanism- both in the lens barrel moving the elements back and forth and in the rangefinder mechanism responding to this movement. If you will read Sean Reid's articles where he rates lenses, you find that he has to take many focus tries at an object to maximize resolution- he even notes that moving the lens barrel ever so slightly- that even the rangefinder patch does not move (showing gear error/lash)- will affect resolution. Therefore there are many reasons for your focusing to be a millimeter off something you intend it to be. Even with a perfectly calibrated rangefinder and lens (they do not exist), I am sure most photog's eyesight (and lots of these guys I bet are over 40 years old) is worse than they think it is, and that is the biggest error to accurate focusing.

 

I won't buy a Noctilux for that reason- I can't depend on my eyes, the gear lash, or the lens mechanics (superb by the way, complements to Leica having the best rangefinder out there) to assure perfect focus, which I must have with that lens. I would say that perfect focus with that lens, for my eyesight and probably others, is half great focusing and half great luck.

[fotografr]

I have always wondered why the discussion of focus shift? back/front focus..yes

 

I select an aperture ( for dof/light whatever ) and focus or use hyperfocus and shoot away.

 

I am missing something?

 

Regards.

 

Actually, yes, you are missing something. When shooting at an aperture of f/1.0 at a distance of 1 meter, there is virtually no room for error. The DOF is about 1 inch. There is no way to shoot using the hyperfocal distance at these settings. If the rangefinder tells you focus is correct and actual focus is off by even a small amount, it will ruin the shot--particularly if you are doing a portrait and trying to keep the eyes in focus.

[Hookeye]

For fear of making an ass of myself; Does the focus shift matter if the increase in DOF is big enough to compensate for it? In Brent's case above - with f/1.0 at 1 meter you need precise focusing, I agree. But if you were to stop down to say, f/5.6 and your focus shift is e.g. 10 cm, but your DOF increase to 25 cm - would the focus shift be noticeable for the user?

 

This has been my belief up to now, -or have I too missed something?

[fotografr]

For fear of making an ass of myself; Does the focus shift matter if the increase in DOF is big enough to compensate for it? In Brent's case above - with f/1.0 at 1 meter you need precise focusing, I agree. But if you were to stop down to say, f/5.6 and your focus shift is e.g. 10 cm, but your DOF increase to 25 cm - would the focus shift be noticeable for the user?

 

This has been my belief up to now, -or have I too missed something?

 

Depending on the focal length you are dealing with, it is true that stopping down will make the shift much less noticeable. Obviously, this is more true with wide angle lenses than normal or telephoto. The reason the shift is so objectionable to many of us is that we buy lenses like the Noctilux so we can shoot with them wide open and enjoy that special bokeh the lens has. The 35 lux ASPH also has a very distinct look when used at f/1.4, and it is a shame to not be able to take advantage of that.

[zlatkob]

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When the increase in DOF is big enough to compensate for the shift, the shift doesn't matter. With some lenses, the increase in DOF is not quite big enough to compensate for the shift, especially with the subject at close distance and especially with the lens around f/4, and then it can be a problem. The solution is to either shoot with the aperture closer to wide open, or stopped down further, or compensate with a slight turn of the focusing ring, or shoot with the subject further away.

[fursan]

Thanks for your replies.

 

Brent,

 

If I use a fixed aperture ( no hyperfocus ), would not that be in focus ( baring front/back focus) .?

 

Thanks

[pklein]

The 35 lux ASPH also has a very distinct look when used at f/1.4, and it is a shame to not be able to take advantage of that.

 

Actually, it's the middle apertures that are the problem. My 35/1.4 Lux Asph is spot-on at f/1.4, and still within the DOF zone at f/2. At f/2.8, 4 and 5.6, the zone of sharpest focus is just barely behind the spot focused on by the rangefinder. It's subtle, but it's there, and it is visible in a 8x10 or even a 5x7 print. Does it really matter? Often it doesn't.

 

By f/8 we're back in the "best sharpness" zone again.

 

When using the "problem" stops, the easiest way to cope is simply to focus on the closest part of whatever you want in focus, or just in front of it. For example, focus on the tip of a person's nose instead of their eyes. You can get used to it, and once you do, life is good again.

 

Or buy new lenses that don't have discernable focus shifts.

 

--Peter

[pklein]

If you'd like to see the effect of the focus shifts, see this thread:

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/24784-m8-focus-shifts-back-focus-2.html

 

Scroll down until you see four pictures of a yardstick.

 

--Peter

[fotografr]

Thanks for your replies.

 

Brent,

 

If I use a fixed aperture ( no hyperfocus ), would not that be in focus ( baring front/back focus) .?

 

Thanks

 

Actually, your aperture is always "fixed" at the point where you set it, so I'm not sure what you mean by that. I'm guessing you mean you are just using the distance setting as opposed to finding the hyperfocal distance. The lens SHOULD be in focus at that point, but it is the focus shift that is causing the front/back focus in some of these lenses. At one aperture, the rangefinder tells you focus is correct and in the image, it will be. But at another aperture, when the rangefinder tells you focus is correct, it actually is not.

[sean_reid]

The answer depends on two things:

 

1) How dramatic is the focus shift?

2) How perfect must the focus be for a given picture.

 

For many of us, much of the time, a slight focus shift doesn't matter. A stronger focus shift might.

 

Cheers,

 

Sean

[Daniel Ortego]

...Or buy new lenses that don't have discernable focus shifts.

 

--Peter

 

So specifically, which lenses are you referring?

Regards,

[Albert4321]

The 35 lux ASPH also has a very distinct look when used at f/1.4, and it is a shame to not be able to take advantage of that.

 

So true, if one wants to stop down the aperture, one should buy the f/2 lens which has less or no focal shift problem, and cheaper + smaller.

 

This is a good thread. I read many focus shift discussion here but never really understood it coming from my Canon experience where everything is tag sharp, no question ask.

 

Now I understand why my 50lux is right on the dot Sharp at f/1.4 (which is more important to me) than stopping down.

 

I shot a bride solo portrait wide open at f/1.4 or f/1.8 and focus on the eye, the pic came out sharp and crisp. Two bride mate came in and sit slightly behind the bride, so I stop down to get more DOF. Guess what, the bride mates are in focus and the bride is not. Does it matter? In this case yes. I think the 35mm Cron would be better for me in this case.

 

But I have no hesitation to use the 50lux (or the 35lux next in my top shopping list) at any aperture

[fursan]

tell me if I got this right:

 

if I focus on an object at a given aperture, I should be ok. Now if I change the aperture

( focus being untouched and on the same object ), there is a possibility that my

original focus could be off.

 

However, if I change the aperture and refocus, I am back to the initial condition of

not having to worry about the focus shift. Correct or I need to apologise for being

thick..once more?

 

Regards.

[colorflow]

tell me if I got this right:

 

if I focus on an object at a given aperture, I should be ok. Now if I change the aperture

( focus being untouched and on the same object ), there is a possibility that my

original focus could be off.

 

 

However, if I change the aperture and refocus, I am back to the initial condition of

not having to worry about the focus shift. Correct or I need to apologise for being

thick..once more?

 

Regards.

 

No, this is not right. Focus shift means that your rangefinder patch is only accurate for a certain aperture, say 1.4, If you change your aperture to 2 or smaller and refocus, your rangefinder patch will be in focus but the optrics of the lens has shifted and you will get out-of-focus shots. i.e. the focusing patch and the actual optics are out of sync.

 

Remember you are not using an SLR. You are not focusing through the lens. The focusing mechanism in a range finder is not connected to or affected by the aperture setting

 

The Nocti does that and so does my Lux 50 asph. I have learned to compensate by turning the focus ring slightly after the patch is in focus. My other Leica lenses also have slight focus shifts, but not discernable in real life.

 

Alan

[sean_reid]

So true, if one wants to stop down the aperture, one should buy the f/2 lens which has less or no focal shift problem, and cheaper + smaller.

 

The 35 Summicron (current model) that I tested also showed minor focus shift. It tends to be more of an issue with fast lenses but it certainly is not limited to F/1.4 models. The 35 Biogon shows minor focus shift as well.

 

Cheers,

 

Sean

[sean_reid]

Remember you are not using an SLR. You are not focusing through the lens. The focusing mechanism in a range finder is not connected to or affected by the aperture setting

 

Alan

 

Exactly.

 

Cheers,

 

Sean

[marknorton]

Keep in mind though that even if you use an SLR, you are still focussing with the lens wide open. My Nikon 85mm f1.4 is a fine lens but it exhibits mild focus shift at the working aperture in just the same way that the lenses we talk about here do.

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Hallo,

Look here: Leica M

[fursan]

Alan,

 

Thank you so much. Got it at last!

 

No, this is not right. Focus shift means that your rangefinder patch is only accurate for a certain aperture, say 1.4, If you change your aperture to 2 or smaller and refocus, your rangefinder patch will be in focus but the optrics of the lens has shifted and you will get out-of-focus shots. i.e. the focusing patch and the actual optics are out of sync.

 

Remember you are not using an SLR. You are not focusing through the lens. The focusing mechanism in a range finder is not connected to or affected by the aperture setting

 

The Nocti does that and so does my Lux 50 asph. I have learned to compensate by turning the focus ring slightly after the patch is in focus. My other Leica lenses also have slight focus shifts, but not discernable in real life.

 

Alan