Need a quick explanation

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I've cleaning and found this in an explanation of finding the SBR of a subject. This was in an old Xeroxed article that was missing many pages, so I have no idea where it came from anymore.

Light - Dark+5=SBR
9-7+5=SBR7

That would make one photo I took a couple of weeks ago have an SBR of 13

Why would you add five to your difference. The explanation must have been on the missing pages.

I just meter the highest and the lowest and figure the difference to get my SBR. have I been doing it wrong?
 

Donald Miller

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Shooter
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From what I understand the reason for the value of 5 has to do with the fact that fact that the BTZS recommends and uses a incident meter for it's application. The hemispherical dome on the incident meter averages values to that value of 5.

With an incident meter, one measures the highlight and shadow values as they present themselves to the light that is striking them. Not as the light from those areas are being reflected back toward the camera.

Additionally the conversion from a reflective reading (spot meter) to incident meter is not a truly linear conversion. I would guess that a linear conversion would get one somewhere into the "ballpark". The BTZS has a conversion formula for reflective to incident readings.
 
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mark

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Ok. It looks like I misread the BTZS book. And the person whom i borrowed it from moved recently. Can someone tell me the formula for spot metering?

It looks like I need to go buy the book and read it again. Wouldn't it be nice if a person could just buy the portion of a book they need. Look at a chapter explanation and then select the ones you need. Then click purchase sections.
 

Donald Miller

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Shooter
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Mark,

I don't have a copy of the book to reference for the formula. I think that Jorge (Mexico) has the conversion formula. Perhaps if you message him then he could provide it to you.
 
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