Claude

55-200 mm Stepping down moves the focus point backward

12 posts in this topic

Hi,

I bought a 55-200 mm lens which is very sharp.  I tested the lens in a live show where the lighting was pretty strong (not to dark e.g. 3200 ISO, f/5,6 1/200 sec). I noticed that in manual or autofocus mode with single point focusing (with theXT-2 stick), the focus is often wrong. I did some tests with a tripod afterward. When I shoot at full aperture, focusing is done at the right spot where the focus point is. By stepping down the aperture and taking another picture (I did let the AF system do it's job for each aperture chosen), I notice a progressive blur on the foreground where focusing is done with an increase in the detail of the background. It is as if the whole thing shifts progressively backwards with an increase in depth of field.

Is this shift in focus and lack of sharpness on focus point while stepping down is common ?  (I mean the shift in focus - An increase in depth of field is normal)

 The situation does not occur if I use multiple point.  The focus is better if the focusing square is smaller thant the subject

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What you have discovered could be "focus shift", where the focus distance varies with aperture.

In Sony's E mount cameras, the behaviour of the aperture during focusing varies by lens model.  Where possible, the camera will focus a lens with the aperture wide open, as this makes focusing faster since it lets the most light onto the sensor,  In lenses that shift focus with aperture, the cameras seem to close down the aperture to the exposure setting before focusing, which unfortunately can slow focusing.

It would be interesting to confirm:

- does another example of the lens behave the same way on the another X-T2 (if so, it's not just a problem with your camera and lens)

- does the lens behave the same way on another body (if so, it's not an issue specifically with the X-T2)

Whatever you find should be reported to Fuji, as it may be another firmware defect.

If it turns out it's just your camera and lens, then obviously you will need to send it for service (I think this is unlikely).

Before you do anything, I would suggest resetting you camera, and then attempting to update the firmware for both the lens and the camera body to make sure everything is up to date.

 

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Thanks for your reply.

I'll do another test tomorrow with an x-a1 body.  I've checked the lens firmware which is up to date.  My X-T2 firmware is not the latest from end of march.  I'll keep you posted with my results,

Regards,

 

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Posted (edited)

Here are some preliminary results. I would have preferred better lighting but the weather did not allow me to do tests outside (sorry for the high ISO of 3200). A 200 mm focal length was used with the 55-200 mm zoom.  These photos were taken in single point mode on a Xt-2 (without upgrading to the latest firmware). The jpg is processed in lightroom without preset.

The focus was made on the nose of the pink sheep and the focusing square was included inside the sheep's head. I did let the AF system do it's job for each aperture chosen and photograph. Comparing the two photos we notice that the nose and the thread in the right eye is less in focus at f / 8 than at f / 4.8. There is a loss of texture in the sheep’s fur front. By observing the ruler, one observes a retreat of the focus of the scale 8-10 cm to cm to 12-15 cm or a recess of 5 cm approximately. This can make the difference between precise or blurred eyelashes and skin texture in a portrait or live shows as I often do. I do not know if the new firmware fixes this problem (if it is a problem or a normal situation).

Unfortunately the conversion to jpg makes this difficult to see and I’ll upload a detail of the face later.  The photos are cropped versions of the originals.

Let me know what you think.

F4_8 -  DSCF6512.jpg

F8 - DSCF6514.jpg

Here are close-up of the sheep's face...and I can't do it because I've Exceeded the 2mb limit…

Edited by Claude
Clarification

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Here are close-up of the sheep's face...and I can't do it because I've Exceeded the 2mb limit…

-------

Today I compared the results with the XA-1 and the 55-200 mm lens. The results are similar to those obtained with XT-2.

Tests with a Nikon D7100 and an 18-140 lens at 140 mm seems to indicate that the focus point  is maintained between full aperture and f / 8 aperture.

So I will test with the new firmware of the XT-2 and probably rent another 55-200 mm to be sure it's not a lens problem but I have doubt.  Here's why:

On another post I've tested a few lens and wrote: "the results at f/4 and f/5.6 did not produce any significant changes".  Now I understand why. Because the first test was done with single point autofocus and the focus point shift even with a flat surface.  At the suggestion of Veejay my other tests included manual focus. 

As Artuk pointed out it's unlikely a lens problem.  I'll keep you posted with my results.

 

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I doubt its a lens problem, as focus shift is a feature of some.lens design.

the camera focusing system.ought to compensate for this issue but clearly it isn't as you have found similar results on 2 bodies.

if you have other zooms lenses it would be interesting o repeat the test - though not all zooms will focus shift with aperture, it depends on their design.

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On 07/04/2017 at 5:10 AM, artuk said:

I doubt its a lens problem, as focus shift is a feature of some.lens design.

the camera focusing system.ought to compensate for this issue but clearly it isn't as you have found similar results on 2 bodies.

if you have other zooms lenses it would be interesting o repeat the test - though not all zooms will focus shift with aperture, it depends on their design.

I've read on this forum that some users have had a similiar experiences with wrong focus or blured photos.

I really hope this can be resolved but if it's part of lens design, I have doubts.  With previous tests I discovered that you need a higher shutter speed than before (other camera e.g. APS-C with lens design for full frame cameras)  to reduce motion blur.  Thus I use High ISO for my tests.

If other people read this post I would appreciate that they do this simple experience and share they're results with a 55-200 zoom and as suggest by artuk with other zoom.

Using a tripod, focus on a main subject or target with single point focus Auto Focus (using the smallest square) making sure that the focusing square covers the whole subject. Then, place smaller objects in the foreground and background (put a few within 5 cm and -5 cm of you're main subject).  You can also use an inclined ruler to have a better Idea of the focus shift (as I did). Try at wide open first, then try stepping down at  f/8 and f/11.  If possible, try to have a fast shutter speed (e.g. 1/100 sec for 50 mm or 1/300 for 150 mm) to minimize camera "shake".  Make note also of you're Camera firmware and lens firmware.  Let me (us) know of you're results.

I'll do more test in a few weeks with a better system to measure the focus shift.  Some of my friends (Canon and Fuji users) offered me to use their camera gear also.

Thanks in advance

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On 10/04/2017 at 4:22 AM, Claude said:

I've read on this forum that some users have had a similiar experiences with wrong focus or blured photos.

I really hope this can be resolved but if it's part of lens design, I have doubts.  With previous tests I discovered that you need a higher shutter speed than before (other camera e.g. APS-C with lens design for full frame cameras)  to reduce motion blur.  Thus I use High ISO for my tests.

If other people read this post I would appreciate that they do this simple experience and share they're results with a 55-200 zoom and as suggest by artuk with other zoom.

Using a tripod, focus on a main subject or target with single point focus Auto Focus (using the smallest square) making sure that the focusing square covers the whole subject. Then, place smaller objects in the foreground and background (put a few within 5 cm and -5 cm of you're main subject).  You can also use an inclined ruler to have a better Idea of the focus shift (as I did). Try at wide open first, then try stepping down at  f/8 and f/11.  If possible, try to have a fast shutter speed (e.g. 1/100 sec for 50 mm or 1/300 for 150 mm) to minimize camera "shake".  Make note also of you're Camera firmware and lens firmware.  Let me (us) know of you're results.

I'll do more test in a few weeks with a better system to measure the focus shift.  Some of my friends (Canon and Fuji users) offered me to use their camera gear also.

Thanks in advance

I've not read on this forum or other forums that there is any problem with 55-200mm AF. It is probably the least mentioned lens on Fuji forums because not only is it trouble free but it is very sharp and fast focusing. I've never had any problems with mine in the several years I've owned it. When using AF, I see no point in focusing then stopping down - why not just AF at the chosen aperture and focal length? When focusing manually I would expect the focus point to drift when changing focal lengths because the variable aperture. However, as I said, I've never had cause to use this lens in manual focus because the AF is excellent even with early Fuji bodies.

I don't have time for testing I'm afraid but those are my experiences.

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Hi VeeJayCee,

>When using AF, I see no point in focusing then stopping down - why not just AF at the chosen aperture and focal length ?

You're right.  And well, something was lost in my translation from french to english.  

I did let the AF system do it's job for each aperture chosen.  So results were better at full aperture than smaller ones.  Sure you have a better depth of field overall but the main focus goes gradually backward each time the aperture is smaller. (Note it's probable that the AF does the focus at full aperture and when you press the shutter step down to the chosen aperture)

This could explain the strange results I've had with lenses tested before (another post) with a flat test target:  f/8 was not better than full aperture using AF.  No problems in manual focus.  So Artuk is probably right in saying that it's a firmware problem on the XT-2.

I agree  that it's time consuming to do these test and I haven't update my firmware yet. I will redo these test again (with another Xt-2 body for comparison) and if not satisfied I will return the whole gear to Fuji until the results are good.  At more than 3 000 $ I expect "Creative Perfection" (Fuji's terminology).

Thanks

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2 hours ago, Claude said:

Hi VeeJayCee,

>When using AF, I see no point in focusing then stopping down - why not just AF at the chosen aperture and focal length ?

You're right.  And well, something was lost in my translation from french to english.  

I did let the AF system do it's job for each aperture chosen.  So results were better at full aperture than smaller ones.  Sure you have a better depth of field overall but the main focus goes gradually backward each time the aperture is smaller. (Note it's probable that the AF does the focus at full aperture and when you press the shutter step down to the chosen aperture)

This could explain the strange results I've had with lenses tested before (another post) with a flat test target:  f/8 was not better than full aperture using AF.  No problems in manual focus.  So Artuk is probably right in saying that it's a firmware problem on the XT-2.

I agree  that it's time consuming to do these test and I haven't update my firmware yet. I will redo these test again (with another Xt-2 body for comparison) and if not satisfied I will return the whole gear to Fuji until the results are good.  At more than 3 000 $ I expect "Creative Perfection" (Fuji's terminology).

Thanks

when the camera is focusing, look into the front of the lens to see if the aperture blades are stopped down (you can see them making a smaller hole), or fully open (a big round hole right through the lens).

If the latter the camera is focusing at open aperture, and then I assume at the end of focusing or when the shutter is released, it is stopping down. If this is the case, then the problem you see is focus shift caused by changing aperture - it happens in some lenses and ideally the lens should be focused stopped down, or the camera needs to compensate or this shift.

if the camera is stopping down t focus then it's not focus shift and sounds like a defect with the lens or camera (either yours specific example or a more generalised issue).

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HI Claude, I have an X-T2 arriving either Saturday or Tuesday depending on Easter holiday postal services so I will be able to check it out using the same gear.

Vic

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8 hours ago, veejaycee said:

HI Claude, I have an X-T2 arriving either Saturday or Tuesday depending on Easter holiday postal services so I will be able to check it out using the same gear.

Vic

VJC, 

You broke down and got an XT2. Welcome to the club :) 

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