Subject: Re: Exposure to the right and tone placement
a long time ago .. in a post far, far, away ... Bruce mentioned that the
true ISO of the camera sensor needs to be established. each sensor in
every camera may be different. the D200 allows one to fine tune it's sensor
in increments of 1/6th of an f/stop.
Sorry to hear about your injury--hope you are on the path to full recovery. Your quote from Bruce is quite apt. In checking my D200, I find that its ISO calibration is significantly different than my D200.
Most people use Kodak 18% cards for testing, but if you take a picture of any card (white, black, or gray) according to the meter reading, the results will be similar and will produce a level in the image according to the calibration of the meter. Thom Hogan says the ISO standard uses the equivalent of 12% reflectance. Applying a gamma of 2.2 and normalizing the exposure to 106% (ISO method) to allow some headroom at the highlights, gives a pixel value of 95 (= 255*[12/106]^[1/2.2]).
The D70 gives a reading of 90, close to predicted; however, the D200 gives a reading of 121. For now, I will use the D200 setting (rather than adjusting the ISO as you point out is possible on this camera) as my D200 pictures were usually underexposed unless ETTR was used.
The ISO 12232 spec for digital still camera ISO is described briefly in this document. Interestingly, they use 18% reflectance (not 12%) in their calculations for convenience. However ISO uses the equivalent of 12% for meters.
If you want to use the D200 reading to place the highlight at 255, increase the indicated exposure by 2.9EV. The equivalent adjustment for the D70 is 2.2EV (these values are calculated by linear regression of 5 exposures bracketed about the meter reading).
In practice, one can merely photograph a card at the metered setting and then bracket upward in 0.3EV increments until the highlight reading is where you want it. For the D200, +2.3 EV gives pixel value 242 with aRGB and the normal tone curve, which is about where I want it to allow a little headroom. +2.7 EV gives 255 and the highlights may be blown; the above equations do not take into account the tone curve applied by the camera in addition to the gamma correction.
Since the camera spot may not be narrow enough, one could use a 1 degree spot meter like Bruce recommends. Indeed in this The Negative, Ansel Adams shows a picture of a Pentax Digital Spot Meter with his own markings applied via a sticker to indicate exposure for Zone 1.
With my Pentax Digital Spot Meter (which is calibrated for 18% to the best of my knowledge), +2EV gives 241 and +2.3 gives 254. As I recall, these results are similar to what Bruce got with his Minolta meter.
Indeed, your mileage does vary.
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