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#211
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wrote in message news In message coh.net, (Philip Homburg) wrote: SNIP it might be useful to start with more common ones, such as D50. I've never heard that terminology before. Is that sunlight? http://www.brucelindbloom.com/CIESpe...alculator.html graphically shows the spectral contribution of several "Reference illuminants" as per CIE. D50 is somewhat similar to a light source of 5000 Kelvin color temperature, a bit warmer than 'average' daylight, the native color temperature of calibration target references. Bart |
#212
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wrote:
In message , Alan Browne wrote: Can you find the "natural" (neutral gain) ISO of the sensor? Define it, and I will find it. Simply put, that region where the sensor needs only unity gain (analog and digital). For example: http://www.dpreview.com/reviews/koni...lumi-graph.gif ....has the 7D noise lowest at ISO 200. This suggests that the unity gain ISO is somewhere in the region of ISO 200 (though not exactly there). Cheers, Alan -- -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. |
#213
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In message ,
Alan Browne wrote: wrote: In message , Alan Browne wrote: Can you find the "natural" (neutral gain) ISO of the sensor? Define it, and I will find it. Simply put, that region where the sensor needs only unity gain (analog and digital). I don't think any ISO uses unity gain. There's nothing magic about unity, anyway. Still has to pass through silicone junctions. For example: http://www.dpreview.com/reviews/koni...lumi-graph.gif ...has the 7D noise lowest at ISO 200. This suggests that the unity gain ISO is somewhere in the region of ISO 200 (though not exactly there). My guess is that it was a mistake, or an anomaly of the in-camera RAW-JPEG conversion. I wouldn't read too much into DPReview charts. Take a blackframe at each ISO, and look at the histogram of the RAW data. How wide are the bell curves? How does it look when converted to 8-bit space, with the mean offset to 128? How does a grey card, externally metered, register level-wise above the middle of the noise curve for that ISO? These are the things that are relevant, IMO; comparing JPEGsfrom cameras that shoot RAW, based on the camera's metering quirks, is useless and misleading. -- John P Sheehy |
#214
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Alan Browne wrote:
wrote: Can you find the "natural" (neutral gain) ISO of the sensor? Define it, and I will find it. Simply put, that region where the sensor needs only unity gain (analog and digital). It's not that simple; the sensor needs unity gain to do what, exactly? The camera makers don't tell us exactly how they are measuring ISO sensitivity, and there are (at least) two different acceptable ways to do it; one using highlights and one using shadows. You'd have to decide what method to use to call something "ISO sensitivity", and it may not be the same one the manufacturer used. -- Jeremy | |
#215
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Jeremy Nixon wrote:
You'd have to decide what method to use to call something "ISO sensitivity", and it may not be the same one the manufacturer used. Yea I guess no one remembers "calibrating" B&W film to their "system" of camera/meter/sensor? Nothing has changed, lots of people still haven't leaned the basics of photography, most of it's trial and error till you calibrate your own workflow. -- Stacey |
#216
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In article ,
Jeremy Nixon wrote: The camera makers don't tell us exactly how they are measuring ISO sensitivity, and there are (at least) two different acceptable ways to do it; one using highlights and one using shadows. You'd have to decide what method to use to call something "ISO sensitivity", and it may not be the same one the manufacturer used. Well, any dSLR that has an ISO 100 when measured using the shadows method will not be a big hit. So we can safely assume that for low ISO the highlights (saturation based) method is used. -- That was it. Done. The faulty Monk was turned out into the desert where it could believe what it liked, including the idea that it had been hard done by. It was allowed to keep its horse, since horses were so cheap to make. -- Douglas Adams in Dirk Gently's Holistic Detective Agency |
#217
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In article , wrote:
I just tried it now with my 550EX flash, in manual mode, based on the guide number; f/22 at 8.23 feet from flash to target with 105mm zoom. The green channel predominates here, with blue at about 80% of the green RAW values, and red at about 50%. The average green value in the white square is about 1520. The blackpoint of the 10D at ISO 100 is about 126, so that leaves us 1494 RAW values for the 90% reflectance square That would put 100% at 1660 above blackpoint. The 10D clips at ISO 100 anywhere from 3997 to 4005 or so (depending on pixel column), so the maximum above blackpoint is about 3875. 1660/3875 = 0.43, or ISO 43, if the standard is 100% reflectance. Now, I shot this in a white hallway 28" wide from 8.2 feet, so there is some extra light bouncing off the walls, so the actual value is actually lower, although the practical value in a narrow white hallway is about right. First of all, if you want to demonstrate a 1 stop headroom, you simply overexpose a stepwedge by one stop and compare it to a normally exposed stepwedge. Playing the numbers game doesn't say much. I don't think that playing with flashes and relying on the guide number is a good way to get repeatable results. -- That was it. Done. The faulty Monk was turned out into the desert where it could believe what it liked, including the idea that it had been hard done by. It was allowed to keep its horse, since horses were so cheap to make. -- Douglas Adams in Dirk Gently's Holistic Detective Agency |
#218
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#219
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In article , wrote:
In message coh.net, If you think a camera's metering is more accurate than the GN of a speedlight flash, you put a lot of faith in the wrong things, IMO. No, I think that a standalone light meter is designed to be accurate (around D50). -- That was it. Done. The faulty Monk was turned out into the desert where it could believe what it liked, including the idea that it had been hard done by. It was allowed to keep its horse, since horses were so cheap to make. -- Douglas Adams in Dirk Gently's Holistic Detective Agency |
#220
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