Green Dot Manual Focusing

Started Dec 15, 2013 | Discussions thread
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user_name
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Green Dot Manual Focusing
Dec 15, 2013

One of the tools Nikon has for those of us using manual focus lenses is the trusty green confirmation dot.

While it is a handy idea in theory, in practice it has some limitations that may make accurate focus difficult or impossible.

The issue is at its worst case when the lens is set to wide open for minimal DOF.

I ran three tests to illustrate the problem.

The first shot is of a test chart using autofocus.  There is absolutely no post processing.  The lens is the Nikkor 85mm f/1.8 G at about 6 to 7 feet from a test target sitting at a 45° angle.

I used a D800 with a remote shutter release on a Gitmo tripod on a concrete/tile floor.  Only the center focus point was used.

The autofocus was run a few times to confirm operation and checked with a loop in Live View mode.

Auto-focused Test Chart

The next test was setting the camera to manual focus.  Live View Mode was deactivated so the viewfinder could be used with the green dot.

Here the lens was defocused counter clockwise and then slowly rotated clockwise until the green dot appeared steady without blinking.

The remote shutter was activated and the shot taken.

Clockwise Manual Focus Advance

Finally, the same test was performed by defocusing clockwise and then slowly returning the focus ring counter clockwise until a steady green dot was displayed.

Counter Clockwise Manual Focus Advance

Two things were observed:

1. Autofocus seems to correctly nail the intended target spot repeatedly, however, several attempts to manual focus yielded significant front focus, regardless of the direction the focus was approached.

2. There is a significant range where the green dot confirms focus depending on the approach direction.

This has been consistently demonstrated on multiple test runs.

Conclusions

My conclusion is that the green focus dot is not reliable in all manual focus instances and when the aperture is wide open the margin of green dot focus error far exceeds the depth of field.  Essentially, it is easy to miss-focus when using the green dot as confirmation and the degree of focus error is not insignificant.

Autofocus vs Manual Focus Bias Anomaly

What I can't explain is the discrepancy between autofocus and manual focus.  Autofocus was done in Live View, which is a phase detect focus mode.  The manual focus green dot is a contrast detect mode and perhaps that explains the forward bias in manual focus mode if the two systems are not accurately calibrated.

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