Lens VR unit IR leakage affects long exposures - cont'd.

Started Feb 17, 2008 | Discussions thread
Yves P. Forum Pro • Posts: 18,674
In other words ...

Turn VR off for long exposures anyways, VR is not going to correct anything slower than say 1/8 of a second anyways ...

Marianne Oelund wrote:

This is the continuation of a thread started by MaxKPhoto, in which
he disclosed his findings regarding IR pollution of long-exposure
images (30 minutes or more), when using the 18-200 VR lens. See his
thread "Design flaw in all Nikkor 18-200 VR's - All models are
defective" here
http://forums.dpreview.com/forums/read.asp?forum=1039&message=26783756 .

The thread contains a considerable amount of speculation, much of
which misses the point, but important contributions were made by
Randy Simms here

http://forums.dpreview.com/forums/read.asp?forum=1039&message=26796958 , where he showed that the 70-200VR also has a similar problem, and by Chuxter here http://forums.dpreview.com/forums/read.asp?forum=1039&message=26804001 , where he showed that the light pollution could not be from a hot surface.

So far, the only lenses that I am aware of which exhibit this
problem, are the 18-200 VR and the 70-200 VR. I have confirmed that
the 200 f/2 VR and the 200-400 f/4 VR do not produce any such
effects. I will leave it as an exercise to other forum participants
to investigate other VR lens models, and I invite them to post their
results on this thread.

The Cause
I carried out an investigation using my 70-200 VR. This lens
produces a very distinct vertical red stripe on long-exposure images,
approximately 7mm to the right of sensor center. I first ruled out
reflections from potential sources within the camera body, by imaging
the mirror box of my D300 over 30 minutes; the result was a
completely black frame showing no IR sources.
Confirming the lens itself as the IR source was done by operating the
lens on the camera, rotated away from its normal mounted position
slightly so that it was de-powered, and observing that the IR
pollution was eliminated.
The final step was to image the lens directly, to pinpoint the
source. This was done by mounting the lens on an F100 film body, and
setting up a second camera to look at the back of the lens through
the open back of the F100 while it was operating. The second camera
used was a D3 at 25,600 ISO, micro-Nikkor 105mm lens, at 10-second
exposure time. By successive approximation of camera position and
focus, I was finally able to obtain this image of the IR sources
within the lens:

This is a crop, aligned so that the center of the crop is at the
center of the subject lens. We see two IR sources (reflecting off of
the diaphragm blades, I believe), one directly left of center, and a
second dimmer source directly above center. The sources therefore
lie on the horizontal and vertical axes, which of course are the axes
along which the VR system moves the elements.
In any servo system such as the VR unit, there must be a means of
position feedback to the control electronics. In various
applications, we typically find potentiometers, strain gauges, LVDT's
or RVDT's, and optical position sensors used for this. Apparently,
the latter are employed in the VR unit. These consist of a light
source, in this case an IR LED, and a phototransistor or transistor
array which uses the light from the LED to read physical position.
In the case of the 18-200 and 70-200 VR units, then, there must be a
small amount of leakage from the IR LED which is finding its way via
reflections, back to the camera's sensor.

Remedy
Unfortunately, this would be beyond simple at-home measures to
remedy, as it would require disassembly of the lens, and installation
of masking to control the stray IR.
The best solution, at least for now, may simply be awareness of the
effect, so that the user can make informed choices regarding lenses
for special applications.

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Yves P.
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