35mm f/1.8 DX wide open back-focus at midway distances ... normal?

[epp_b]

I've been noticing my 35mm f/1.8 DX has some notable back-focus at medium level distances when used wide open at f/1.8.

This is a sample I shot with a tripod in bright overcast light:

At near focus and far focus, it's acceptably sharp. It's only at mid distances (~15ft +/-10ft) where this happens. Is this fairly normal for this lens wide open or should I be talking to Nikon about warranty service?

 

[tirediron]

Very difficult to draw any accurate conclusions from images that small.

 

[jake337]

What body are you using? Any bodies ability to focus accurately below f2.8 is meh.

 

[cgipson1]

What body are you using? Any bodies ability to focus accurately below f2.8 is meh.

Profile shows: Nikon D7000 | Nikon D40 | Tokina 11-16 | 35/1.8 | 50/1.8 | 55-200 VR | 75-150/3.5 Series E | SB-600 | Alienbees Cybersyncs

Betting D7000!

 

[tirediron]

What body are you using? Any bodies ability to focus accurately below f2.8 is meh.

Care to elaborate? This is news to me.

 

[cgipson1]

have you tried adjusting the AF-Fine tune in the D7000 to compensate?

 

[cgipson1]

What body are you using? Any bodies ability to focus accurately below f2.8 is meh.

Care to elaborate? This is news to me.

Sorry, Jake.. have to agree with John on this one. Focus wide open needs to be even more precise than smaller apertures... due to severe lack of DOF.

 

[Derrel]

Maybe a dumb question, but the camera is NOT being allowed to select the AF point in the AF-done shots, right??? You are NOT in any kind of AF-A mode or any auto AF area modes, right?

I ask because the STRONG pattern of the lattice is exactly the kind of ,well, strongly detailed, very linear subject that an AF system would definitely want to LOCK-ON to, if given the chance.

 

[epp_b]

Maybe a dumb question, but the camera is NOT being allowed to select the AF point in the AF-done shots, right??? You are NOT in any kind of AF-A mode or any auto AF area modes, right?

Nope, I'm either in AF-S with single point selection for still subjects (like this one) or AF-C 3D for sports. I never use AF-A.

have you tried adjusting the AF-Fine tune in the D7000 to compensate?

Yes, but unfortunately, fine tune assumes an equal bias for all distances. If I adjust it, it compensates nicely for mid-focus, but then near and far focus are out of wack.

 

[jake337]

Sorry, Jake.. have to agree with John on this one. Focus wide open needs to be even more precise than smaller apertures... due to severe lack of DOF.

My mistake. I must have read something wrong somewhere. I had always thought consumer bodies had trouble autofocusIng accurately wide open. Maybe its just the experience I've had with my D90 that I can never achieve the same as sharpness, wide open, as when I zoom in with live view and focus manually. I think it may be, at a distance of 10-15 feet, the center point covers more than just the eye so if seems to be hard to get tack sharp focus on the eyes wide open.

 

[KmH]

See the D7000 user's manual page 93 - Getting Good Results using Auto Focus. Every Nikon DSLR has that same page, though it may have a different page number in another camera model's user manual.

Subjects containing many fine details, like the tree trunk, and regular geometric patterns, like the lattice in the background, are situations where auto focus doesn't work very good.

 

[epp_b]

OK, I've done some shots with an actual testing sheet. It's definitely back-focusing. I also compared it to my 50/1.8 which has absolutely no problems at all getting it dead-on. But, like I said, fine tuning fixes fixes some distances and disrupts accuracy at others.

So, my question remains: is this fairly par for the course on this lens or should I send it in for warranty?

 

[Derrel]

Sounds like your 35mm f/1.8 lens might have a serious cocaine habit that's been leading to erratic, sketchy behavior...and needs to be sent to rehab...

...no wait,wait, I meant Khloe Kardashian's hubby Lamar Odom...