Why is my lens sharper wide open?

[jimmyjamjojo]

Bit of a noodle scratcher for me - My Fuji 55-200mm has always felt a little "off" to me. Nothing too obvious, but I've always believed it to be a little soft at the long end - This is usually after trying to use it for a landscape shot focusing at infinity. I can live with that, zooms do offer a compromise after all.

I decided recently (after having the thing for several years) to do some actual tests to make sure it was in fact focusing correctly or that the OIS wasn't faulty.

I got some strange results...

These shots were taken on a tripod, with OIS turned off, at base iso, auto shutter speed, and a delayed release.

Overall scene -



Crop @ F4.8 (wide open) -



Crop @ F8 -



Crop @ F11 -




Crop @ F16 -



So yeah, none are amazingly sharp, but it seems the lens is REALLY soft at F8. That explains why I was seeing poor results when using it for landscape shots, and why I had managed to get some very crisp results at 200mm at other times - they were wide open.

Both camera (X-T3) and lens are up to date, and it was like this on my X-T1 too. Visual inspection of the diaphragm doesn't show anything out of place. I've repeated these shots a number of times and get the same results every time - Sharpest wide open, worst at f8 improving slightly at 11-f16.

What are the chances fuji will fix this? I don't mind paying so long as it isn't crazy expensive. Anyone had the same/similar issue?

 

[sk66]

It would be unusual for a lens like that, but it could be an example of the focus shifting due to the aperture stopping down; and f/11-16 compensating some due to the greater DoF.

I would run the test again with a slant ruler in the image to see if the focus is shifting (as opposed to just being softer). And run it again using stopped down metering/focus (live view/exposure preview/etc) to see if that works better (eliminates focus shift).

 

[jimmyjamjojo]

It would be unusual for a lens like that, but it could be an example of the focus shifting due to the aperture stopping down; and f/11-16 compensating some due to the greater DoF.

I would run the test again with a slant ruler in the image to see if the focus is shifting (as opposed to just being softer). And run it again using stopped down metering/focus (live view/exposure preview/etc) to see if that works better (eliminates focus shift).

This was initially focused while wide open, then switched to manual focus so it would remain the same throughout the test.

Either way - I did previously do the slant ruler test as I originally suspected it was a focusing issue. I found that the lens focuses accurately, it just gets softer overall.

 

[Dave Canon]

You should not be taking landscapes with the focus set at infinity. Certainly my Canon long zooms and probably other makes focus beyond infinity to allow for changes due to heat etc. In any case I always choose a point in the landscape I want to be correctly focussed then choose an aperture that will then cover the DOF I want. You can test this by pointing your lens at a distance object which you would consider to be infinity and manually focussing. You may well find that the setting is slight closer than the infinity mark. To be honest for speed I used autofocussing with a centre spot and back button focussing so I could easily change the composition after focusing.

Dave

 

[jimmyjamjojo]

You should not be taking landscapes with the focus set at infinity. Certainly my Canon long zooms and probably other makes focus beyond infinity to allow for changes due to heat etc. In any case I always choose a point in the landscape I want to be correctly focussed then choose an aperture that will then cover the DOF I want. You can test this by pointing your lens at a distance object which you would consider to be infinity and manually focussing. You may well find that the setting is slight closer than the infinity mark. To be honest for speed I used autofocussing with a centre spot and back button focussing so I could easily change the composition after focusing.

Dave

Sorry, I was a little fast and loose with my language there - By "focus at infinity" I do infact mean focusing on something more than a few hundred meters away, usually with autofocus. I've done a bit of astro so know all about how inaccurate most lens markings are. Thanks for the heads up though!

 

[Terrywoodenpic]

Do the test again using the electronic shutter and a 10 second delay to eliminate any chance of movement. And do it at three different focal lengths , widest, middle and max tele.

 

[jimmyjamjojo]

Do the test again using the electronic shutter and a 10 second delay to eliminate any chance of movement. And do it at three different focal lengths , widest, middle and max tele.

I did try at widest and longest, and found the same results. I was also using electronic shutter as i was in the office and supposed to be working...

Good shout though, need to eliminate any possibility - I wasn't on ground floor, although it is a sturdy building and nobody was walking around. I'll repeat on solid ground, using the app to trigger it.

 

[Mintchocs]

It’s possible that this lens design suffers from focus shift.

 

[sk66]

This was initially focused while wide open, then switched to manual focus so it would remain the same throughout the test.

This would show a focus shift due to aperture changing. Forcing it to focus using f/8 (i.e. stopped down if possible) will eliminate that possibility.

Either way - I did previously do the slant ruler test as I originally suspected it was a focusing issue. I found that the lens focuses accurately, it just gets softer overall

Then I don't have any reasonable explanation for that. About the only thing I can come up with might be a sticky aperture blade causing an uneven obstruction; but that's a stretch.

 

[Terrywoodenpic]

I did try at widest and longest, and found the same results. I was also using electronic shutter as i was in the office and supposed to be working...

Good shout though, need to eliminate any possibility - I wasn't on ground floor, although it is a sturdy building and nobody was walking around. I'll repeat on solid ground, using the app to trigger it.

none of the shots look especially sharp to me. I get better from my XC 50-230 hand held
XE2, 1/250 @ f7.1 ISO 250 XC 50-230 @230mm