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Inconsistent Incident vs. Reflected Spot Light Reading


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#1 Marquette Trishaun

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Posted 11 June 2007 - 09:46 AM

Hello All,

Recently I was doing color testing for an upcoming production.

The incident light readings using my Sekonic Light meter were perfect yielding very predicatble results.

Secondly I needed to achieve similar results using a reflected spot reading, due to some areas of the shot I simply will not be able to get to. So I devised a test to make sure I exposed properly.

Here is my test, and here is my results:

I used a brown wooden wall for the experiment.

1) My incident light reading up close on the wall with the spear pointed towards the camera was balanced to a 5.6.

2) When I positioned myself at the camera and took a reflected reading of the wooden wall, the meter yielded a wopping f-stop of 8.

In this case, I was expecting the meter to yield f-stop on the other side like say a 2.8 or something because it percieved that the dark wood needed more light to register at 18% grey midtone.

It is possible that my meter is out of cal. I point out that the only reason I am not using a grey card is because I will not be able to reach some places.

-Thanks in advance,
Marquette
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#2 Stuart Brereton

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Posted 11 June 2007 - 10:16 AM

The only way to check your meter is with a surface of known reflectance - a gray card. Perform your test again, using the gray card not the wall. If the readings still don't match, it's time to get your meters calibrated.
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#3 Michael Nash

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Posted 12 June 2007 - 01:30 AM

The 508 Cine is well known for having a discrepancy between the spot and incident readings. The good news is that you can calibrate just one and not the other. In other words, you can tune the spot function to match the incident reading (on a properly lit gray card, of course).

The other problem you can get with the 508 is that light can leak into eyepiece and throw off your reading. You have to be very careful to cup your hand around the eyepiece when taking spot readings with this meter.
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#4 Marquette Trishaun

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Posted 12 June 2007 - 10:47 AM

The 508 Cine is well known for having a discrepancy between the spot and incident readings. The good news is that you can calibrate just one and not the other. In other words, you can tune the spot function to match the incident reading (on a properly lit gray card, of course).

The other problem you can get with the 508 is that light can leak into eyepiece and throw off your reading. You have to be very careful to cup your hand around the eyepiece when taking spot readings with this meter.



Thanks alot Michael, I thought something was a little funny. Do you have any recomendations for companies that can perform this calibration.

Also, if I were upgrade to newer model in the same vain, do you know of similar issues with it also. Overall I like Sekonics functionality and would like to stay in this family.

-Marquette
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#5 Michael Nash

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Posted 13 June 2007 - 12:32 AM

You can do the compensation yourself. Rotate the eyepiece to the spot position, hold down the ISO and ISO2 buttons and scroll the wheel. You'll notice a little +/- indicator appear in the upper right of the display as you change the value. Compare the reading of a gray card until both the incident and spot readings match. My spot is adjusted to -0.4.

Of course this assumes the incident meter is calibrated properly in the first place. I usually just check mine against other people's meters on set, and rarely come up with more than .2 stop difference.

The 508 has a few issues that make it "buggy," but I haven't heard of those same problems with other models like the 608.
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#6 Marquette Trishaun

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Posted 13 June 2007 - 07:34 PM

You can do the compensation yourself. Rotate the eyepiece to the spot position, hold down the ISO and ISO2 buttons and scroll the wheel. You'll notice a little +/- indicator appear in the upper right of the display as you change the value. Compare the reading of a gray card until both the incident and spot readings match. My spot is adjusted to -0.4.

Of course this assumes the incident meter is calibrated properly in the first place. I usually just check mine against other people's meters on set, and rarely come up with more than .2 stop difference.

The 508 has a few issues that make it "buggy," but I haven't heard of those same problems with other models like the 608.



Worked like a charm. Thanks a million.

-Marquette
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#7 e gustavo petersen

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Posted 17 June 2007 - 11:59 PM

I hate to ask, but did you check to see if the exposure "Adjustment" on the spot meter was active. The Adjustment (which is different than calibration) can be set either + or - on both the Spot and Incident. Speaking from experience, my gaffer occasionally forgets to compensate for the adjustments made on the incident side when switching over to the spot side. Just a suggestion.

For what it's worth, I just purchased a new L758cine and it's off by 1/2 stop.
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#8 e gustavo petersen

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Posted 18 June 2007 - 12:52 AM

I was looking for a grey card vendor and found this website that talks about why there might be a different readings in incident and reflectance. It doesn't address Marquette's problem but I think it's worth reading.

http://www.bythom.com/graycards.htm
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#9 Marquette Trishaun

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Posted 18 June 2007 - 12:49 PM

I was looking for a grey card vendor and found this website that talks about why there might be a different readings in incident and reflectance. It doesn't address Marquette's problem but I think it's worth reading.

http://www.bythom.com/graycards.htm



Thanks for the great info, pretty much puts me back at square one, LOL. Not that everyone would care, but I definitely think puts a lot of old information floating around out there as fact to rest.

-Marquette
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#10 Stuart Brereton

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Posted 18 June 2007 - 01:23 PM

That article has been linked before, I think. I checked with Minolta, and they told me that all their meters are calibrated to 18% grey. What other manufacturers do, who knows?
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