Date: Wed, 21 Aug 1996 19:07:16 -0700
From: nms@inamess.vip.best.com (Nick Silva)
Subject: Bizarre solution to M645 exposure problem!

Ever since I bought the Mamiya 645 Pro 2 months ago, I've had exposure problems. I usually bring any new eqpt up to Twin Peaks (in SF) to test for lens sharpness, and overall function. Well my first shots from there were 2 stops overexposed, and the meter was reading wrong, so I returned the cmaer for repair, assuming it was broken out of the box. Mamiya went over it with a fine tooth comb, and could find nothing wrong with it. I got it back on Monday, and went up to Twin Peaks again. Same problem as before! The meter read 2 stops over! I cursed the techies at Mamiya, I cursed the product, I cursed MF, and then I decided to get scientific about it. So I took the camera off the tripod, and pointed it around at various things: all normal readings...

I pointed the camera back at the scene I had just metered on the tripod...normal reading. I remounted the camera on the tripod ... 2 stops over. I removed the camera ... normal reading. I remounted the camera ... 2 stops over. Unbelievable. So that's when I started thinking about the RF and TV signals being transmitted from the big tower there, and how the tripod might act as an antenna, and cause a small current to enter thru the ground socket and perhaps change the ground refernce voltage. But it's a carbon fiber tripod! Still, I was on a quest.

So I borrowed another 645 Pro from the store, and I took my 3 tripods up the hill. They were the Gitzo 1228, a Slik U212, and a Tiltall. All 3 tripods and both cameras exhibited this phenomenon, but to varying degrees. The Gitzo was off the most, anywhere from 1-3 stops. The other 2 did not affect the meter as much, at the most 1-2 stops. Funny thing is, the cameras did not even have to *touch* the tripod to have their readings affected! As I moved the camera closer, the meter would start overexposing by up to a stop, then jump even more once mounted.

As a control, I then went halfway down the hill, and repeated the test. The effect was less, with the Gitzo giving 1-2 stops. I then went downtown, and tested again. No difference between on/off camera. I tested again when I got home. Again, no difference.

Anybody out there who's very technically minded have any idea why this is happening?

Nick Silva

Date sent:        Tue, 26 Mar 2002 22:36:31 -0500
From:             Marvin Born 
Send reply to:    mborn@k8xu.com
To:               w.j.markerink@a1.nl
Subject:          TV tower and tripods. 

Comments regarding the tripod in a RF field.

The TV signal has a wave length  that is related to the height of
the tripod and the frequency if the channel.    As you suggested
the tripod acts an an antenna, collects the RF energy and then
re-radiates it into you camera and anything else metal or
conductive in the area, including your body.


The formula is  468/frequency in Mhz. equals the wave length in
feet.     For a half wave length.  or more likely in your case,
234/freq in Mhz. equals a 1/4 wave length in feet. .    You did
not mention the channel of the TV station.
but if you had the tripod up full length, I would guess it is
between channel 2 and channel 6.

Typically,  any odd multiple of a  1/4 wave length makes a nice
antenna   so find the channel number and convert to MHz and use
the formula.

channel 6 is 82 to 88 mhz.   or 5.7 feet for a 1/2 wave,  2.85
feet for a 1/4 and 11.4 for a full wave
channel 2 is 54 to 60
channel 3 is 50 to 66

channel 10 would be 192 to 198
channel 13  is 210 to 216

(To be technically correct,  the formula should be 500 / freq,
but the difference of 468 to 500 corrects for the velocity of the
signal through free space vs travel through a conductor.   I
don't have any idea of the velocity  factor of a tripod so I
used the standard formula for wire. )

PS this is a nice web site.    lots of good stuff to read.  I
work in the broadcast industry and use a Hasselblad 500CM at
work.     For exactly the reason you just found.  I has no RF
problems.

M Born

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