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Author Topic: Does focus shift vary with distance?  (Read 9967 times)

AFairley

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Does focus shift vary with distance?
« on: June 05, 2015, 12:52:48 pm »

Does aperture change induced focus shift vary with distance?  I.e., can a lens exhibit focus shift at far distances but not close distances or vice versa?  I'm wondering whether an auto-focus correction value for a focus-shifting lens determined for a specific aperture will be good over a range of distances or only at the tested distance.
« Last Edit: June 05, 2015, 12:57:22 pm by AFairley »
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kers

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Re: Does focus shift vary with distance?
« Reply #1 on: June 05, 2015, 01:30:12 pm »

What i noticed is that wide angle lenses have sometimes very awkward areas of sharp/unsharp, not simply to grasp with a curve.
Yes i found it does make a difference where you put the focus.
you will learn a lot form a lens by making a serie of photographs of a suitable subject and change focus from close to beyond infinity.
at every focuspoint make a serie from wide open to f16.
You will be surprised by the outcome...
Normal and telelenses usually are more predictable.
« Last Edit: June 05, 2015, 01:39:02 pm by kers »
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Pieter Kers
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AFairley

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Re: Does focus shift vary with distance?
« Reply #2 on: June 05, 2015, 04:36:29 pm »

Thanks, Pieter.  I am aware of the variable curvature of the focus plane with some designs - the lens I am specifically interested in, the Nikon 28mm f1.8G has such a wavy MTF curve.  But I am specifically interested in focus shift at the center of the lens in this case.
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kers

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Re: Does focus shift vary with distance?
« Reply #3 on: June 05, 2015, 05:58:10 pm »

The 28mm i don't know...
The nikkor 24mm 1.4 par example @f2.8 is at some close focus point able to be sharp  from close to infinity but only in the centre... the sides are blurred. Good to know if you need that.
On the other hand it can have a reasonable flat focus at infinity @ f2. Not something you would predict or find out in the shop.
Focus is crucial and very sensitive with this lens.
Do not know what you are looking for but maybe try the sigma 24mm 1.4. i could use it for a short period.
It is very straightforward. ; @ near infinity :sharp in the centre @1.4.  this central sharp part becomes larger when stopping down and @ f5.6-8 it is completely sharp. So @ 1.4 you have a aberration free very sharp centre part. say 15mm. Must say autofocus is not very fast. and some reviewers find the autofocus not consistent- do not know about that. Something you learn after spending some time with the lens.
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Pieter Kers
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AFairley

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Re: Does focus shift vary with distance?
« Reply #4 on: June 05, 2015, 09:01:45 pm »

I'm talking about focus shift in the sense that if you focus on a point wide open and then stop down, the point of focus at that point in the frame moves forwards or backwards.
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Some Guy

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Re: Does focus shift vary with distance?
« Reply #5 on: June 06, 2015, 01:22:58 am »

Diffraction plays into it with the aperture.

See this:  http://www.reikan.co.uk/focalweb/index.php/2013/06/lens-focus-shift-reikan-focal/

I know with the Sigma ART AF tuning dock, they allow for setting up 4 tuning ranges for the AF over four given distances with their ART prime lenses.

SG
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kers

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Re: Does focus shift vary with distance?
« Reply #6 on: June 06, 2015, 03:56:24 am »

do not know about the 28mm1.8-
I have only one lens with considerable focusshift_ the 14-24 f2,8.
From 2.8 to f4 the focus jumps backwards - after that it is stable.
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Pieter Kers
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Bart_van_der_Wolf

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Re: Does focus shift vary with distance?
« Reply #7 on: June 06, 2015, 04:21:06 am »

I'm talking about focus shift in the sense that if you focus on a point wide open and then stop down, the point of focus at that point in the frame moves forwards or backwards.

Yes, there are two different, but related, kinds of issues. The one you describe, when leaving (manual) focus untouched, stopping down will move the plane of best focus. This has to do with the changed contribution of edge lens rays to the focused image. Another issue is that the AF mechanism may pick a different focus plane wide open, then stopping down to the actual working aperture will result in a different focus plane.

This is caused by a combination of issues, where the AF sensor also is affected by the focus it detects based on edge rays, which would be different from focus detected while physically being stopped down, or when contrast detection in the center would be used. Field curvature is part of these issues, and also the fact that phase detect AF uses edge rays to do it's depth detection (just like a range finder with two viewfinder images taken from a different angle that need to be aligned).

Cheers,
Bart
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Rhossydd

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Re: Does focus shift vary with distance?
« Reply #8 on: June 06, 2015, 05:33:27 am »

Yes, it can change.
If you think you have an issue with auto focus accuracy and have a camera that allows you to fine tune the AF accuracy, do have a look at Reikan FoCal. It will perform tests on issues like this and the results are well presented for you to make an informed choice of what might be the best settings to choose.
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Bart_van_der_Wolf

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Re: Does focus shift vary with distance?
« Reply #9 on: June 06, 2015, 06:51:41 am »

Yes, it can change.
If you think you have an issue with auto focus accuracy and have a camera that allows you to fine tune the AF accuracy, do have a look at Reikan FoCal. It will perform tests on issues like this and the results are well presented for you to make an informed choice of what might be the best settings to choose.

Yes, AF micro-adjustment will tell what to do for a particular working aperture relative to the AF wide open aperture, but it won't help much if one cannot set multiple corrections for multiple working apertures if the lens exhibits a lot of focus shift. The 50mm F/1.2 comes to mind... It would allow to find a common middle ground for the most frequently used apertures though, which is already something.

Cheers,
Bart
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Rhossydd

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Re: Does focus shift vary with distance?
« Reply #10 on: June 06, 2015, 07:52:11 am »

Yes, AF micro-adjustment will tell what to do for a particular working aperture relative to the AF wide open aperture, but it won't help much if one cannot set multiple corrections for multiple working apertures if the lens exhibits a lot of focus shift.
Agreed, but FoCal can help show where the problems are and that allows you to make an informed choice about what correction to make, if any.
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bjanes

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Re: Does focus shift vary with distance?
« Reply #11 on: June 06, 2015, 09:07:56 am »

Yes, it can change.
If you think you have an issue with auto focus accuracy and have a camera that allows you to fine tune the AF accuracy, do have a look at Reikan FoCal. It will perform tests on issues like this and the results are well presented for you to make an informed choice of what might be the best settings to choose.

Focus shift also occurs with manual focus and is caused by spherical aberration, which tends to be most severe with fast lenses and is mitigated by stopping down, which eliminates rays from the peripheral areas of the lens. See vanwalree.com for an excellent explanation. Note that spherical aberration is different from curvature of field in which case the center of the image can be sharp and the edges are out of focus. Spherical aberration affects the entire image.

Diglloyd (paid site) also has an excellent discussion with many examples. To answer the OP's question, Diglloyd states that focus shift does vary with distance and is usually more severe at close distances with most lenses, but can be more severe at distance with some designs.

Bill
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AFairley

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Re: Does focus shift vary with distance?
« Reply #12 on: June 06, 2015, 12:28:19 pm »

Thanks all for posting, but only Rossyd actually answered my original question, which was whether the degree of aperture-change-induced focus shift (in a lens that exhibits it) varies with the distance to object focused on.  And no one has answered the question of whether, if the degree varies, this is true of all lenses that exhibit focus shift or only some.
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Rhossydd

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Re: Does focus shift vary with distance?
« Reply #13 on: June 06, 2015, 12:34:59 pm »

And no one has answered the question of whether, if the degree varies, this is true of all lenses that exhibit focus shift or only some.
I would have thought that it's pretty obvious that different lenses will perform differently. Manufacturing tolerances mean that no two lenses, even of the same design, will perform absolutely identically.
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bjanes

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Re: Does focus shift vary with distance?
« Reply #14 on: June 06, 2015, 01:55:25 pm »

Thanks all for posting, but only Rossyd actually answered my original question, which was whether the degree of aperture-change-induced focus shift (in a lens that exhibits it) varies with the distance to object focused on.  And no one has answered the question of whether, if the degree varies, this is true of all lenses that exhibit focus shift or only some.

Diglloyd (paid site) also has an excellent discussion with many examples. To answer the OP's question, Diglloyd states that focus shift does vary with distance and is usually more severe at close distances with most lenses, but can be more severe at distance with some designs.

Didn't you see my post where I reported that focus shift does vary with distance and is usually more severe at near distances? Lenses with f/2.8 or smaller (larger f/number) apertures usually do not have much focus shift and well corrected fast lenses will have less focus shift. According to Diglloyd, the Canon 85mm f/1.2L II shows negligible focus shift.

Bill
« Last Edit: June 06, 2015, 02:00:12 pm by bjanes »
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smthopr

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Re: Does focus shift vary with distance?
« Reply #15 on: June 06, 2015, 02:10:19 pm »

Interesting note: panavision primo motion picture lenses have 2 witness marks for focusing. One for the widest apertures, and one for everything else. I've never heard that subject distance is something to consider here, only iris size...
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kers

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Re: Does focus shift vary with distance?
« Reply #16 on: June 06, 2015, 02:19:45 pm »

As far as i know only Sigma ART with the USB Sigma Dock able to calibrate focus for i think 3 distances. I do not know par example a Nikon lens on a Nikon body changes autofocus depending on the distance of focus- but in theory it could be done.

if i want to be sure about the focus i use liveview @100% - with the convenient splitscreen on the d810- to check sides and centre-  and use it at the aperture i want to make the photograph with.
An other nice thing of the splitscreen is that it allows you to determine the depth of field more quickly.

« Last Edit: June 06, 2015, 02:23:24 pm by kers »
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Pieter Kers
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AFairley

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Re: Does focus shift vary with distance?
« Reply #17 on: June 06, 2015, 08:05:00 pm »

Didn't you see my post where I reported that focus shift does vary with distance and is usually more severe at near distances? Lenses with f/2.8 or smaller (larger f/number) apertures usually do not have much focus shift and well corrected fast lenses will have less focus shift. According to Diglloyd, the Canon 85mm f/1.2L II shows negligible focus shift.

Bill

Holy crap, I read your post and the last sentence didn't register!   :o. Thanks  for the Digiloyd reference.
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BrianVS

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Re: Does focus shift vary with distance?
« Reply #18 on: June 06, 2015, 08:38:37 pm »

Spherical aberration means that the focal length of the lens varies across the aperture. As you close the aperture down, the focal length of the center of the lens becomes dominant. The focal length marked on a lens is for infinity focus, as you focus closer the effective focal length of the lens increases. For a lens with a large degree of spherical aberration, this means the delta focal length across aperture will increase. So yes- the shift is greater with close focus as the focal length increases. 1/focal length= 1/backfocus+ 1/distance. Plug in the focal length for the edge of the lens and focal length for the center of the lens, the delta between them increases as you get closer to the subject.
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David Eichler

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Re: Does focus shift vary with distance?
« Reply #19 on: June 08, 2015, 02:47:54 am »

Thanks all for posting, but only Rossyd actually answered my original question, which was whether the degree of aperture-change-induced focus shift (in a lens that exhibits it) varies with the distance to object focused on.  And no one has answered the question of whether, if the degree varies, this is true of all lenses that exhibit focus shift or only some.

Just goes to show that one should be very careful about asking for advice on the Internet. But how do you actually know that Rossyd's information is correct? Just because he addressed your question most directly? In any case, I believe he has the correct answer to your inquiry.
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