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Thread: Digital metering and 18% grey

  1. #1
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    Digital metering and 18% grey

    I've been doing some technical experimenting with the grdII, and am slightly confused as to whether the results have to do with some error I'm making or just the fact that digital cameras meter differently to film.

    1st point: The GRD II spot meter seems to be very accurate, in the sense that it gives the exactly same iso and exposure value as my handheld sekonic.

    2nd point: I'd expect whatever surface it's metering to render 18% grey on the computer.

    So I take a photo of something, remember what surface I metered off, go to Aperture, do the default B&W conversion. And what happens is that the metered area is 50 % grey.

    Could the discrepancy be in the Aperture conversion? I think it's unlikely to be that large.

    (A further "error" I found is that my monitor renders 18% grey as about one stop off what a printed grey card looks like. But this should be beside the point.)

    So the question is: Do I have some concept wrong here? OR Is the difference due to the linearity (50% !!!?) of digital vs. the non-linearity of film?

    I feel like I could learn something here, but don't know where to look it up. Any clarification is greatly appreciated.

  2. #2
    Member Ebe's Avatar
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    May 2009
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    Re: Digital metering and 18% grey

    This will shed some light on the subject, or start a controversial thread.

    Contrary to popular belief, the overall, average reflectivity of an ‘average’ scene is 12 to 14 percent. This is the assumption on which traditional broad-area reflected-light exposure meters were based. An ANSI standard was created, ** ANSI PH3.49-1971 Advisory and not mandatory. Reflected light meter calibration to 12.5% reflectance, allowable error plus or minus 2%.

    Light meters, and camera metering systems, are not all calibrated to the same standard.
    Some are as low as 10%, and a few that became popular for use with the zone system were at 18%.
    However far more are calibrated using the ANSI standard 12.5%, others split the difference between 12.5 and 18 and use 14%. You need to test your equipment to determine proper exposure with your meter, camera, and processing procedures. (You might never know what target value your meter is calibrated to)

    A full frame exposure using the cameras light meter of a solid gray, white or almost black card should put
    the histogram spike between 1/3 and 4/10 on the histogram scale.

  3. #3
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    Re: Digital metering and 18% grey

    Thanks a lot for your answer Kurt.

    My point was that the Ricoh GRDs, in particular, place as you say "white grey or nearly black" surface on 55-60% grey - nowhere near 18%. This is true of the Aperture conversion, but also for the default B&W jpg.

    I still don't understand the underlying reason for this. It is certainly one of the reasons (but not by any means the only one), why the B&W conversion is so plasticky compared with film.

    Is it true only for compacts, because of the high noise in shadows? I don't have a digital SLR at hand to check.

    Below is a "zone system" table for the GRD that I came up with, using the default Aperture B&W conversion with 0 and 100% boost in the raw controls (Aperture has a very handy raw boost slider, which I often set very low).

    The default exposure is at zone 6 - the numbers are % grey.

    (There are apparently only two stops with detail above zone 6, but zone 9 can be brought back nicely with highlight recovery. So there are roughly 10 stops of dynamic range, accepting lots of noise in the deep shadows.)

    Sorry for being so set on the GRDs, these are the only digital cameras I have.


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