Is this focus shift or lens just out of whack?

[nek724]

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Hello all,

 

So I did some experiment (by accident) with two of my lenses that I had since probably almost a decade now. They're the 28mm Elamrit ASPH v1, and 50mm Summicron v4. I shot with them for years on film M6 and thought they're good, unless I misfocus of course. However, recently acquired an M11, and messing around with them on liveview it's giving me another look.

 

Firstly the 28mm, I placed a figurine in dead center and shot it at f2.8, focus at 0.7m and it seemed perfect. I did the same settings, but move the camera, placing the figurine in the right corner and took the same shot, but the figurine is out of focus. At first, I though this is what people called corner sharpness etc, but I see the focus area on the table I was working on so I tried to find the focus area, and turns out it was behind the figurine. I then pulled out the measuring tape, and turns out I achieved focus on the figurine when the camera was at 0.85m. I tried the same with the 50mm and same, it shifted when the figurine is in the corner, but not as bad at just 1cm so 0.71m.

 

I'm imagining the focus area of the 28mm like a U shape now. The center focus is at the bottom tip of the U, then corners focus is behind if that makes sense.

 

Or is this normal?

 

Thanks!

Kenneth

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[Stuart Richardson]

Yes, that is field curvature, but usually it is the other way around (focus in the corners is closer to the camera). Sharpness is not necessarily a complete flat plane. Lenses that are can be called flat-field lenses. Older Leica lenses tend to have more field curvature than the latest APO designs. It is also often greater in wide angles. If you google it, I am sure you will find a good explanation.

I will say, it could also be something to do with the distance to the camera of the figure, depending on how you moved it. I think it is easier to test this by photographing a 3 dimensional landscape and seeing where the plane of focus lies.

Edited January 16 by Stuart Richardson

[nek724]

@Stuart Richardson Thanks for the explanation! Using film, you have time that go by so one would have missed this detail, but with liveview, I could learn and be aware to avoid this next time, but that is annoying if I focus on something and recompose, now knowing it's not easy as just recompose and shoot. However, the corners seems darken shooting wide open anyways in real world scenario so might not be so noticeable to the viewer.

39 minutes ago, Stuart Richardson said:

it could also be something to do with the distance to the camera of the figure, depending on how you moved it.

I didn't touch the figurine. Only move the camera to the left, so placing the figurine to the right of the frame.

39 minutes ago, Stuart Richardson said:

test this by photographing a 3 dimensional landscape and seeing where the plane of focus lies.

If focusing at near infinity I think it wouldn't be this bad, I'd assume. However, this test was only shot at 0.7m at wide open, I think the DOF is at it's extreme.

Edited January 16 by nek724

[Stuart Richardson]

9 minutes ago, nek724 said:

I didn't touch the figurine. Only move the camera to the left, so placing the figurine to the right of the frame.

That's what I mean, by moving the camera you might have slightly moved the figure out of the focus plane. Depending on the angles etc, there might have been a slight change. It is hard to test for this if you move the camera (even rotate). Best to test at a normal shooting distance with a three dimensional scene where you can keep everything consistent.

[nek724]

10 minutes ago, Stuart Richardson said:

That's what I mean, by moving the camera you might have slightly moved the figure out of the focus plane.

Right, so now, just recomposing the angle, gotta step back 15cm (one step), that's not what I expected to do.

[lct]

A bit of focus shift is normal at about f/4 on Summicron 50/2 v4 & v5.

[Jeff S]

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1 hour ago, lct said:

A bit of focus shift is normal at about f/4 on Summicron 50/2 v4 & v5.

But, since he didn’t change aperture, focus shift is not an issue with his test as conducted.  The title of the thread suggests the OP might not understand focus shift. More like lens shift.

[nek724]

When I first noticed this, I was thinking is this what people refer to as soft corners? But that can't be because it's like out of focus and I'm standing at the correct distance. Well, different lens with different method to be used correctly. I'm still in disbelief with the 28mm, 15cm in difference!?

 

Same time, recently I've been eyeing the 28mm Summicron v2. What suppose to be sharper corners they say. Would that mean it'll fix this issue? Anyone with 28mm Summicron v2 can let me know? Thanks!

[lct]

Depends on focus distance and aperture too. If you want sharp corners with either Elmarit 28/2.8 asph v1 or Summicron 28/2 asph v1 or v2, better choose f/8 at least, preferably f/11. F/16 would be softer due to difraction. As for Summicron v1 and v2, i have both versions but hardly see significant differences re sharp corners. I rarely focus at infinity though, so that could be the reason. BTW @nek724, you may wish to show a photo here, it will be easier to understand your problem, if any.

[Jean-Michel]

Testing lenses can be fraught with all kinds on inconsistencies. 

Some years ago, I built a target to a check and adjust the micro-focus adjustment on my then Canon 5d2 and lenses. I later used the same to check M lenses and an M9.

You can buy commercial targets, or, as I did, download a do it yourself target from : https://squit.co.uk/photo/focuschart.html

I printed the target on photo paper and dry-mounted onto 4 ply mounting board material. You do need to be careful in assembling  the target.

I suppose that you could build a second one and position it at the left to right edge of the image frame to see how much the focus  plane moves forward or rearward at the edge.

In my case, the only M lens that needed an adjustment was my version 1 35 mm Summicron (with goggles). I must have used it for decades and be unaware of the small maladjustment as I mostly used it at f/4 or smaller, and only printed to 8 by 12 inches on 11  by 14 paper — larger prints meant using medium or large format cameras. At the time, I sent the lens to Dan Goldberg (DAG) and he fixed it very nicely.

 

 

[adan]

"Focus shift" has nothing to do with corners vs. centers.

It is strictly about the plane of focus (in the exact same part of the picture) shifting/changing with different apertures. (•) = constant place focused on.

f/1.4   -----------------|---•-----------  front focus
f/2.0  ---------------------|-----------  correct focus
f/2.8  ---------------------•---|-------   back focus

It comes from spherical aberration, in which the outer edges of the lens focus at one distance, and the center of the lens focuses at a different distance (with a continuum in between).

Progressively stopping down the aperture removes the light coming from the edges of the lens, so that the location of the "best apparent sharpness" shifts/changes its distance.

Lenses at the factory can have their focus-cam adjusted, to provide "best apparent sharpness" at any one specific aperture a user finds "most convenient and useful." 

Some users may prefer correct focusing at f/1.4, and again at f/4-f/5.6 (where DoF hides any focus error).

(Which is my own preference - if I pay for an f/1.4 lens, I want to use it accurately at f/1.4 in dim light - or stop down a lot (e.g. to f/6.8) in bright light anyway. I have no use for the "in between" apertures of f/2.0 or f/2.8 or f/4.0. 😉 )

Others may prefer calibration for f/2.0, and make little mental/finger corrections for f/1.4 or f/2.8, from experience.

[evikne]

I once made this test that revealed focus shift on one of my lenses. After a CLA at Leica it was calibrated as in Adan's example, with perfect focus at f/2. Because the lens is of an older design and not quite optimal wide open, I prefer to use it at f/2, so it suits me perfectly. 

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Edited January 17 by evikne

[Olaf_ZG]

I have a question, if one focus f.e. at 2.8, and doesn’t change aperture. Will focus be correct or impacted by focus shift?

Or does focusshift only occurs when - after focusing - one changes aperture? Thanks.

[Kl@usW.]

vor 5 Minuten schrieb Olaf_ZG:

I have a question, if one focus f.e. at 2.8, and doesn’t change aperture. Will focus be correct or impacted by focus shift?

Or does focusshift only occurs when - after focusing - one changes aperture? Thanks.

Yes. ‘Shift’ refers to the change in the plane of focus that occurs after focusing has been performed at a given aperture and then the aperture is subsequently changed.

[TomB_tx]

4 hours ago, Olaf_ZG said:

I have a question, if one focus f.e. at 2.8, and doesn’t change aperture. Will focus be correct or impacted by focus shift?

Or does focusshift only occurs when - after focusing - one changes aperture? Thanks.

A lens with significant focus shift will be in focus accurately at one fstop, but not at others. For example, my original Nokton 1.4 (ver 1) would focus accurately at f1.4, but would miss best focus by f2.8. It would shift a bit more at f4.  By f8 all would look pretty good due to DOF increase. If using live view (which I didn't have back then) you can focus accurately at any fstop, but if you change fstop after focusing on LV it will shift.

That said, I get a lot of good pictures on lenses with focus shift, if I just ignore the issue. Lots of other factors affect the image, and focus shift is a minor one, I loved that Nokton 1.4 before I read about the focus shift... BTW - shift is not an issue with the ver 2 Nokton 35 1.4.

[Jean-Michel]

7 hours ago, Olaf_ZG said:

I have a question, if one focus f.e. at 2.8, and doesn’t change aperture. Will focus be correct or impacted by focus shift?

Or does focusshift only occurs when - after focusing - one changes aperture? Thanks.

Hi,

There is no correlation between the focus you achieve with the rangefinder and  the f/stop the you select. The aperture ring is not connected to the moving lens barrel. Whatever focus shift is there at a given f/stop will still be there.

As TomB_tx writes, if you use live view you can focus more accurately at any f/stop. Of course, if you set your lens at, say, f/5.6 or f/8, exact focus becomes a bit of a guess since the depth of field will show some parts of the image in front and behind the actual plane of focus as in focus. 

[lct]

55 minutes ago, Jean-Michel said:

[...] if you use live view you can focus more accurately at any f/stop. Of course, if you set your lens at, say, f/5.6 or f/8, exact focus becomes a bit of a guess since the depth of field will show some parts of the image in front and behind the actual plane of focus as in focus. 

Not a problem with focus magnification. I can focus WA lenses at working apertures up to f/11 this way. Here Summicron 28/2 at f/8, FF & crop.

Edited January 18 by lct