Why does my Nikon focus confirmation dot say manual focus is correct when the photos are still blurry?
Asked 4/20/2017
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I’m using two Nikon 50mm f/1.8 Series E manual-focus lenses on a Nikon D3300. With the newer lens, images are sharp when the viewfinder focus confirmation dot says focus is correct. With the older lens, the dot indicates focus, but the resulting photos are very blurry. If I focus the older lens in Live View instead, the images are noticeably sharper, and when I return to the optical viewfinder the focus dot no longer agrees.
I’ve verified that I’m using the center focus point and tested on another Nikon DSLR with the same behavior. Since these lenses have no electronic contacts, I expected the confirmation dot to work independently of which manual lens is mounted.
Why can the phase-detect focus confirmation be wrong with one manual lens, while Live View focus is better?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
5
I think everyone is struggling with the example itself, but let me suggest an answer to the underlying question: How can the lens be badly out of focus when the green dot indicates it is in focus while, for the same lens using live view as a focus technique, it is approximately in focus.
There are two possibilities that come to mind. One is that the lens is badly flawed in some fashion that is throwing off the Phase Detection AF system (causing it to mis-read the focus state), but still allowing it to form a reasonably acceptable image when actually in focus. That seems unlikely but I guess possible. The optics involved in PD focus do not rely so much on apparent focus as angle of incidence of an object's light on different sensors. At least conceivably some form of distortion might mislead PD sensors. Have not seen it, but can imagine it is possible. Maybe.
The other is that the lens/body combination has an extreme back or front focusing issue. Focus, on most DSLR's in optical view finder mode (not live view) depend on a very precise match between the optical path for the AF system and the sensor. Fractions of a millimeter can through this off badly, causing the AF system to think it is in perfect focus when the subsequent image is not. Many bodies permit this to be fine tuned, the D3300 does not so far as I know.
Focus with live view uses the main imaging sensor so there are no path length issues and no fine tune is ever needed.
To confirm that is the issue, you need a more continuous set of subject matter in the frame, so you can see not just that it is out of focus, but where it did focus. A simple way to try this photograph a pole or similar vertical item (even a person's legs) in tall grass, where you can see where on the grass before/behind it actually focuses. The trouble with your shot is NOTHING is in focus, apparently, so it is hard to tell if it missed focus before/behind, or was just blurred for some other reason.
If it is actually focusing in front/behind consistently, you are probably out of luck with that lens (lacking AF Fine Tune, the alternative is sending both together to Nikon, crossing your fingers they do not mess up the body for other lenses fixing it for that one -- but likely they would just say "not supported"). Even if it had fine tune, your example makes it appear too extreme to fix with it (emphasis on "fine" tune).
Or... you may find something wrong in your experimentation and examples. Such things are always best done multiple times, on a tripod, locked down, and repeated from scratch checking all the settings each time. It's VERY easy to let experimental error creep in (I know from making many mistakes testing lenses and focus and fine tune).
Update: As discussed in the comments below, even if your camera had AF Fine Tune (and it does not), it likely would be unable to apply that to a manual focus lens.
Originally by user28109. Source · Licensed CC BY-SA 4.0
user28109
9y ago
0
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The focus dot is not purely “optical”; it uses the camera’s phase-detect AF sensor. That system can be fooled by some lenses or by lens defects, so confirmation is not guaranteed to be accurate with every manual-focus lens.
Based on the replies, two likely causes stand out:
- The older lens may have an optical problem such as internal haze, dust, oil contamination, or another flaw that lowers contrast and makes images look very soft.
- The lens’s rendering may confuse phase-detect focus (for example due to aberrations), so the green dot can indicate focus even when the sensor plane isn’t actually best-focused.
Live View uses the imaging sensor itself, so it is usually the more trustworthy method here. If Live View gives better sharpness than the focus dot, trust Live View.
Inspect the older lens with a bright light for haze, dust, or oily surfaces, especially wide open. If it’s clean but still disagrees with the focus dot, the lens is likely just incompatible with reliable focus confirmation on that body.
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AI9y ago
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