Help: Discrepancy between reported and real focal length.

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fpimentel
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p.1 #1 · Help: Discrepancy between reported and real focal length.

Last year I sent to repair a Nikon 17-35mm f/2.8 that needed a new AFS engine.
The lens was repaired and when I got it back I did check it and I did some general testing (without printing the images) to be sure that it was working correctly. Generally, when I shoot landscapes (and I need a wide-angle lens) I use my Nikon 14-24mm f/2.8 and the 17-35mm basically plays the role of a “back-up lens.”

This summer I used the Nikon 17-35mm for the first time (after it was repaired) to do some basic shooting of landscape photos in Maine. When I came back to NYC and I did process the RAW files I noticed something weird. Pictures that I remembered having shoot at 17mm of focal length did show incorrect focal lengths of 22 and 24mm. The same happened with pictures that I took at 35mm. The EXIF information says that those photos were taken at 30mm or 32mm while I am sure that I did move the focus ring up to 35mm. Today I did some testing using both lenses, shooting at 17mm (full frame).

The images (NYC-lower Manhattan seen from New Jersey) that I have look quite similar and were taken with a Nikon D800, manual mode, F/16 and a shutter speed of 1/500 on a tripod to be sure that I was going to have an identical angle of view. While the EXIF of the picture taken with the 14-24 says 17mm for focal length, the one taken with the 17-35mm says 24mm (even if the focus sing was on 17mm). And both photos seems to be identical regarding the plane, the same wider angle of view and the same “effective focal length.”

The puzzle that I am trying to solve is why if the real focal length seems to be 17mm, the lens is reporting an incorrect one of 24mm? I am sure that before I sent the lens to be repaired, the lens was reporting the correct focal length.

These are the two samples, the first one using the 17-35mm, the second one using the 14-24mm. Anyone has an idea of why this discrepancy between the real and the reported focal length?

Photo 1: Nikon 17-35mm at 17mm
NIKON D800 24 mm f/16.0 1/500 sec 720 ISO 0.0 EV

Photo 2: Nikon 14-24mm at 17mm
NIKON D800 17 mm f/16.0 1/500 sec 720 ISO 0.0 EV

Sep 02, 2012 at 01:54 AM

niXer
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p.1 #2 · Help: Discrepancy between reported and real focal length.

Sounds like the CPU chip might have been replaced by accident with the 24-70s.

Maybe that or the contacts are dirty.

Sep 02, 2012 at 02:37 AM

fpimentel
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p.1 #3 · Help: Discrepancy between reported and real focal length.

I cannot open the lens and check out the chip (that is well beyond my knowledge about lenses) but I can clear the contacts. Thanks, FP.

Sep 02, 2012 at 02:55 AM

EB-1
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p.1 #4 · Help: Discrepancy between reported and real focal length.

There are contacts within the lens that encode the focal length based on the zoom rotation. They are probably not making contact or (less likely) shorted part of the time. Sometimes there is a FPC that can be damaged as well and cause similar problems.

EBH

Sep 02, 2012 at 03:08 AM

fpimentel
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p.1 #5 · Help: Discrepancy between reported and real focal length.

EB-1 wrote:
There are contacts within the lens that encode the focal length based on the zoom rotation. They are probably not making contact or (less likely) shorted part of the time. Sometimes there is a FPC that can be damaged as well and cause similar problems.

EBH

As I indicated, the lens was repaired and the AFS engine replaced. To do that the lens had to be opened and disassembled. Maybe when it was put back together the focus zoom ring was not well aligned or something was misplaced? Even if the warranty for the repair has expired, I would like to make a coherent case to the Nikon-authorized service center that did the repair.

FP

Sep 02, 2012 at 05:04 PM

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