Lens keeps changing focus distance upon repeated AF?

Energize

Energize

Energize

Caporegime
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I've noticed that my lens will slightly change the focus distance upon repeated half-presses of the shutter, I focus perfectly in the liveview and then switch to af through the ovf and sometimes the distance scale remains static and sometimes it changes slightly, which at f/1.4 results in a soft image. What is causing this?
 
I had the same issue with a Canon 550D + 50D + 85mm 1.8.
Inaccurate AF is a super annoying issue, you can try sending it in to whichever manufacturer you shoot with, but I doubt it will come back resolved tbh.
 
take two(or more) pictures it sounds like inconsistent focus rather than a back front focusing issue which would make it harder to fix with micro adjust if you have it.

Still worth doing some repeated stable testing to see if the issue is random or a more consistent issue you could tune out.
 
As above

You should be able to do a similar test as below with a cereal box and a ruler.
See how many sharp ones you get out of 10. It's not unusual to get the odd one slightly soft, or on the 'edge' of the focal plane, but ideally you want enough in focus that you can trust your not going to 'miss' important moments.
If your results show allot of inconsistency, then the only thing you can do asside from getting a technician to give it the once over, is calibrate the AF tuning to where the lens focuses most of the time, or if it's completely random try and set it in the middle of the two extremes of the errors.

Below is what I get with my 85 @1.4, focusing again after every shot.
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a1ex2001 said:
take two(or more) pictures it sounds like inconsistent focus rather than a back front focusing issue which would make it harder to fix with micro adjust if you have it.

Still worth doing some repeated stable testing to see if the issue is random or a more consistent issue you could tune out.
Click to expand...

The lens back focuses as well, I have it set to -15 to get the correct focus most of the time.
 
Energize said:
The lens back focuses as well, I have it set to -15 to get the correct focus most of the time.
Click to expand...

I posted a link to an article to why this isn't really a problem because of the tolorences of sensor and lens. That is why the CPS was created and why you have micro adjust.

If however after micro adjust it still can't focus then you have a problem.
 
What is your target that you are focussing on? Remember that the focus sensors extend beyond the markings in the viewfinder. It can be easy to get slight differences if there are multiple contrast elements within the focus area.
 
Raymond Lin said:
I posted a link to an article to why this isn't really a problem because of the tolorences of sensor and lens. That is why the CPS was created and why you have micro adjust.

If however after micro adjust it still can't focus then you have a problem.
Click to expand...

I think it's at this stage. -15 is outside of manufacturing tolerances so technically it is a fault though easily fixed. Inconsistent focus though is another matter, a particularly frustrating one, especially if the technician send it back as being ok.

No doubt these guy's are working to KPI's, so with any issue that isn't the norm and immediately obvious, there is likely a temptation to return it saying it's fine.
 
Energize said:
I've noticed that my lens will slightly change the focus distance upon repeated half-presses of the shutter, I focus perfectly in the liveview and then switch to af through the ovf and sometimes the distance scale remains static and sometimes it changes slightly, which at f/1.4 results in a soft image. What is causing this?
Click to expand...



Do some repeated shots. Use a solid tripod, remote release and mirror lock up. Take a large set of photos against a static high contrast subject. Take a photo, then reset the focus to either extreme but take not on whether you are pulling near or far focus. Say take 15 photos when you manually pull the focus to the MFD, then another 15 when you push the focus passed infinity.

You will expect some failures depending on the camera and the lens, but most should be similar within some noise margin which is approximated by a Gaussian probability function. You should also see a consistent but small difference between focus from near or from far, but the difference should b small enough not to be a problem.


Also worth playing with liveview, as that will remove any calibration errors.
Also worth focusing and then switching to MF and take some test shots to see if your tripod is up to scratch because if you still have issues then the rest of the results will be inconclusive.
 
An Exception said:
What lens is it?
Click to expand...

Nikon 50mm f/1.4D.

D.P. said:
Do some repeated shots. Use a solid tripod, remote release and mirror lock up. Take a large set of photos against a static high contrast subject. Take a photo, then reset the focus to either extreme but take not on whether you are pulling near or far focus. Say take 15 photos when you manually pull the focus to the MFD, then another 15 when you push the focus passed infinity.
Click to expand...

What do you mean by mfd?
 
MFD = minimum focus distance.

basically manually turn the focus ring until it is at the shortest focus setting, e.g. 30cm or some such.
T
 
An Exception said:
Umm, at 1.4 the lens is soft anyway?
Tbh, there isn't much more anyone can help you with unless you post some samples and preferably do a real test as already been advised, unless for some reason test charts is uncool or something...
Click to expand...

I'm doing that now, hence asking what MFD stood for.

I don't see what the resolution of the lens at f/1.4 has to do with the autofocus.
 
Try the lens at f2.8...Make sure you be on One-Shot if on front button for people. If using back button then Al-Servo works well.
 
Energize said:
I'm doing that now, hence asking what MFD stood for.

I don't see what the resolution of the lens at f/1.4 has to do with the autofocus.
Click to expand...

As you haven't posted any examples I'm stabbing in the dark. Having owned this lens, I know it's soo soft at F1.4, that it looks out of focus.
 
Upon further examination it seems that this is not a lens issue, the body appears to back focuse all lenses by 15 points and has variable AF.
 
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